Daves Old Computers - TRS-80 (Z80 based)

The TRS (Tandy Radio Shack) line of computers includes many models based on the Z80 processor. The first TRS machines could only run ROM BASIC, then disk drives were added, and TRS-DOS was created.

Although these Z80 based machines are what are commonly referred to as "TRS-80", several other distinct computer architectures were also marketed under this name, including: TRS-80 Color Computer, TRS-80 Model 100/102/200 and the TRS-80 Pocket Computer.

Click any photo to view a large high-resolution image.

TRS-80 Model I

This is the original TRS-80 - first released in 1977, this machine consists of a main unit incorporating the keyboard, CPU, memory and supporting devices, and an monitor which was a rebadged RCA TV from which the tuner had been removed.

Program storage was originally by cassette tape, however a separate expansion unit was later produced which supported more RAM, and the connection of floppy disk drives. A fully configured TRS-80 was quite a maze of wires and boxes.

Views: main-unit bottom, monitor back, users manual.

This is the Model I expansion unit. It sits under the monitor and provides the Model 1 with more memory, floppy diskette capability as well as additional peripheral interfaces.

TRS-80 Model II

The TRS-80 Model II is a complete departure from the original TRS-80 design. It includes an integrated monitor, and an 8" diskette drive. Unlike other TRS-80 models, this machines does NOT have BASIC in ROM. It was designed as a business machine (with a price to match), and loaded all of its system software from disk. Internally, this system is bus oriented, with cards for the CPU, Memory and disk controller. It is also expandable, and options such as a 68000 CPU card were available.

Unlike other "All in one" TRS-80 Models, the back of the Model II contains all of the I/O connectors.
One Model-II donated by Willem Boon.

If the whopping 500k of disk storage offered by the Model II's internal single-sided 8" diskette drive was not enough, you could add this external drive expansion unit with up to three more drives.

Like the Model II, this unit is larger than it appears in these photos... Remember that these are full sized 8" diskette drives!

Here is a view from the back.

1983 Tandy/RadioShack Catalog Model II pages: One, Two.

TRS-80 Model III

The TRS-80 Model III is pretty much the original TRS-80 design, repackaged into a single unit which incorporates the keyboard, monitor, CPU & expansion devices as well as two diskette drives.

The back of the Model III is quite plain. All switches, controls and I/O connections except for cassette are located on the bottom of the machine.

Here is a look inside, and a closeup of the system board.

TRS-80 Model III documentation.

1983 Tandy/RadioShack Catalog Model III pages: One, Two, Three.

TRS-80 Model 4

The TRS-80 Model 4 looks like the Model III, however it has major improvements which make it much more powerful. The most important of these is the ability to MAP out the ROMs and run with 64k RAM. This allows the computer to run CP/M - the industry standard operationg system at the time. It also allowes up to 128k RAM and has several other enhancements. The white case was a departure of previous "grey" TRS-80 models.

The back of the Model 4 is quite plain. All switches, controls and I/O connections except for cassette are located on the bottom of the machine.

Donated by Jim Fare.

Inside the TRS-80 Model 4. Note that the monitor is attached to the cover, not the main computer frame. Also note that there is only one power supply (The Model III has two power supplies).

The main logic board is located vertically at the rear of the machine. There are two more boards located between it and the panel that you can see behind the diskette drives - these are the diskette controller, and the serial I/O interface board.

This is a TRS-80 Model 4P - the only "portable" Z-80 based TRS-80 ever made. The 4P is essentially a Model 4 in a portable case, although it does omit the Model III BASIC mode ROMS (which can be loaded from disk).

Here it is closed, and here is a view from the back showing the I/O connectors.

The Tandy Model 4D is the last and most powerful TRS-80 made. It is similar to the standard Model 4, except that it has double-sided disk drives, and an improved gate-array mainboard. Note that it now bears the TANDY name instead of TRS-80.

One small but handy improvement is that the serial port has been redirected to be accessable from the back of the machine.

I have been unable to determine exactly when the TRS-80 line was discontinued, however this machine does appear in the 1987 Tandy Computer Catalog.

Here is an original TRSDOS boot disk for the TRS-80 Model 4.

1984 Tandy/RadioShack Catalog Model 4 pages: One, Two, