TRS-80

:This article is about the Tandy TRS-80 computer. For the Chicago-based electronica group called TRS-80, see TRS-80 (group).

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TRS-80 (also affectionately or derisively known as the "Trash-80") was the designation for several lines of microcomputer systems produced by the Tandy Corporation and sold through its Radio Shack stores in the late-1970s and 1980s.

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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.

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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 Is.

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Hardware

The Model I looked like a very thick keyboard (like the later Commodore VIC-20) and used a Zilog Z80 processor. The basic model originally shipped with 4k of RAM, and later 16k.

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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.

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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.

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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.

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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.

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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.

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Writing to the screen directly (rather than by using the runtime calls in the BASIC ROMs) caused "snow" on the screen because no bus arbitration logic was used to arbitrate between CPU writes to the screen RAM and display logic reads from the same RAM. 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.

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Peripherals

User data was stored on cassette tape. To upgrade to a floppy disk based system one 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 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.

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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.

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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.

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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).

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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.

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Level I Basic was Li-Chen Wang's free Tiny Basic, hacked by Radio Shack to add functionality.

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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.

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

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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.

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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.

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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.

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Clones

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 Model's I/II and Team Computers (LNW also produced an alternate version of the Expansion Interface), and the Dutch Aster CT-80. There was also a poor clone, the Colour Genie, which amongst other changes had improved graphics, so programs relying on the original graphic capabilities of the TRS-80 couldn't be run.

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EACA in Hong Kong made a clone that was marketed around the world under different names, in Australia and New Zealand it was the Dick Smith System-80 in North America it was PMC-80, and in Western Europe it was video-genie/">Video Genie and later Genie I and II (it had a different expansion bus so EACA also had its own Expansion Interface). It came in two versions: the home edition, which had an inbuilt cassette drive, and the 'business edition', which eschewed the cassette drive in favour of a numeric keypad

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The Model II

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

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In 1980, Tandy produced the Model II which was designed as a business machine. It was not an upgrade of the Model I, but an entirely different system. The Model II was built using the faster Z-80A chip and contained a built-in 8-inch floppy disk drive, as well as 64k of memory.

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The 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.

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The 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.

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