TRS-80

For the Chicago-based electronica group, see TRS-80 (group).

File:Trs80 2.jpg

TRS-80 Model I.

TRS-80 (also affectionately or derisively known as the "Trash-80") was the designation for several lines of desktop microcomputer systems produced by the Tandy Corporation and sold through its RadioShack stores in the late-1970s and 1980s. These systems were designed for hobbyist, home, and small-business use.

Early Z80-based home systems

History

Announced at a press conference in August 3, 1977, the Tandy TRS-80 Model I was Tandy's entry into the home computer market, meant to compete head on against the Commodore PET 2001 and the Apple II. At $599 for a complete package including cassette storage, the computer was the most expensive single product Tandy's Radio Shack chain of electronics stores had ever offered. Company management was unsure of the computer's market appeal, and intentionally kept the initial production run to 3,000 units so that, if the computer failed to sell, it could at least be used for accounting purposes within the chain's 3,000 stores.

Tandy ended up selling 10,000 the first month and 55,000 its first year. Before its January 1981 discontinuation, Tandy sold more than 250,000 Model I's.

Hardware

The Model I combined the motherboard and keyboard into one unit, in what was to be a common case-design trend throughout the 8 and 16 bit microcomputer eras, although it had a separate power supply unit. It used a Zilog Z80 processor clocked at 1.77 MHz. The basic model originally shipped with 4K of RAM, and later 16K.

Keyboard

The transfer of information about what keys were being pressed was unusual, in that instead of transferring data via an I/O device or chip, the hardware mapped the keyboard to a pre-defined location in memory, i.e., there was no 'real' memory at this location, but performing a read from the keyboard area of the memory map would return the state of a particular set of keys.

A version of the computer was produced which replaced the nameplate with a numeric keypad.

Many users complained about the TRS-80 keyboards which were mechanical switches and suffered from "Keyboard Bounce" resulting in multiple letters being typed accidentally. A Keyboard De-Bounce tape was distributed, which slowed down polling of the keyboard to compensate. Eventually, this was added to a later ROM revision.

Video

It was accompanied by a white on black display, which was a modified RCA XL-100 Black and White television. The actual color of the system was light bluish (the standard "P4" phosphor used in black-and white televisions), and green and amber filters or replacement tubes, to make the display easier on the eyes, were a common after market item. Because of bandwidth problems in the interface card that replaced the TV's tuner, the display would lose horizontal sync if large areas of white were displayed; a simple hardware fix (involving less than half an hour's work) could be applied to correct that. A display unit that was green by default also existed, made from another modified television chassis, but its casing was entirely incompatible with the Model I expansion unit (The expansion unit was designed specifically for the XL-100 screen, with recesses in its casing to hold the feet of the display).

Layout of characters and pixels on the TRS-80 display

The video hardware could only display text at 64 or 32 characters wide by 16 lines resolution in upper case. This was because the video memory system used but a single kilobyte of video memory, seven bits wide, with the seventh bit used to differentiate between text and "semigraphics" characters. Aftermarket Lowercase upgrades (which were very popular and referred to as the "Electric Pencil Modification" after a popular Wordprocessor of the time) added the 8th bit and through use of a switch, one could go back and forth between the original 7 bit or 8 bit video.

Primitive graphics ("text semigraphics," rather than a true bitmap) could be displayed because the upper 64 characters of the 128 character set displayed as a grid of 2×3 blocks (very similar to Teletext). BASIC programs could write directly to the resulting 128×48 grid.

Any access to the screen memory, writing to it using the BASIC statement PRINT or accessing the screen memory directly, caused "flicker" on the screen. The bus arbitration logic would block video display while access was given to the CPU (causing a short black line). Normal BASIC programs would not be much affected by this, but fast programs made in assembly might be badly affected by this effect if the programmer didn't take it into consideration. This was not as bad as a Timex ZX81, where the entire screen flickered, and many software authors were able to minimize this effect. Not withstanding this primitive display hardware many arcade-style games were available for the Tandy TRS-80.

Peripherals

User data was stored on cassette tape. The cassette tape interface was extremely sensitive to audio volume changes, and the machine only gave the very crudest indication as to whether the correct volume was set, via a blinking character on screen when data was actually being loaded - to find the correct volume, one would sometimes have to attempt to load a program once adjusting volume until the machine picked up the data, then reset the machine, rewind the tape and attempt the load again. Automatic gain control or indicator circuits could be constructed to compensate for this, (fortunately the owner's manual provided complete circuit diagrams for the whole machine, including the peripheral interfaces, with notes on operation) and there was also an alternative tape interface that one could build in order to receive transmissions from the BBC's "Chip Shop" programme in the UK, an experiment in transmitting free software for several different BASIC home microcomputers, in a common tape format, over the radio. A special program (loaded using the conventional tape interface) was needed to access the custom interface over the expansion port and then load the recorded software.

To upgrade to a floppy disk based system the owner had to buy the "Expansion Interface" (or "E/I") that added a "single density" floppy disk interface. This was based on a Western Digital 1771 single density floppy disk controller chip, but it lacked a separate external "data separator", and was thus very unreliable. There was also the ability to expand to up to a total of 48k of RAM, a second cassette tape port to use two tapes simultaneously, a serial interface (option) and a centronics printer interface. The Expansion Interface was the most troublesome part of the system, having gone through several modifications (a pre-production version is said to have looked completely different, and to have had a card cage) before on-board buffering of the bus connector lines cured its chronic problems with random lockups and crashes. Its edge card connectors tended to corrode due to the use of two different metals in contact, and would periodically have to be cleaned with a pencil eraser. The E/I required a second power supply unit identical to that of the TRS-80 itself, but was designed with a recess in its casing capable of holding both PSU units and removing some of the clutter around the system. The E/I-TRS-80 cable was also only a couple of inches in length, meaning the user had no choice but to use the official layout of placing the E/I directly behind the computer and placing the monitor on top of the E/I - this caused problems if one owned a monitor whose case did not fit the E/I mounting holes, as mentioned above. The short length of interconnect cable also meant that the unit could easily be disconnected accidentally if the main unit was moved.

One unusual peripheral offered was a "screen printer": an electrostatic rotary printer that scanned the video memory through the same bus connector used for the E/I, and printed an image of the screen onto aluminum-coated paper in about a second. Unfortunately, it was incompatible with both the final, buffered version of the E/I, and with the "heartbeat" interrupt used for the real-time clock under Disk BASIC. This could be overcome by using special cabling, and by doing a "dummy" write to the cassette port while triggering the printer.

A Data Separator and/or a Double Density disk controller (based on the WD 1791 chip) were made available by Percom (a Texas Peripheral Vendor), LNW, Tandy and others. The Percom Doubler added the ability to boot and use Double Density Floppies (they provided their own modified TRSDOS called DoubleDOS), and included the Data Separator. The LNDoubler added the ability to read and write from 8" Diskette Drives for over 1.2mb of Storage.

All TRS-80 disk formats were soft-sectored with index-sync (as opposed to the Apple II formats, which were soft-sectored without index sync, with many Apple drives lacking even an index hole detector), and except for some very early Shugart drives (recognizable by their spiral-cam head positioner), all TRS-80 floppy drives were 40-track double-density models. The combination of 40 tracks, double-density, and index-sync gave a maximum capacity of 180 kilobytes per single-sided floppy disk, considerably higher than most other systems of the era. On the other hand, the use of index-sync meant that in order to turn a floppy disk into a "flippy," it was necessary not only to cut a second write-enable notch, but also to punch a second index hole window in the jacket (at great risk to the disk inside). Or one could purchase factory-made "flippies," or use the back side for Apple Computer systems (as some software publishers of the era did).

[Edit: the drives sold by Radio Shack were 35-track models with a 160K capacity.]

BASIC

There were two versions of the BASIC programming language produced for the Model I. Level I BASIC fit in 4K of ROM, and Level II BASIC fit into 12K of ROM. Level I was single precision only and had a smaller set of commands. Level II introduced double precision floating point support and had a much wider set of commands. Level II was further enhanced when a disk system was added, and the Disk Based BASIC was loaded.

