TRS-80

TRS-80 Model I
Radioshack TRS80-IMG 7206.jpg
TRS-80 Model I with Expansion Interface
Manufacturer Tandy Corporation
Type Home computer
Release date August 3, 1977; 41 years ago (1977-08-03)[1]
Discontinued January 1981 (1981-01)
Units sold 100,000+ (As of October 1979)[2]
Operating system TRSDOS, NewDos/80
CPU Zilog Z80 @ 1.774 MHz
Memory 4 KB ~ 48 KB

The TRS-80 Micro Computer System (TRS-80, later renamed the Model I to distinguish it from successors) is a desktop microcomputer launched in 1977 and sold by Tandy Corporation through their RadioShack stores. The name is an abbreviation of Tandy/RadioShack, Z-80 microprocessor.[3] It is one of the earliest mass-produced and mass-marketed retail home computers.[4]

The TRS-80 has a full-stroke QWERTY keyboard, the Zilog Z80 processor (rather than the more common Intel 8080), 4 KB DRAM standard memory (when many 8-bit computers shipped with only 1 KB RAM), small size and desk footprint, floating-point BASIC programming language, standard 64-character/line video monitor, and a starting price of US$600[1] (equivalent to US$2500 in 2018).

An extensive line of upgrades and add-on hardware peripherals for the TRS-80 was developed and marketed by Tandy/RadioShack. The basic system can be expanded with up to 48 KB of RAM (in 16 KB increments), and up to four floppy disk drives and/or hard disk drives. Tandy/RadioShack provided full-service support including upgrade, repair, and training services in their thousands of stores worldwide.

By 1979, the TRS-80 had the largest selection of software in the microcomputer market.[5] Until 1982, the TRS-80 was the best-selling PC line, outselling the Apple II series by a factor of five according to one analysis.[3]

In mid-1980, the broadly compatible TRS-80 Model III was released. The Model I was discontinued shortly thereafter, primarily due to stricter FCC regulations on radio-frequency interference to nearby electronic devices.[6][7] In April 1983 the Model III was succeeded by the compatible Model 4.

Following the original Model I and its compatible descendants, the TRS-80 name later became a generic brand used on other technically unrelated computer lines sold by Tandy, including the TRS-80 Model II, TRS-80 Model 2000, TRS-80 Model 100, TRS-80 Color Computer and TRS-80 Pocket Computer.

History

Tandy/RadioShack TRS‑80 Model I

In the mid-1970s, Tandy Corporation's RadioShack division was a successful American chain of more than 3,000 electronics stores. After buyer Don French purchased a MITS Altair kit computer, he began designing his own and showed it to vice president of manufacturing John Roach. Although the design did not impress Roach, the idea of selling a microcomputer did. When the two men visited National Semiconductor in California in mid-1976, Steve Leininger's expertise on the SC/MP microprocessor impressed them. National executives refused to provide Leininger's contact information when French and Roach wanted to hire him as a consultant, but they found Leininger working part-time at Byte Shop and he and French began working together in June 1976. The company envisioned a kit, but Leininger persuaded the others that because "too many people can't solder", a preassembled computer would be better.[8][9][10]

Tandy had 11 million customers that might buy a microcomputer, but it would be much more expensive than the US$30 median price of a RadioShack product, and a great risk for the very conservative company.[10] Executives feared losing money as Sears did with Cartrivision,[11] and many opposed the project; one executive told French, "Don't waste my time—we can't sell computers." As the popularity of CB radio—at one point comprising more than 20% of RadioShack's sales—declined, however, the company sought new products. In December 1976 French and Leininger received official approval for the project but were told to emphasize cost savings; for example, leaving out lowercase characters saved US$1.50 in components and reduced the retail price by US$5. In February 1977 they showed their prototype, running a simple tax-accounting program, to Charles Tandy, head of Tandy Corporation. The program quickly crashed as the computer could not handle the US$150,000 figure that Tandy typed in as his salary, and the two men added support for floating-point math to its Tiny BASIC to prevent a recurrence. After the demonstration Tandy revealed that he had already leaked the computer's existence to the press, so the project was approved.[10][12]

MITS sold 1,000 Altairs in February 1975, and was selling 10,000 a year. Leininger and French suggested that RadioShack could sell 50,000 computers,[13] but others disagreed and suggested 1,000 to 3,000 per year at the target US$199 price. Roach persuaded Tandy to agree to build 3,500—the number of RadioShack stores—so that each store could use a computer for inventory purposes if they did not sell.[5][8][9][12]

Having spent less than US$150,000 on development, RadioShack announced the TRS-80 (Tandy RadioShack) at a New York City press conference on August 3, 1977. It cost US$399 ($1650 today), or US$599 ($2477 today) with a 12" monitor and a RadioShack tape recorder as datacassette storage; the most expensive product RadioShack previously sold was a US$500 stereo. The company hoped that the new computer would help RadioShack sell higher-priced products, and improve its "schlocky" image among customers. Small businesses were the primary target market, followed by educators, then consumers and hobbyists; despite its hobbyist customer base, RadioShack saw them as "not the mainstream of the business" and "never our large market".[9][14][13][15][16]

Although the press conference did not receive much media attention because of a terrorist bombing elsewhere in the city, the computer received much more publicity at the Personal Computer Faire in Boston two days later. A front-page Associated Press article discussed the novelty of a large consumer-electronics company selling a home computer that could "do a payroll for up to 15 people in a small business, teach children mathematics, store your favorite recipes or keep track of an investment portfolio. It can also play cards." Six sacks of mail arrived at Tandy headquarters asking about the computer, over 15,000 people called to purchase a TRS-80—paralyzing the company switchboard—and 250,000 joined the waiting list with a $100 deposit.[9][5][13][17][18]

Despite the internal skepticism, RadioShack aggressively entered the market. The company advertised "The $599 personal computer" as "the most important, useful, exciting, electronic product of our time".[19] Company president Lewis Kornfeld stated when announcing the TRS-80, "This device is inevitably in the future of everyone in the civilized world—in some way—now and so far as ahead as one can think", and Tandy's 1977 annual report called the computer "probably the most important product we've ever built in a company factory". Unlike competitor Commodore—which had announced the PET several months earlier but had not yet shipped any—Tandy had its own factories and distribution network, and even small towns had RadioShack stores. The company announced plans to be selling by Christmas a range of peripherals and software for the TRS-80, began shipping computers by September, and opened its first computer-only store in October. Still forecasting 3,000 sales a year, RadioShack sold over 10,000 TRS-80s Model Is in its first one and a half months of sales, and over 200,000 during the product's lifetime;[18][8][20][9][12][5][16][11][21]:4[14] one entered the Smithsonian's National Museum of American History.[22] By mid-1978 the waits of two months or more for delivery were over,[23] and the company could state in advertisements that TRS-80 was "on demonstration and available from stock now at every RadioShack store in this community!".[19]

The first units, ordered unseen, were delivered in November 1977, and rolled out to the stores the third week of December. The line won popularity with hobbyists, home users, and small-businesses. Tandy Corporation's leading position[24] in what Byte Magazine called the "1977 Trinity" (Apple, Commodore and Tandy) had much to do with Tandy's retailing the computer through more than 3,000 of its RadioShack storefronts in the USA.[25] Tandy claimed it had "7000 [RadioShack] stores in 40 countries".[26] The pre-release price for the basic system (CPU/keyboard and video monitor) was US$500 and a US$50 deposit was required, with a money-back guarantee at time of delivery.

By 1978, Tandy/RadioShack promoted itself as "The Biggest Name in Little Computers".[27][26] By 1980 InfoWorld described RadioShack as "the dominant supplier of small computers".[28] Kilobaud Microcomputing estimated that it was selling three times as many computers as Apple Computer, with both companies ahead of Commodore.[29] By 1981 hundreds of small companies produced TRS-80 software and accessories,[30] and Adam Osborne described Tandy as "the number-one microcomputer manufacturer" despite having "so few roots in microcomputing".[31] Roach became Tandy's CEO that year, Leininger became director of strategic planning, and French founded a software company. Selling computers did not change the company's "schlocky" image; the RadioShack name embarrassed business customers, and Tandy executives disliked the "Trash-80" nickname for its products. By 1984 computers accounted for 35% of sales, however, and the company had 500 Tandy RadioShack Computer Centers.[8][12][16][32]

Following the Model III launch in mid-1980, Tandy initially claimed that the Model I had not been dropped.[33] However, it had been discontinued by the end of the year. Tandy cited one of the main reasons as being the prohibitive cost of redesigning it to meet stricter FCC regulations covering the significant levels of radio-frequency interference emitted by the original design.[7][6] The Model I radiated so much interference that, while playing games, an AM radio placed next to the computer could be used to provide sounds.[34]

Hardware

Tandy/RadioShack TRS‑80 Model I PCB
Tandy/RadioShack TRS‑80 Model I Rear Panel Connectors
Tandy/RadioShack TRS‑80 Model I Level II ROM Upgrade PCB

The Model I combines the mainboard and keyboard into one unit, which became a design trend in the 8-bit microcomputer era, although the Model I has a separate power supply unit. It uses a Zilog Z80 processor clocked at 1.78 MHz (later models shipped with a Z80A). The initial Level I machines shipped in late 1977-early 1978 have only 4k of RAM. After the Expansion Interface and Level II BASIC were introduced in mid-1978, RAM configurations of 16k and up were offered (the first 16k was in the Model I itself and the remaining RAM in the EI).

The OS ROMs, I/O area, video memory and OS work space occupy the first 16 kB of memory space on the Model I. The remaining 48 kB of the 64 kB memory map space is available for program use, subject to the amount of physical RAM installed. Although the Z80 CPU can use port-based I/O, the Model I's I/O is memory-mapped aside from the cassette tape and RS-232 serial ports.

The memory map of the Model I and III render them incompatible with the standard CP/M OS, which loads at hexadecimal address $0000 with TPA (Transient Program Area) starting at $0100. A customized version of CP/M is available for both computers, but the intended portability advantage of CP/M is nullified.[35]

Keyboard

Tandy/RadioShack TRS‑80 Model I ALPS Keyboard PCB

The keyboard design is unusual. Instead of transferring data through an I/O chip, the hardware maps the keyboard to dedicated locations in the processor's memory. Performing a read from the keyboard area of the memory returns the state of a particular set of keys.

Many users complained about the TRS-80 Model I keyboard, which uses mechanical switches and suffers from "keyboard bounce", resulting in multiple letters being typed per keystroke.[36] The problem was described in Wayne Green's editorial in the first issue of 80 Micro.[37] A Keyboard De-Bounce tape was distributed, which alters the system software to reduce the effect of bounce and to slow down polling of the keyboard. The change was subsequently added to a firmware ROM revision. The keyboard hardware was also modified to minimize bounce.

The root cause of the key bounce was found to be dust and dirt entering the keyboard's switches, and contaminating the switch contacts. This problem was most common among heavy smokers (smoke and ash particles). The key switches can be cleaned, rectifying the problem, but the bounce recurs again when the keyboard is re-exposed to the contaminating environment.

Later production runs of the Model I computer replaced the nameplate with a numeric keypad. Older versions could be upgraded with a numeric keypad add-on by keyboard replacement, which also remedied the key bounce problem because other parts of the system would be updated by the service technician during installation.

Later computers came with keyboards that included numeric keypads.

Video and audio

Layout of characters and pixels on the TRS-80 display

The TRS-80 Model I was accompanied by a modified RCA black-and-white television. The color of the screen text is faintly blue (the standard P4 phosphor used in black-and white televisions). Green and amber filters, or replacement tubes to reduce eye fatigue were popular aftermarket items. Later models came with a green-on-black display.

Complaints about the video display quality were common. As Green wrote, "hells bells, [the monitor] is a cheap black and white television set with a bit of conversion for computer use".[37] (The computer could be purchased without the RadioShack monitor.)[18] CPU access to the screen memory causes visible flicker. The bus arbitration logic blocks video display refresh (video RAM reads) during CPU writes to the VRAM, causing a short black line. This has little effect on normal BASIC programs, but fast programs made with assembly language can be affected. Software authors worked to minimize the effect, and many arcade-style games are available for the Tandy TRS-80.

Because of bandwidth problems in the interface card that replaced the TV's tuner, the display loses horizontal sync if large areas of white are displayed. A simple half-hour hardware fix corrects the problem.

Like the Apple II, an unmodified Model I cannot display lowercase letters. The video RAM uses 2102 SRAM chips, which are 1Kb x 1, but the PCB has seven installed, allowing only 128 characters to be displayed (0 to 127). Leaving out an eighth chip saved $1 in manufacturing costs, which would have increased the purchase price of the computer by $15. By installing an eighth 2102 (done by piggybacking it onto chip #7), it is possible to display all 256 characters (which includes lowercase letters). The 1978 manual for the popular word processor Electric Pencil came with instructions for modifying the computer. Although the modification needs to be disabled for Level II BASIC, its design became the industry standard and was widely sold in kit form,[38] along with an eighth 2102 chip with descenders for the lowercase letters. Later models came with the hardware for the lowercase character set to be displayed with descenders.

Since there is only 1k of video RAM, the Model I's display has 64x16 characters instead of the more common 40x25 or 80x25. Both 64x16 and 32x16 video modes are supported, which is in contrast to 40x25 used by Apple and 80x24 used by some dumb terminals. The choice of 64x16 was part in economics, part in physics and part in usability. To support 80x24, twice as much static ram would be required (7 additional RAM chips, 9 chips total with the extra address decoding needed), plus there was no room on the board to put any extra chips. The Model I was originally intended to be used with a user supplied black-and-white television. With no color burst signal and with the RF stage skipped, a black-and-white TV easily displays up to 64 columns well, but 80 columns would create an unviewable image. With 1K of RAM address space, a maximum of 16 lines is possible. From a usability standpoint, 16 lines of 64 characters are more versatile than 24 lines of 40 characters, as 64 characters is the full width of a typewritten page (at the typical typewriter's 10 characters per inch with 1 inch page borders commonly used at the time).

With higher density RAM chips and dedicated purpose build monitors, higher resolution crisp displays are obtainable; 80x24 character displays are available in the Model II, Model 4, and later systems.

The Model I has no built-in speaker. Square wave tones can be produced by outputting data to the cassette port and plugging an amplifier into the cassette "Mic" line. Most games use this ability for sound effects. An adapter was available to use Atari joysticks.[39]

Peripherals

Cassette tape drive

User data was originally stored on cassette tape. RadioShack's model CTR-41 cassette recorder was included with the US$599 package.[21]:3–4 The software-based[8] cassette tape interface is slow and erratic;[36] Green described it as "crummy ... drives users up the wall", and the first issue of 80 Micro had three articles on how to improve cassette performance.[37] It is sensitive to audio volume,[18] and the computer gives only a crude indication as to whether the correct volume was set, via a blinking character on screen while data is loaded. To find the correct volume at first use, the load is started and the volume is adjusted until the TRS-80 picked up the data. Then it is halted to rewind the tape and restart the load. Users were instructed to save multiple copies of a software program file, especially if audio tape cassettes instead of certified data tape was used. Automatic gain control or indicator circuits can be constructed to improve the loading process (the owner's manual provides complete circuit diagrams for the whole machine, including the peripheral interfaces, with notes on operation).

An alternative to using tape was data transmissions from the BBC's Chip Shop programme in the UK, which broadcast software for several different microcomputers over the radio. A special program was loaded using the conventional tape interface. Then the radio broadcast was connected to the cassette tape interface. Tandy eventually replaced the CTR-41 unit with the CTR-80 which had built-in AGC circuitry (and no volume control). This helped the situation, but tape operation is still unreliable.

TRS-80 Model I computers with Level I BASIC read and wrote tapes at 250 baud (about 30 bytes per second); Level II BASIC doubles this to 500 baud (about 60 bytes per second). Some programmers wrote machine-language programs that increases the speed to up to 2,000 bits per second without a loss of reliability on their tape recorders. With the Model III and improved electronics in the cassette interface, the standard speed increased to 1,500 baud that works quite reliably on most tape recorders.

For loading and storing data from tape, the CPU creates the sound by switching the output voltage between three states, creating crude sine wave audio.

The first version of the Model I also has a hardware problem that complicated loading programs from cassette recorders. Tandy offered a small board which was installed at a service center to correct the issue. The ROMs in later models were modified to correct this.

Model I Expansion interface

Only the Model I uses an Expansion interface; all later models have everything integrated in the same housing.

The TRS-80 does not use the S-100 bus like other early 8080 and Z80-based computers.[9] A proprietary Expansion Interface (E/I) box, which fits under the video monitor and served as its base, was offered instead. Standard features of the E/I are a floppy disk controller, Centronics parallel port for a printer, and additional cassette connector. Optionally, an extra 16 or 32 kB of RAM can be installed and a daughterboard with an RS-232 port.[40] The 40-conductor expansion connector passes through to a card edge connector, which permits the addition of external peripherals such as an outboard hard disk drive, a voice synthesizer, or a VOXBOX voice recognition unit.[41][42]

Originally, printing with the Model I requires the expansion interface, but later Tandy made an alternative parallel printer interface available.

The Model I Expansion Interface is the most troublesome part of the TRS-80 Model I system. It went through several revisions. The E/I connects to the CPU/keyboard with a 6-inch ribbon cable which is unshielded against RF interference and its card edge connector tends to oxidize due to its base metal contacts. This demands periodic cleaning with a pencil eraser in order to avoid spontaneous reboots, which contributes to its "Trash-80" sobriquet. Aftermarket connectors plated with gold solves this problem permanently. Software developers also responded by devising a recovery method which became a standard feature of many commercial programs. They accept an "asterisk parameter", an asterisk (star) character typed following the program name when the program is run from the TRSDOS Ready prompt. When used following a spontaneous reboot (or an accidental reset or program crash), the program loads without initializing its data area(s), preserving any program data present from the pre-reboot session. Thus, for example, if a VisiCalc suffers a spontaneous reboot, to recover data the user enters "VC *" at the TRSDOS Ready prompt, and Visicalc restores the previous computing session intact.

The power button on the EI is difficult to operate as it is recessed so as to guard against the user accidentally hitting it and turning it off while in use. A pencil eraser or similar object is used to depress the power button and the EI has no power LED, making it difficult to determine if it is running or not.

The expansion unit requires a second power supply, identical to the base unit power supply. An interior recess holds both supplies.

The user is instructed to power on and power off all peripherals in proper order to avoid corrupting data or potentially damaging hardware components. The manuals for the TRS-80 advise turning on the monitor first, then any peripherals attached to the EI (if multiple disk drives are attached, the last drive on the chain is to be powered on first and work down from there), the EI, and the computer last. When powering down, the computer is to be turned off first, followed by the monitor, EI, and peripherals. In addition, users are instructed to remove all disks from the drives during power up or down (or else leave the drive door open to disengage the read/write head from the disk) because of potential spurious electrical activity being generated (a common problem on many early floppy drives).

The EI displays a screen full of garbage characters on power up and unless a bootable system disk is present in Drive 0, it hangs there until the user either presses the Reset button on the back of the computer, which causes it to attempt to boot the disk again, or Break+Reset was pressed, which drops the computer into BASIC. Due to the above-mentioned problems with potentially corrupting disks, it is recommended to power up to the garbage screen with the disk drives empty, insert a system disk, and then hit Reset.

InfoWorld compared the cable spaghetti connecting the TRS-80 Model I's various components to the snakes in Raiders of the Lost Ark.[36] RadioShack offered a "TRS-80 System Desk"[43] that concealed nearly all the cabling. It can accommodate the complete computer system plus up to four floppy drives and the Quick Printer. Since the cable connecting the expansion interface carries the system bus, it is short (about 6 inches). The user has no choice but to place it directly behind the computer with the monitor on top of it. This causes problems for a non-Tandy monitor whose case did not fit the mounting holes. Also, the friction fit of the edge connector on the already short interconnect cable makes it possible to disconnect the system bus from the CPU if either unit is inadvertently moved during operation.

Floppy disk drives

RadioShack introduced floppy drives in July 1978, about six months after the Model I went on sale. The Model I disk operating system TRSDOS was written by Randy Cook under license from RadioShack; Randy claimed to have been paid $3000 for it. The first version released to the public was a buggy v2.0. This was quickly replaced by v2.1.[44] requires buying the Expansion Interface, which included a single-density floppy disk interface (with a formatted capacity of 85k) based on the Western Digital 1771 single-density floppy disk controller chip. The industry standard Shugart Associates SA-400 minifloppy disk drive was used. Four floppy drives can be daisy-chained to the Model I. The last drive in the chain is supposed to have a termination resistor installed but often it is not needed as it is integrated into later cables.[45]

Demand for Model I drives greatly exceeded supply at first.[37] The drive is unreliable, partly since the interface lacked an external data separator (buffer).[36] The early version(s) of TRS-DOS are also buggy, and not helped by the Western Digital FD1771 chip that cannot reliably report its status for several instruction cycles after it receives a command. A common method of handling the delay was to issue a command to the 1771, perform several "NOP" instructions, then query the 1771 for the result. Early TRS-DOS neglects the required yet undocumented wait period, and thus false status often returns to the OS, generating random errors and crashes. Once the 1771 delay was implemented, it is fairly reliable.

In 1981, Steve Ciarcia published in BYTE the design for a homemade, improved expansion interface with additional RAM and a disk controller for the TRS-80.[46]

A data separator and a double density disk controller (based on the WD 1791 chip) were made by Percom (a Texas peripheral vendor), LNW, Tandy and others. The Percom Doubler adds the ability to boot and use Double Density floppies using a Percom-modified TRSDOS called DoubleDOS. The LNDoubler adds the ability to read and write from 5¼" diskette drives for a total of 1.2 MB storage. Near the end of the Model I's lifespan in 1982, upgrades were offered to replace its original controller with a double density one.

The first disk drives offered on the Model I were Shugart SA-400s which supported 35 tracks and was the sole 5.25" drive on the market in 1977-78. By 1979, other manufacturers began offering drives. Models 3/4/4P uses Tandon TM-100 40-track drives. The combination of 40 tracks and double-density gives a capacity of 180 kilobytes per single-sided floppy disk. The use of index-sync means that a "flippy disk" requires a second index hole and write-enable notch. One could purchase factory-made "flippies". Some software publishers formatted one side for Apple systems and the other for the TRS-80.

The usual method of connecting floppy drives involves setting the drive letter via jumper blocks on the drive controller board, but Tandy opted for a slightly more user-friendly technique where all four select pins on the drives are jumpered and the ribbon cable is missing the Drive Select line. Thus, the user does not need to worry about moving jumpers around depending on which position on the chain a drive was in.

A standard flat floppy ribbon cable is usable on the Model I, in which case the drives is jumpered to their number on the chain, or even an IBM PC "twist" cable, which requires setting each drive number to 1, but only permits two drives on the chain.

Although third party DOSes allow the user to define virtually any floppy format wanted, the "lowest common denominator" format for TRS-80s is the baseline single density, single sided, 35-40 track format of the Model I.

Third-party vendors like Aerocomp made available double-sided and 80 track 5-1/4 inch and later 3-1/2 inch floppy drives with up to 720 kB of storage each. These new drives are half-height and therefore require different or modified drive housings.

Exatron Stringy Floppy

An alternative to cassette tape and floppy disk storage was provided by Exatron. The device is a continuous loop tape drive, dubbed the "stringy floppy" or ESF. It requires no Expansion Interface, plugging directly into the TRS-80's 40-pin expansion bus, is much less expensive than a floppy drive, can read and write random-access data like a floppy drive unlike a cassette tape, and it transfers data at up to 14,400 baud. Exatron tape cartridges store over 64 kB of data. The ESF can coexist with the TRS-80 data cassette drive. It was popular with TRS-80 enthusiasts, selling over 4,000 units by 1981. Exatron also made a complementary RAM expansion board that installed in the TRS-80 keyboard to increase memory to 48 kB without the E/I.[47]

Hard drive

RadioShack introduced a 5 MB external hard disk for the TRS-80 Model III/4 in 1983. It is the same hard disk unit offered for the Model II line, but came with OS software for Model III/4. An adapter is required to connect it to the Model I's E/I.[48] The unit is about the same size as a modern desktop computer enclosure. Up to four hard disks can be daisychained for 20 MB of storage. The LDOS operating system by Logical Systems was bundled, which provides utilities for managing the storage space and flexible backup. The initial retail price for the first (primary) unit (US$2495) is equivalent to US$6300 in 2018. Later, a 15MB hard disk was offered in a white case, which can be daisychained for up to 60 MB. Like most hard disks used on 8-bit machines, there is no provision for subdirectories, but the DiskDISK utility is a useful alternative that creates virtual hard disk ".DSK" files that can be mounted as another disk drive, and used like a subdirectory would. To display the directory/contents of an unmounted DiskDISK virtual disk file, a shareware DDIR "Virtual Disk Directory Utility"[49] program was commonly used.

Printers

The "Quick Printer",[50] is an electrostatic rotary printer that scans the video memory through the bus connector, and prints an image of the screen onto aluminum-coated paper in about one second. Unfortunately, it is incompatible with both the final, buffered version of the expansion interface, and with the "heartbeat" interrupt used for the real-time clock under Disk BASIC. This can be overcome by using special cabling, and by doing a "dummy" write to the cassette port while triggering the printer.

Two 3rd party printers were for 57 mm metal coated paper, selling for approximately DM 600 in Germany, and a dot-matrix printer built by Centronics for normal paper, costing at first DM 3000, later sold at approximately DM 1500 in some stores. It has only 7 pins, so letters with descenders such as lowercase "g" do not reach under the baseline, but are elevated within the normal line.

RadioShack offered an extensive line of printers for the TRS-80 family, ranging from basic 9-pin dot matrix units to large wide-carriage line printers for professional use, daisy-wheel printers, ink jet printers, laser printers and color plotters. All have a Centronics-standard interface and after the introduction of the Color Computer in 1980, many also had a connector for the CoCo's serial interface.

FP-215 is a flatbed plotter.[51]

Software

BASIC

Three versions of the BASIC programming language were produced for the Model I. Level I BASIC fits in 4 KB of ROM, and Level II BASIC fits into 12 KB of ROM. Level I is single precision only and had a smaller set of commands. Level II introduced double precision floating point support and has a much wider set of commands. Level II was further enhanced when a disk system was added, allowing for the loading of Disk BASIC.[9]

Level I BASIC is based on Li-Chen Wang's free Tiny BASIC with some additional functions added by RadioShack.[11] It has an excellent manual[9][52][53]—the User's Manual for Level 1 by David A. Lien—which presents lessons on programming with text and humorous cartoons, making the subjects very easy to understand. Lien wrote that it was "written specifically for people who don't know anything about computers ... I want you to have fun with your computer! I don't want you to be afraid of it, because there is nothing to fear".[54][18] Level I BASIC has only two string variables (A$ and B$), 26 numeric variables (AZ), and one array, A(). Code for functions like SIN(), COS() and TAN() is not included in ROM but printed at the end of the book. The only error messages are "WHAT?" for syntax errors, "HOW?" for arithmetic errors such as division by zero, and "SORRY" for out of memory errors.

Level I BASIC is not tokenized; reserved words are stored literally. In order to maximize the code that fits into 4K of memory, users can enter abbreviations for reserved words. For example, writing "P." instead of "PRINT" saves 3 bytes.

Level II BASIC, introduced in mid-1978, was licensed from Microsoft and is required to use the expansion bus and disk drives. RadioShack always intended for Level I BASIC to be a stopgap until Level II was ready, and the first brochure for the Model I in January 1978 mentioned that Level II BASIC was "coming soon". It is an abridged version of the 16k Extended BASIC, since the Model I has 12k of ROM space. According to Bill Gates, "It was a sort of intermediate between 8k BASIC and Extended BASIC. Some features from Extended BASIC such as descriptive errors and user defined functions were not included, but there were double precision variables and the PRINT USING statement that we wanted to get in. The entire development of Level II BASIC took about four weeks from start to finish." The accompanying manual is more terse and technical than the Level I manual. Original Level I BASIC-equipped machines could be retrofitted to Level II through a ROM replacement performed by RadioShack for a fee (originally $199). Users with Level I BASIC programs stored on cassette have to convert these to the tokenized Level II BASIC before use. A utility for this was provided with the Level II ROMS.

Disk BASIC allows disk I/O, and in some cases (NewDos/80, MultiDOS, DosPlus, LDOS) adds powerful sorting, searching, full screen editing, and other features. Level II BASIC reserves some of these keywords and issues a "?L3 ERROR", suggesting a behind-the-scenes change of direction intervened between the creation of the Level II ROMs and the introduction of Disk BASIC.

Microsoft also marketed an enhanced BASIC called Level III BASIC written by Bill Gates,[55] on cassette tape. The cassette contains a "Cassette File" version on one side and a "disk file" version on the second side for disk system users (which was to be saved to disk).[56] Level III BASIC adds most of the functions in the full 16 KB version of BASIC plus many other TRS-80 specific enhancements. Many of Level III BASIC's features are included in the TRS-80 Model III's Level II BASIC and disk BASIC.

Level I BASIC was still offered on the Model I in either 4k or 16k configurations after the introduction of Level II BASIC.

Other applications

Blackjack and backgammon came with the TRS-80 for free, and at its debut RadioShack offered four payroll, personal finance, and educational programs, all on cassette.[9][21]:3 The more than 2,000 RadioShack franchise stores as of September 1982 sold third-party hardware and software, but the more than 4,300 company-owned stores were at first prohibited from reselling or even mentioning products not sold by RadioShack itself.[57][58][59] Its own products' quality was often poor. A critical 1980 80 Micro review of a text adventure described it as "yet another example of RadioShack's inability to deal with the consumer in a consumer's market". The magazine added, "Sadly, too, as with some other RadioShack programs, the instructions seem to assume that the reader is either a child or an adult with the mentality of a slightly premature corned beef."[60]

Green stated that year that although "there are more programs for the 80 than for all other systems combined" because of the computer's large market share, "RadioShack can't advertise this because they are trying as hard as they can to keep this fact a secret from their customers. They don't want the TRS-80 buyers to know that there is anything more than their handful of mediocre programs available", many of which "are disastrous and, I'm sure, doing tremendous damage to the industry".[61][62] An author wrote in a 1979 article on the computer's "mystery of machine language graphics control" that "RadioShack seems to hide the neat little jewels of information a hobbyist needs to make a treasure of the TRS-80". He stated that other than the "excellent" Level I BASIC manual "there has been little information until recently ... TRS-80 owners must be resourceful", reporting that the computer's "keyboard, video, and cassette" functionality were also undocumented.[52] The first book authorized by Tandy with technical information on TRSDOS for the Model I did not appear until after the computer's discontinuation.[59] By 1982 the company admitted—after no software appeared for the Model 16 after five months—that it should have, like Apple, encouraged third-party developers of products like the killer app VisiCalc.[63] (A lengthy 1980 article in a Tandy publication introducing the TRS-80 version of VisiCalc did not mention that the spreadsheet had been available for the Apple II for a year.[64]) By 1985 the company's Ed Juge stated that other than Scripsit and DeskMate, "we intend to rely mostly on 'big-name', market proven software from leading software firms".[65] A full suite of office applications became available from the company and others, including the VisiCalc and Multiplan spreadsheets and the Lazy Writer, Electric Pencil, and Scripsit word processors.

Despite the TRS-80's limited graphics and sound capability, independent software companies such as Big Five Software produced unlicensed versions of arcade games like Namco's Galaxian, Atari's Asteroids, and Exidy's Targ.[39] Some companies ported games from other home computers of the era, such as the original Zork adventure game. Many games are unique to the TRS-80, including shooters like Cosmic Fighter and Defence Command and strange experimental programs such as Dancing Demon,[66] a game in which the player composes a song for a devil and choreographed his dance steps to the music.[67] Microchess for the Model I has three levels of play and could be run in the 4kb of memory that was standard with this model.

Utility software such as Stewart Software's Toolkit offered the first sorted directory, decoding or reset of passwords, and the ability to eliminate parts of TRSDOS that were not needed in order to free up floppy disk space. They also produced the On-Line 80 BBS, a TRSDOS based Bulletin Board System.

Perhaps because of the lack of information on TRSDOS,[59] by 1982 perhaps more operating systems existed for the TRS-80 than for any other computer.[68] TRSDOS is limited in its capabilities, since like Apple DOS 3.3 on the Apple II, it is mainly conceived of as a way of extending BASIC to support disk drives. Numerous alternative OSes appeared, including NewDOS, a third-party rival sold by a company called Apparat Personal Computers, which went out of business in 1987. Others included DoubleDOS, DOSPlus, MicroDOS, NEWDOS/80, UltraDOS (later called Multidos), and VTOS. The last versions (6.x) of TRSDOS are actually renamed LS-DOS (aka LDOS).

CP/M is a standard operating system for business use on Z80-based machines, and versions run on all TRS-80s. Omikron Systems' Mappers board remaps the BASIC ROM to run unmodified CP/M programs on the Model I.[69]

Reception

Dan Fylstra, among the first owners, wrote in BYTE in April 1978 that as an "'appliance' computer ... the TRS-80 brings the personal computer a good deal closer to the average customer", suitable for home and light business use. He concluded that it "is not the only alternative for the aspiring personal computer user, but it is a strong contender."[18] Jerry Pournelle wrote in 1980 that "the basic TRS-80 is a lot of computer for the money. It comes ready to run right out of the box, and it can be set up by three boys – ages 9, 11, and 13 ... The Tandy/RadioShack documentation is excellent, and there are a lot of good programs available". He noted that while "just about every component of my TRS-80 has taken a trip to the local store to be fixed", "none of that cost me anything; it wasn't even inconvenient, especially with local RadioShacks all over the place ... Given the price of the TRS-80, Tandy's quality control is better than you'd expect." Pournelle criticized the quality of Tandy's application and system software—including the "needlessly complex" TRSDOS—and high cost of its peripherals. He reported, however, that with the Omikron board, additional memory, and 8- and 5 1/4-inch disk drives, "for a total cost of under $5000, you have a 48 K-byte machine capable of running all the TRS-80 programs, CP/M software, and top-grade text editors like Word Master, Magic Wand, Electric Pencil, and the Proteus editor ... all without building a single kit".[69]

Three years later Pournelle was less positive about the computer. He wrote in May 1983, "As to our TRS-80 Model I, we trashed that sucker long ago. It was always unreliable, and repeated trips to the local RadioShack outlet didn't help. The problem was that Tandy cut corners".[70] Pournelle wrote in July:[59]

I'm a little bitter about my experiences with Tandy. I had genuinely thought that the Model I was the machine of the future: an inexpensive home computer that could be expanded by stages until it would do professional work. Of course it was never that. First, Tandy tried to fence in Model I users through that goofy operating system, and then it wouldn't let RadioShack stores sell non-Tandy software. ... It had never been all that well designed, and when sales took off much faster than anticipated, the quality control system couldn't cope.

Compatible successors

Tandy replaced the Model I with the broadly compatible Model III in 1980. (The TRS-80 Model II is an entirely different and incompatible design). The Model III was succeeded by the backward-compatible Model 4 in 1983.

Model III

TRS-80 Model III

Tandy released the TRS-80 Model III on July 26, 1980. The improvements of the Model III over the Model I include:[71] built-in lowercase, a better keyboard with repeating keys, an enhanced character set, a real-time clock, 1500-baud cassette interface, a faster (2.03 MHz) Z-80 processor, and elimination of the cable spaghetti (due to its all-in-one enclosure). Furthermore, a Model III with two floppy drives requires the use of only one electrical outlet (a two-drive Model I requires five outlets: one each for the CPU/keyboard, Expansion Interface, Video Monitor, and the drives) and avoided the complicated power on/off sequence of the Model I. Shortly after the Model III's introduction, Model I production was discontinued as it did not comply with new FCC regulations as of January 1, 1981 regarding electromagnetic interference.[20][36][72][6]

Tandy distinguished between the high-end Model II[15] and Model III, describing the latter as "an administrative system, good for things like word processing, data management and VisiCalc operations" and suitable for small businesses.[73] The lowest-priced version of the Model III was sold with 4KB of RAM and cassette storage. The computer's CPU board has three banks of sockets (8 sockets to a bank) which can accept type 4116 DRAMs, so memory configurations come in 16KB, 32KB, or 48KB RAM memory sizes. Computers with 32k or 48k of RAM can be upgraded with floppy disk drive storage. There is space inside the computer cabinet for two full-height drives. Those offered by Tandy/RadioShack are single-sided, 40 track, double-density (MFM encoding) for 180k of storage. Third-party suppliers offered double-sided and 80-track drives, though to control them they had to modify the TRSDOS driver code or else furnish an alternative third-party DOS which could (see below). The installation of floppy disk drives also requires the computer's power supply to be upgraded. There is no internal cooling fan in the Model III; it uses passive convection cooling (unless an unusual number of power-hungry expansions were installed internally, such as a hard disk drive, graphics board, speedup kit, RS-232 board, etc.).

Tandy claimed that the Model III was compatible with 80% of Model I software.[73] The Model III's memory map and system architecture is mostly the same as the Model I, but the disk drives and printer port were moved from memory mapped to port I/O, thus Model I software that attempts to manipulate the disk controller directly or output to the printer (in particular Model I DOSes and application packages such as Visicalc and Scripsit) will not work. Under the supplied TRSDOS 1.3 operating system Model I disks can be read in the Model III, but not vice versa.[20] The optional LDOS OS (by Logical Systems Inc.) use a common disk format for both Model I and Model III versions.

Customers and developers complained of bugs in the Model III's Microsoft BASIC interpreter and TRSDOS.[73] Tandy/RadioShack (and TRS-80 magazines like 80 Micro) periodically published many software patches to correct these deficiencies and to permit users to customize the software to their preferences.

Differences in the WD1771 and WD1791 floppy controllers created problems reading Model I disks on a Model III (the double density upgrade in the Model I include both chips while a Model III had only the WD1791). The WD1771 supports four data markers while the WD1791 only supports two, and some versions of TRS-DOS for the Model I also use them. In addition, they are used by copy protection schemes. Software was available to allow Model I disks to be read on a Model III.

The WD1791 supports the 500 bit/s bitrate needed for high density floppy drives, but the controller is not capable of using them without extensive modifications.

TRS-DOS for the Model III was developed in-house by RadioShack rather than being contracted out like the Model I's DOS. None of the code base from Model I DOS was reused and the Model III DOS was rewritten from scratch; this also created some compatibility issues since the Model III DOS's API was not entirely identical to the Model I DOS. This was primarily to avoid legal disputes with Randy Cook over ownership of the code as had occurred with Model I DOS and also because RadioShack originally planned to include several features in the Model III such as 80 column text support that were not included. Two early versions, 1.1 and 1.2, were replaced by version 1.3 in 1981 which became the standard Model III OS. TRS-DOS 1.3 is not format compatible with 1.1 and 1.2; a utility called XFERSYS is provided which converted older format disks to TRS-DOS 1.3 format (this change is permanent and the resultant disks cannot be read with the older DOS versions).

The Model III's boot screen was cleaned up from the Model I. Instead of displaying garbage on screen at power up, it displays a "Diskette?" prompt if a bootable floppy was not detected. The user can insert a disk and press any key to boot or Break+Reset to go into BASIC.

While Model I DOS is fairly flexible in its capabilities, Model III DOS is hard coded to only support 180k single sided floppies, a problem fixed by the many third party DOSes. To that end, when RadioShack introduced hard disks for the TRS-80 line in 1982, the company licensed LDOS rather than attempt to modify Model III DOS for hard disk support.

BASIC on the Model III is 16k in size and incorporates a few features from Level I Disk BASIC

DOS 1.3 was given a few more minor updates, the last being in 1984, although the version number was unchanged. This includes at least one update that writes an Easter Egg message "Joe, you rummy buzzard" on an unused disk sector, which is reputedly a joke message left by a programmer in a beta version, but accidentally included in the production master.[74]

The Model III keyboard lacks CONTROL. Many application programs use @, while others use ⇧ Shift+. Often CLEAR is used in combination with number and alpha keys. The Model III keyboard also lacks ⇪ Caps Lock; to caps-lock the alpha keys the user presses ⇧ Shift+0. Under LDOS typeahead is supported.

Aftermarket hardware were offered by Tandy/RadioShack and many third-party manufacturers. The usual selection of add-ons and peripherals available for the Model I were offered: outboard floppy drives (two additional drives could be connected to a card-edge connector on the back panel), an outboard hard disk drive (LDOS was furnished as Tandy's hard drive OS vice TRSDOS), a high-resolution graphics board[75] (resolution 512x192 pixels), an RS-232C serial port on an internal circuit card, and a parallel printer (connected by a card-edge connector). A particularly popular hardware/software add-on was the Orchestra-90[76][77] music synthesizer. It can be programmed to play up to five voices with a range of six octaves stereophonically. A great many Orch-90 (as it was often called) music files were available for download from CompuServe. The Orch-90 was licensed from a company called Software Affair, which also produced the Model I-compatible Orchestra-85 from 1981.

Because TRSDOS 1.3 was found wanting by many users, Tandy offered (at additional cost) Logical System's LDOS Version 5 as an alternative. As with the Model I, other third-party sources also offered TRSDOS alternatives for the Model III, including Apparat's NEWDOS, Alphabit's MultiDOS, and Micro Systems Software's DOSPlus. These are compatible with TRSDOS 1.3 and ran the same applications programs, but offer improved command structures, more and better system utilities, and enhancements to the Microsoft BASIC interpreter. After writing the original Model I TRS-DOS, Randy Cook began work on his own DOS, titled VTOS, which was superseded by LDOS and also created some frustration for users as it is the only TRS-80 DOS to be copy protected.

Although mostly intended as a disk-based computer, the Model III was available in a base cassette configuration with no disk hardware and only 16k of RAM. RadioShack also offered a Level I version with 4k or 16k, otherwise identical to Model I Level I BASIC, but with the addition of LPRINT and LLIST commands for printer output. Upgrading to a disk machine necessitate installing at least 32k of RAM, the disk controller board, and an additional power supply used for the disk drives.

As with the Model I's EI, the RS-232 port on the Model III was an extra cost option and not included in the base price of the computer.

Like the Model I, the Model III sold well in the educational market. Many school administrators valued the Model III's all-in-one hardware design because it made it more difficult for students to steal components.[citation needed] InfoWorld approved of the Model III's single-unit design, simplified cable management, and improvements such as lack of keyboard bounce and improved disk reliability. The reviewer, a former Model I owner, stated "I'm impressed" and that "had the Model III been available, it's probable that I wouldn't have sold it". He concluded, "If you're looking for a computer that's not too expensive but that performs well, you would be wise to test the Model III—you might end up buying it."[36]

Don French, who had left RadioShack to found FMG Software after designing the Model I, expressed his disappointment in the new machine while trying to convert CP/M to run on it. "I've encountered numerous problems with the floppy drive and its interface. RadioShack will sell a Model III to anyone. They're trying to market it as a business computer when the existing software is woefully inadequate. 48k just isn't enough. You run out of memory before you get going. They're selling a medical package that takes up nine disks. I think the Model III is a very poorly conceived machine."[78]

Model 4

TRS-80 Model 4 (standard version)
TRS-80 Model 4P

The successor to the Model III was the TRS-80 Model 4 (April 1983, with "4" written as an Arabic numeral). It has faster Z80A 4 MHz CPU,[79] a larger video display 80 columns x 24 rows, bigger keyboard, and can be upgraded to 128KB of RAM. The Model 4 is fully compatible with Model III software and CP/M application software. A diskless Model 4 (with 16KB RAM) cost $999, with 64KB RAM and one single-sided 180K disk drive $1699, and two drives $1999; an upgrade for Model III owners cost $799 and provided a new motherboard and keyboard.[53] Tandy sold 71,000 in 1984.[80] The new computer uses the same all-in-one cabinet as the Model III, adopting a more contemporary-looking beige color scheme instead of the 1970s-style "Star Wars" black and gray used on the Model I/III. The Model 4's case also switched from spray-painted translucent plastic to molded plastic, ensuring that the coloring is permanent and not prone to peeling off.

The Model 4's first appearance in the RadioShack catalog read as follows: "Yes, it looks like a Model III, but it's much much more. Compare the price and features of our amazing new Model 4 to any other computer in its class. You'll find that for power, versatility, and convenience it is a true breakthrough. To add the same features to other computers, you'd have to pay a whole lot more."[81]

The Model 4 can run CP/M without modification. Digital Research produced for Tandy/Radio Shack a version of its CP/M 3.0 for the Model 4,[82] but it is buggy and actually provides a smaller Transient Program Area than the non-banked CP/M 2.2. A third party, Montezuma Micro, supplied a version of CP/M 2.2 that was customized for the Model 4's hardware and has a utility for reading/writing CP/M disk formats of many other brands of computer. A Borland Sidekick-like utility (Monte's Window) was sold separately that ran in the 64KB of banked RAM.

The Model 4 uses WD1770/1773 floppy controllers instead of the WD1791, which allows for a larger gap between the index hole and first sector; later releases of TRS-DOS and LDOS were modified for compatibility with the controller.

The Model 4 shipped with TRSDOS 6, an enhanced version of the popular LDOS (itself an enhancement to older versions of TRSDOS) by Logical Systems. When the Model 4 boots into TRSDOS 6 the video display switches into 80×24 mode and the entire 64KB address space is mapped as RAM. The Model 4 is also capable of running all Model III software when a Model III operating system disk is detected and loaded during bootup, with a 64×16 video mode and Model III ROMs mapped from address zero. Model 4 features, including the internal speaker, are unavailable in Model III mode.[53] Users experienced in Z80 assembler can access Model 4 features like the larger screen and banked RAM in Model III mode through its machine I/O ports.

TRSDOS 6 provides utilities and filters to the Model 4's new hardware features. Its RAM disk program is called Memdisk and can use the optional extra 64KB of ram, or a specified portion of the Z-80's main 64KB. This can hold TRSDOS's most-often needed modules and utilities with about 30KB to spare, freeing all floppy drives for data diskettes. Installing the system on the Memdisk also speeds the computer's operation greatly, as system overlays load from memory rather than from disk. TRSDOS also offers a SYSRES command which specified TRSDOS overlay modules resident in the main 64KB, permitting more free disk space on the system diskette in drive zero, which also enlarges the free space available on a system Memdisk. TRSDOS also provides a print spooler that runs as a background task while other applications are in use; it can take little user memory in an expanded machine since it can use the extra 64KB as its buffer memory. The BACKUP utility is versatile, if sometimes confusing for beginners due to its many parameters. A FORMS filter makes tailored printouts possible for applications lacking capability for formatted printing. A keyboard filter, Keystroke Multiplier, lets the user define macro strings to the CLEAR and ALPHA keys. A simple (non-symbolic) machine language debugger is standard. TRSDOS has an @DEBUG SVC available, which a programmer can insert into a program to invoke the debugger on command. Job Control Language serves as the equivalent to MS-DOS's batch processor. A JOBLOG facility records all TRSDOS commands issued. A capable terminal program, COMM/CMD, services the RS-232 serial port. TRSDOS 6, like previous versions, is supplied with a PATCH utility which allowed non-programmers to install machine code program modifications.

A redesigned version of Microsoft BASIC that resembled the company's GW-BASIC for MS-DOS was bundled with TRSDOS 6 (though it still uses line-oriented editing like Model III BASIC). New features support variable names longer than two characters, WHILE...WEND loop structures, program chaining, and user defined functions (DEF FN). Some features from the Model III BASIC were dropped such as the ability to compress BASIC statements by omitting spaces (this feature, also found in Commodore BASIC, reduced the memory footprint of programs). It also lacks the commands for setting, resetting, and testing graphics blocks on the display.

For veteran Model III BASIC programmers Model 4 BASIC has two disadvantages. First, variable names had to be separated from BASIC keywords with spaces, unlike Model III Disk BASIC which permits them to be run together. This is because the Model 4 interpreter's variable names can be up to 40 characters in length, and the Model III interpreter's variable names have to be one or two characters in length (the interpreter accepts longer names but only the first two characters are significant for uniquely identifying the variable). The Model 4 version of BASIC therefore has to search for the delimiting space to find the end of the variable name. What this means for the programmer converting his old Model III program into Model 4 BASIC is tedious editing because Model III programmers, to save memory and speed execution, typically compact their code by using two-character variable names and eliminating spaces separating variables and keywords. The second disadvantage is that Model 4 BASIC lacks the command available in Model III BASIC for sorting arrays (CMD"O"). This is a problem for programmers maintaining code for business applications, and caused many BASIC coders to write a replacement capability. Later third-party software publishers made products available to fill the gap for non-programmers. Another solution preferred by some was to continue running their programs in Model III mode under Model III BASIC, but activating the Model 4's faster speed, larger video display, and extra keys by manipulating its hardware with machine code. For those programming in languages other than interpreted BASIC, but never updated for the Model 4, this is their only option for accessing the new Model 4 features (Microsoft eventually updated their entire line of language products for the Model 4).

One notable program available only for the Model 4 was marketed by Radio Shack as DoubleDuty.[83] This is one of the first task-switching programs available for any microcomputer. It uses the upper 64KB of a 128KB machine to keep resident a second TRSDOS application, which can be switched instantly with another application loaded into the main 64KB. A third partition is available for TRSDOS library commands, such as DIR. DoubleDuty first appeared in Radio Shack's 1985 Computer Catalog (RSC-12), the same year that IBM's Topview, Apple's Switcher, and Quarterdeck's DESQview first became available. DoubleDuty was written by Randy Cook, the author of the first version of TRSDOS for the original Model I.

