Inside a Classic: The TRS-80 Model 100

Released in 1983, the TRS-80 Model 100 quickly became a favorite of journalists who enjoyed its long battery life (20 hours on four alkaline AAs), its excellent full-stroke keyboard, and its internal 300 baud modem, which allowed them to send in reports from the field.

Notice the keyboard wear from years of a user's thumb hitting the space bar. Most Model 100's you'll encounter sport this kind of wear -- they were so useful that people didn't want to let them go.

The underside of the Model 100 seems simple enough, with two removable covers and a very important switch. We'll get to those soon. The Model 100 is 11.8 inches long, 8.5 inches wide and 2 inches tall.

Here you can see a 25-pin RS-232 serial port and a Centronics printer port. The port labeled "phone" is where users hooked up a special cable to interface with the internal modem. To the right of that is a cassette tape interface. You'd plug in an everyday tape recorder and record your data on the cassette. External disk drives were available for the 100, but not common.

Peeking in from the left is the reset button, which the user needs from time to time due to a few pesky bugs in the ROM code, reminding us that even non-Windows systems can crash.

The Model 100's designers blessed the unit with ample expansion potential, especially for a compact computer of its size. You could connect an external floppy disk drive (the "Chipmunk" third-party drive was a popular option) or the Model 100's Disk/Video interface, which provided both disk-based storage and the ability to connect to a CRT monitor.

Unfortunately, most of its powerful expansion options had to squeeze through an inconveniently located 40-pin expansion bus on the bottom surface of the unit, seen here. Immediately below that, you'll notice a free slot for a user-provided software ROM chip, which proved essential in extending the 100's usefulness. It allowed users to add software for spreadsheets, bar code scanners and the like.

Four AA batteries is all it takes to power the Model 100 for up to 20 hours. The 100 stores user files in battery-backed RAM for instant access, which made them subject to loss when the batteries ran out. To prevent this, a small NiCD battery soldered to the motherboard constantly recharges and maintains the memory between battery changes. To wipe that memory out, the user flips the "memory power" switch seen here.

Only four screws separated me from the inside of the 100, which splits conveniently into two sections, with the motherboard on the left, and the I/O half on the right.

To the upper right, you'll notice the 30-pin umbilical cord that keeps the Model 100's display alive with power and fresh display data. You can see the 3.6v rechargeable memory battery I mentioned earlier next to the display's cable.

The black connector for a bar code reader made the 100 especially useful for portable data collection. Next to that, you'll find switches to change internal modem settings.

The Model 100's brain, an 8-bit 2.4 MHz 80C85 CPU, is a simplified, low power version of the 8080 manufactured by OKI.

The unit's RAM modules store both run-time program data and battery-backed user file storage. This unit originally shipped with 24 kilobytes of RAM and was expanded to 32K via an open socket.

A quarter-century ago, Bill Gates was still getting his hands dirty with tasks that, today would be left to entry-level Microsoft programmers. Case in point: Gates, with another colleague, programmed the Model 100's built-in software, which is contained within the chip you see before you. To this day, the Model 100 remains his last major programming project at Microsoft.

In an interview with the National Museum of American History, you can still hear Gates' pride in the project. "It is a cool user interface, because although most of the code is a BASIC Interpreter, we did this little file system where you never had to think about saving anything. You just had this menu where you pointed to things. It was a great little editor and scheduler. We crammed it all into a 32K ROM."

In this shot of the fully-extracted motherboard, you can get a general survey of the layout, including the curious purple RAM modules. The orange, red, and black cable hanging off the board in the upper right pipes in fresh juice from an optional AC adapter. The black circle to the right of that adjusts the unit's display contrast, while the main power switch lies just below.

On the I/O half of the split slab, we see the back of the LCD display unit (top) and the underside of the keyboard (below). The LCD data cable hangs off to the left, while another cable for the system power LED protrudes toward the camera. The two cables to the left of that connect the all-important keyboard to the motherboard.

The Model 100 sports a serviceable monochrome LCD panel that can display eight lines of 40 characters each, with an overall 240x64 resolution. Such a small resolution might seem limiting today, but for the size and convenience of the Model 100 in 1983, it was enough. In fact, Gates told the National Museum of American History that it was the display that really got him excited about the project: "Up to then, all we had was four lines by 20 characters. I didn't think using 4 by 20 you could do much that was interesting. But, when [my friend Kazuhiko Nishi] said we could go 8 by 40, then I got to be pretty fascinated with the idea of a portable machine."

Unlike most of today's LCDs, you'll find no backlight here. On the bright side, one can easily read the screen in full sunlight.

Foam padding lines the display bezel in the Model 100's empty top half. To the left of that, you'll notice the unit's only speaker -- a piezoelectric buzzer in a blue plastic case. Even if the Model 100 had a music program, you couldn't play Beethoven's Ninth Symphony on it. The speaker is capable only of beeps.

Aside from the computer's small size and ample battery life, the Model 100's keyboard, more than any other feature, guaranteed the machine's success. The Model 100's designers had the incredible good sense to include an impressive 56-key full-stroke typewriter keyboard in a reasonable, un-cramped size and with a layout that "just feels right." Despite the millions of words typed on this particular unit, its keyboard still feels as good as the day it was manufactured.

Reassembled and wrapped in its snug pleather case, this Model 100 will return to the shelves of my Vintage Computing and Gaming site. But not all Model 100s are out to pasture. Rick Hanson of Club 100 says Model 100s are still in use around the country, mostly as data collection devices or cheap, programmable microcontrollers.