Level I Basic was Li-Chen Wang's free Tiny Basic, hacked by Radio Shack to add functionality. It achieved a measure of noteworthiness due in large part to its outstanding manual, which presented lessons on programming with text and colorful graphics, making the subjects very easy to understand. The basic had only two stringvariables (a$ and b$), 25-23 variables and just the possibility of one array. Basic code for functions like sin(), cos() and tan() was not included in ROM but printed at the end of the book.

Level II BASIC was licensed from Microsoft. It was a cut down version of the 16K Extended BASIC, since the Model I had 12K of ROM space. The accompanying manual was not nearly as colorful and suited for beginning programmers as the Level I Basic manual. Users had to take their TRS-80 computers to the Radio Shack dealers to have the machines upgraded from Level I to Level II BASIC.

See "TRS-80 architect.htm" (TRS-80 architect reminisces about design project) for a complete discussion.

The Disk Based BASIC added the ability to perform disk I/O, and in some cases (NewDos/80, MultiDOS, DosPlus, LDOS) added powerful sorting, searching, full screen editing, and other features.

Microsoft also marketed a tape-cassette based enhanced BASIC called Level III BASIC. This added most of the functions in the full 16K version of Basic.

The first models of the Model I also had problems reading from the cassette drives. Tandy eventually offered a small board which was installed in a service center to correct earlier models. The ROMS in later models were modified to correct this.

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.

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 was different and EACA also made its own Expansion Interface to fit it. There were 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 would have a numeric keypad instead of the inbuilt cassette player. EACA's Colour Genie was also based on TRS-80 Model I but with improved graphics and other changes it was not very compatible.

In Brazil there were several manufacturers of different Model I/III/IV clones. Digitus made the DGT-100 and DGT-1000, Prologica made the highly-successful CP300 and CP500 series, (insert name here) made the Sysdata Jr. Dismac made the D8000/D8001/D8002 series. Prologica also made the CP400 / CP 400II which were copies of the TRS80-Color, with the external case being almost a copy of the Timex 2068

Tandy sold the LNW-80 computers with a Tandy Brand in Mexico.

Model III

As a follow on to the Model I, in July 1980 Tandy released the Model III, a more integrated and much improved Model I. The improvements of the Model III included built-in lower case, a better keyboard, and a faster Z-80 processor. With the introduction of the Model III, Model I production was eventually discontinued as the Model I's did not comply with new FCC regulations regarding radio interference. In fact, the Model I's radiated so much RFI (Radio Frequency Interference) that many game companies made their games so you could put an AM radio next to the computer and use the interference to get sounds. The TRS-80 Model III also came with the option of integrated disk drives.

Model IV

The successor to the Model III was the Model 4 (April 1983), which included the capability to run CP/M. (This had previously only been possible via a hardware modification that remapped the BASIC ROMS away from memory address zero.) Prior to the Model 4, CP/M support was only possible with a third-party add-on, sold as the Mapper board. The Model 4 also had the ability to display high-resolution graphics with an optional board. The Model 4 also came in a "luggable" version known as the Model 4P (1984) which was portable. It was a self-contained unit that looked like a small sewing machine.

Business systems

Model II

In October 1979, Tandy began shipping the Model II, which was targeted to the small-business market. It was not an upgrade of the Model I, but an entirely different system, built using the faster Zilog Z80A chip running at 4 MHz, with the computer, disk drive, and monitor built into a single cabinet, and a detached keyboard. It was available with 32K or 64K of RAM; hard disk drives and additional floppy drives were available as options. It ran TRSDOS-II and BASIC. TRSDOS-II was not very compatible with TRSDOS for the Model I, thus the Model II never had the same breadth of available software as the Model I. This was somewhat mitigated by the availability of the CP/M operating system from third parties such as Pickles & Trout.

Tandy offered a desk custom-designed for the Model II for US$370. It could hold an additional three 8" disk drives or up to four 8.4MB hard drives.

An upgrade to the Model 16 was eventually offered. The Model II was replaced by a cost-reduced Model 12, which did not include an expansion card cage, though one was available as an option.

Model 16, Model 16B, and Tandy 6000

Tandy later released the TRS-80 model 16, which was a follow on to the Model II; an upgrade was available to Model II owners. The Model 16 had both the 8-bit Z-80 and 16 bit 68000 microprocessors, and could run either TRSDOS-16 or Xenix (Microsoft's version of Unix). The Model 16 evolved in to the Model 16B then the Model 6000, along the way switching to half-height 8-inch floppy drives and gaining an internal hard drive.

Because the business systems were designed for work and not for home use, there is a lot less affection and nostalgia directed at them than at the Z-80 and Color Computer (Coco) systems.

Other systems

TRS-80 Color Computers

Tandy also produced the TRS-80 Color Computer (Coco) using a Motorola 6809 processor. This machine was clearly aimed at the home market, where the Model 2 and above were sold as business machines. It competed directly with the Commodore 64.

TRS-80 Model 100 line

In addition to the above, Tandy produced the TRS-80 Model 100 series, which were the first commercial line of laptop computers. This series includes TRS-80 Models 100, 102 and 200. These units were popular with journalists and included a built-in modem. The Model 100 was designed by the Japanese company Kyocera with software written by Microsoft. Some sources have claimed that the Model 100 featured the last code that Bill Gates ever wrote. The Model 100 had an internal 300 baud modem, a basic programming language and a very limited text editor. It was possible to use the Model 100 on essentially any phone in the world with the use of an optional acoustic coupler that fit over a standard telephone handset. Due to this feature and the text editor, the Model 100/102 was very popular with journalists in the early 1980s. These computers lacked any kind of a disk drive except for an optional external drive.

TRS-80 MC-10

This short-lived and little-known Tandy computer was similar in appearance to the Sinclair ZX81.

It was a small system based on the Motorola 6803 processor and featured 4k of RAM. A 16k RAM expansion pack that connected on the back of the unit was offered as an option as was a thermal paper printer.

Programs loaded using a cassette which worked much better than those for the Sinclair. A magazine published offered programs for both the COCO and MC-10 but very few programs were available for purchase. Programs for the MC-10 were not compatible with the COCO.

TRS-80 Pocket Computers

TRS-80 was also used for a line of Pocket Computers which were manufactured by Sharp or Casio, depending on the model.

TRS-80 PC-Compatible Computers

In the early-1980s, Tandy began producing a line of computers that were more or less PC compatible. Two of these systems were referred to as TRS-80 Model 2000 and Tandy 1000. As margins decreased in PC clones, Tandy was unable to compete and stopped marketing their own systems.

Originally, Tandy offered computers manufactured by Tandon Corporation, and then started producing their own line of systems.

The TRS-80 Model 2000 system was similar to the Texas Instruments Professional Computer in that it offered better graphics, a faster processor (80186) and higher capacity disk drives (80 track double sided 800k 5.25 drives). The industry was moving away from MS-DOS compatible computers (like the Sanyo MBC-550 and the TIPC) and towards fully compatible clones (like the Compaq, Eagle, Columbia MPC and others).

The later Tandy 1000 systems and follow-ons were also marketed by DEC, as Tandy and DEC had a joint manufacturing agreement.

Emulators

• Windows: HT1080Z EACA Emulator for Windows XP
• Windows: trs32 Model 1/3/4 Emulator for Win XP (shareware)
• Windows: Windows (shareware) Several TRS-80 Emulators; emulated in 80486+ Assembly
• Linux/Unix: xtrs (open-source)
• Java: Java applet
• Mac OS X: M.E.S.S. Emulator includes TRS-80 emulator
• Mac Classic: TRS-80 Emulator for Mac OS (no sound support) for Mac OS 7.5.5 or higher

External links

• Ira Goldklang's TRS-80 Revived Pages
• The History of the TRS-80
• Radio Shack Model I and II catalog RSC-3
• Jeff Hellige's TRS-80 Model 2000 page
• The TRS-80 Model 2000 at Obsolete Computer Museum
• Tim Mann's TRS-80 Pages
• Semi-Virtual Diskette (SVD)
• Club 100 (The Model 100 Users Group)
• Bill Gates with Tandy 200