The Model 4's memory mapping and OS more closely resemble the TRS-80 Model II than it does the Model III. Like the Model II, there is no ROM-based OS and all OS software was RAM-resident and loaded from disk at bootup. There is only a small boot loader ROM which checks for the presence of a Model 4 OS disk and if one is not detected, it defaults to Model III mode. Also like the Model II, the OS uses vectored API calls instead of absolute addresses, and software developers were encouraged to use the API calls rather than low-level hardware access to ensure compatibility with any future iterations of TRSDOS 6.

Interfacing with the computer's hardware is likewise done differently than previously. Rather than accessing the memory-mapped keyboard and video directly, this is done entirely through the TRSDOS SVCs. This method permits I/O redirection over all the computer's devices, including the disk drives. This makes possible, for example, to "print" a document to a disk file so it can be printed at a later time when a printer was physically available. Another frequent use of I/O redirection is redirecting video output to the printer for permanent hardcopy of a program run. TRSDOS supplies the LINK, ROUTE, and FILTER commands to enable these capabilities. This method also ensures consistent communication between memory resident modules attached to the OS's logical devices. Some applications programmers, however, circumvent this device-independent approach by physically accessing the computer hardware, such as Radio Shack's SuperScripsit word processor; its programmers insisted on having the CTL255 routine built into the keyboard driver expressly for this purpose. Another offender was Anitek Software's Lescript. These two applications ignore any filter programs attached to the keyboard device, depriving the user of some capable terminate-and-stay-resident programs, such as Misosys' Sidekick-like Pro-WAM Window Application Manager.

The disk drive storage on the Model 4 are identical to the Model III, consisting of one or two single-sided full-height 5.25" drives (various brands of disk drive were used in the Model 4, while the Model III had exclusively Tandons), providing 180k of storage with TRSDOS. An additional two drives can connect via the external floppy port. Any floppy drive with the Shugart-style 34-pin interface can be used; thus it is possible to upgrade a Model 4 to use double-sided, 80 track, or even 3.5" 720 KB (low density) floppies. External hard disks were available using the computer's 50 pin expansion card-edge, which also permit other external hardware requiring direct access to the Z80 buses. These include Atari style joystick adapters or the line of data acquisition devices marketed by Alpha Products. A parallel printer can be added using another card-edge connector provided for this purpose. An RS-232 serial port was optional on the original versions of the Model 4.

The keyboard has CTRL, ⇪ Caps Lock, and three function keys. The video display can be dumped to the printer by pressing ⇧ Shift+:. CTRL+R repeats the last TRSDOS command. TRSDOS supports a typeahead feature with an 80 character key buffer. It uses the TRSDOS event tasker, so operations that disable Z80 interrupts (such as floppy disk access) can result in missed keystrokes.

The video display RAM and keyboard matrix are not memory-mapped as on the previous Models I and III. Rather, whenever a program called functions requiring video RAM to be modified, or the keyboard matrix to be read, TRSDOS executes code that switches out (made inaccessible) the uppermost three kilobytes from the Z80 address space (hexadecimal F400 - FFFF). This is replaced with the keyboard matrix from the address range F400 -F7FF, and the 2 KB static RAM of video memory from F800 - FFFF. The video display actually requires only 1920 bytes to render the 80x24 text screen; the remaining memory holds the typeahead buffer and the code that accessed it. While this frees 3 KB of Z80 address space for programs, it is marginally slower than writing directly into video RAM. This banked keyboard/video address space is compatible with the external 32 KB memory banks; it is not necessary to switch in Bank 0, the Z80's upper 32 KB of base memory, to access the keyboard or video memory. However, some third-party memory expansion kits do not allow this.

The video hardware supports characters in reverse video and Model I/III style block graphics. Due to the different screen resolution (640x240 pixels rather than 512x192 pixels), the block graphics characters are not of uniform height. The ten vertical pixels in each character cell are divided into three graphic blocks, the upper two of which were four pixels high, and the bottom graphics block takes the remaining two vertical pixels. These irregular graphic blocks make video games in Model 4 mode unattractive; game programmers prefer running in Model III video mode. A 40 character wide display mode is available using a double-width raster image. By manipulating the video hardware in machine code, the Model III video modes (64 and 32 character columns with 16 rows) are available as well. An alternate character set is available which included the entire Greek alphabet and special symbols.

As with the Model III, the Model 4 was available in a baseline model with no disk drives and only 16k. It uses a 16kx1 DRAM known as the 8040517. This chip, also used in 16k TRS-80 CoCo 2s, is functionally identical to a 4116 DRAM, but only uses +5V power instead of the 4116's tri-voltage power. 64k and 128k models use standard 4164 DRAMs. Unlike previous machines, the RS-232 port is standard equipment and no longer an extra cost option. The RS-232 and printer ports are also moved to the back instead of the underside of the computer, making them more easily accessible.

The Model 4 was announced in the same press release (April 1983) as was the TRS-80 Model 100 laptop. The two computers were often marketed by Tandy/Radio Shack as a complementary pair. Model 100 cassette tapes are readable on the Model 4 with its cassette interface and a TRSDOS 6 utility called TAPE100/CMD, which store Model 100 programs and data as TRSDOS disk files. Programs and files can also be transferred back and forth via an RS-232 serial cable. Both the Model 4 and the Model 100 came with terminal software as a built-in feature.

The Model 4 has the ability to display 640×240 or 512×192 high-resolution monochrome graphics with an optional board which attaches to a socket connector on the logic board. The Radio Shack Model 4 graphics board sold for $249 and include a modified version of Microsoft BASIC (called BasicG) with commands for drawing basic geometric shapes and manipulating arrays in graphics RAM. The graphics screen memory is separate from the usual character screen, and the two can be displayed together or separately. A third-party company called Micro Labs based in Richardson, Texas sold a compatible graphics board which was popular because it was sold for only $199 (initially). It comes with a different BASIC providing for graphics commands called GBasic. Whereas the Radio Shack board has 32KB of graphics RAM, the Micro Labs board has 20KB. Since only 19,200 bytes are required to render a 640 x 240 pixel screen, the additional RAM on the Radio Shack board is available for windowing the viewable screen around a larger virtual area. The graphics RAM is accessible through four Z-80 I/O ports and is especially easy to access for uses other than graphics. For example, a public domain utility called Grafdisk adds the graphics memory to regular banked memory to create a larger TRSDOS ramdisk (96K on the Radio Shack board).

Early versions of the Model 4 mainboard were designed to accept a Zilog Z800 16 bit CPU upgrade board to replace the Z80 8 bit CPU but this option was never released. In 1987 a company called H.I. Tech produced an enhanced CPU board, the XLR8er, using the Hitachi HD64180 Z-80 compatible processor. It runs with a 6.144 megahertz clock rate and adds 256K of memory that could be used as a RAM disk. When combined with the upper 64K of Model 4 banked RAM, a ramdisk of up to 384K can be mounted. Later, software was developed that can access XLR8er RAM as standard TRSDOS 32K banks through the @BANK supervisor call. This makes the extra memory accessible to standard TRSDOS applications coded to use banked RAM. The Hitachi CPU also executes many Z-80 instructions in fewer clock cycles than the Zilog chip; 8Mhz performance was claimed but in reality most software realize a performance improvement of only 25 to 30 percent. Exceptions are programs that made heavy use of the Z-80 block instructions like LDIR and CPIR. This group of instructions take 21 transition states per iteration to execute on a Z-80; on the HD64180 they take only 14. These sorts of programs realize an effective clock speed of 7 megahertz (75% speedup) with the XLR8er board. Enthusiast created programs can access the 64180's DMA channels to attain an even higher rate of data transfer.

The Model 4 include a sound generator, a first for the TRS-80 line as the Model I/III require the user to output sounds to the cassette port, which is then connected to a stereo amplifier for sound output. However its sound capabilities are extremely limited, with just seven tones that can be produced, ranging from C to G♭ on the musical scale, and there is no E. Each tone had 32 different durations it can be sounded for. TRSDOS includes a filter program, CLICK/FLT, that beeps the speaker each time a key was pressed, to provide audible feedback for the typist. One Radio Shack spokesman described the Model 4's sound generator as "being intended for business alerts".

BYTE in October 1983 noted the lack of native software for the Model 4 but praised its backwards compatibility and TRSDOS 6's new features. The magazine concluded that the Model 4 "provides a lot of flexible computing power ... Radio Shack has a guaranteed winner".[53] Creative Computing chose the Model 4 as the best desktop computer under $2000 for 1984, stating that the $1299 price for a system with two disk drives was "a real bargain".[84]

Gate Array Model 4

The original version of the Model 4 (Radio Shack catalog number 26-1069) does not use gate array logic chips on its CPU board, but rather Programmable Array Logic chips (PALs). Starting from late 1984, a revised version was produced which came to be known as the Gate Array Model 4 (catalog number 26-1069A). This change greatly reduced the chip count and allows the circuitry for the Floppy Disk Controller and the RS-232 serial port to be included on the CPU board (making this new Model 4 a single-board computer, unlike the original 26-1069). The upgrade to 128K does not require the special PAL chip available only from Radio Shack, allowing users to expand the memory themselves with third-party RAM chips. The Gate Array machine shipped with a green video screen instead of the black-and-white screen, and the arrow keys on the keyboard are grouped together into a single cluster (the old Model 4 had two arrows on each side). Veteran TRS-80 game players were quite unhappy with the new arrangement of arrow keys.

The position of the RS-232 port's DB-25 connector was improved. On the non-gate version this points straight down at the surface on which the computer rested. The gate array's connector points directly to the computer's rear, making the cable connection much easier and reliable.

An improvement was made in the computer's speed of execution. The original Model 4, though advertised as a 4 MHz machine, actually performs at an effective speed of 3.5 MHz because wait states are inserted for the slower PAL support circuitry. The Gate Array CPU board allows the Tandy engineers to clock the Z-80 at an actual 4 MHz clock rate without any wait states. This difference in operating speed makes many third-party hardware modifications, particularly speedup kits, troublesome to install on the older Model 4.

Model 4P

A "luggable" version known as the Model 4P (September 1983, Radio Shack catalog number 26-1080) is a self-contained unit with a case design similar to that of a portable sewing machine. It has all the features of the desktop Model 4 except for the ability to add two outboard floppy disk drives and the interface for cassette tape storage (audio sent to the cassette port in Model III mode are sent to the internal speaker). It was sold only with the two internal single-sided 180KB drives. It was later made with the Gate Array technology (catalog number 26-1080A). 80 Micro published an article describing a simple motherboard modification to enable the installation of two external floppy drives.

The 4P's CPU board lacks the Model III ROM chips containing the Model III Microsoft BASIC interpreter. Instead the computer is furnished with a floppy disk labelled "Model III/A". This is called the "Model III ROM Image" disk. If the operator wants to boot a Model III DOS, he inserts this disk into the boot drive after powering up. Once it is loaded, he replaces the ROM Image disk with his Model III DOS boot disk and presses reset. From then on the computer behaves exactly like a Model III.

The 4P has a slot for an internal modem board. The Radio Shack modem board uses its own proprietary command set and only supports communications at 300 baud. Later a third-party company called Teletrends produced a 1200 baud modem board that uses the industry-standard Hayes AT command set.

It has an internal fan; its compact design does not permit it to use passive convection cooling as did the desktop Model 4.

A notable aspect of the 4P's video display is the solid, fully formed appearance of its text characters. This resulted from the fact that its 9-inch screen has the same 640x240 resolution as the desktop's 12-inch screen, producing a greater density of pixels.

Tandy discontinued the 4P by spring 1985, stating that "even though you won't find a more enthusiastic and devoted group of owners than our Model 4P folks, transportables just weren't moving well for any company that also sold a desktop version".[65]. The best-selling transportables at this time were those of the Kaypro line by Non-Linear Systems, which ran the popular CP/M OS and bundled the MicroPro line of applications including the bestselling Wordstar.

Model 4D

Tandy's first MS-DOS computer was the Tandy 2000 in 1983. By 1985 it also sold the popular 1000. The company stated that year that although Tandy had discontinued the 4P and other Model 4 variants, it intended to produce the computer "until the marketplace tells us it is no longer a product" and promised "a new double-sided drive version this fall".[65] Tandy also had outstanding contracts with public school districts throughout the US for continued support of the TRS-80s in classroom use.

The final version of the Model 4 is the Model 4D (Radio Shack catalog number 26-1070) in 1985. It is a Gate Array desktop machine featuring dual TEC-branded (not TEAC) FB-503 disk drives[85] with a capacity of 360KB each (40 track, double-sided). Rather than using a lever-style latch as had previous Model 4 drives (various brands were used throughout its production run), the new drives use a twist-style latch that provides for more reliable clamping. They are half-height drives mounted with full-height faceplates. These drives can format 42 tracks (or cylinders) with no difficulty, though this was unadvertised and not officially approved by Tandy or Logical Systems. This technique increases the available storage on each disk to 378KB. To create such a floppy disk, the user needs to specify the (CYL=42) parameter on the command line when invoking the FORMAT utility.

The Deskmate productivity suite was bundled with the 4D. It supplies simple applications including a word processor, filer, spreadsheet, calendar, and mail manager.

Model 4Ds sold during 1987 and later shipped with an updated version of its operating system, now called LS-DOS 6.3 after its third-party developer Logical Systems. It provides scores of enhanced features, the most important of which is the ability to handle file dates through December 31, 2011. The original TRSDOS 6 licensed to Radio Shack can only handle dates through December 31, 1987. Files are now time-stamped as well. Another useful feature modifies the BASIC interpreter to access LS-DOS Supervisor Calls using integer variables, without having to resort to high memory subroutines coded in Z-80 assembler. Basic commands like LIST, EDIT, and PRINT are accessible via single keystrokes. Also welcome in LS-DOS 6.3 is the TED/CMD simple text editor. The TRSDOS non-interactive BUILD command had previously been the only method of creating plain text files. TED's chief virtue is that it occupies only 3KB of disk space while offering decent cursor movement and block capabilities. This is no small convenience for a system with only floppy disk storage.

The Model 4D is the last computer descended from Radio Shack's original Model I from 1977. It is not branded as a Radio Shack product, however. The badge mounted on its front cover brands it as the "Tandy TRS-80 Model 4D". This change in marketing resulted from Tandy corporation's desire to enhance its stature in the marketplace, because it was perceived by some in the computer press that the old "Radio Shack" moniker connoted an image of inferior quality. The Model 4D is the last computer to bear the "TRS-80" name. It retailed for $1199 at its introduction in 1985. During 1987–1988 the retail stores removed the Model 4Ds from display but they were kept in the yearly computer catalog and were available by special order through 1991, when they were closed out for $599. Parts and repair service remained available for several years longer.

Clones

TRS-80 Model I clone, the DGT-100 by DIGITUS Ind. Com. Serv. de Eletrônica Ltda.

Many clones of the TRS-80 Model I came on the market: the Lobo Max-80 (Lobo also produced their own version of the Expansion Interface), the LNW-80 Models I/II and Team computers (LNW also produced an alternate version of the Expansion Interface), and the Dutch Aster CT-80, a computer that could run both TRS-80 and CP/M software, and also had all the improvements of the later Model III.[86]

EACA in Hong Kong made a Model I clone that was marketed around the world under different names with modifications. In Australia and New Zealand it was the Dick Smith System 80, in North America it was PMC-80 and PMC-81, in Hungary the HT-1080Z, in South Africa the TRZ-80, and in Western Europe it was Video Genie. The expansion bus is different and EACA also made its own Expansion Interface to fit it. There are several versions, and it was later split into a 'home' and a 'business' version, Genie I and II, and System-80 Mark I and II, where the II has a numeric keypad instead of the built-in cassette player. EACA's Colour Genie is also based on TRS-80 Model I but with improved graphics and other changes, reducing its compatibility.[86]

In Brazil several manufacturers developed clones for models I/III/IV. Dismac series D8000/D8001/D8002 (all three Model I clones) are the first personal computers manufactured in industrial scale in South America. Digitus made the DGT-100 and DGT-1000, Prológica made the highly successful CP300 and CP500 series (both Model III clones), Sysdata Eletrônica Ltda. made the Sysdata Jr. Prologica also made the CP400 / CP 400II which are copies of the TRS-80 Color Computer, with the external case being almost a copy of the Timex Sinclair 2068.[86]

In Germany, S.C.S. GmbH in Mörfelden-Walldorf offered the Komtek-I Model I clone.[86] Noteworthy was its four relay switching outputs.

In the Soviet Union, some ideas from the TRS-80 were used in development of the Корвет (Corvette) home/school computer.

Meritum (from Poland) is TRS-80 compatible. Based also on Iron Curtain similar parts like U880 instead of Z80,[87] it was advertised as based on successful and popular design, despite being made more than ten years after the TRS-80 premiere.[88]

See also

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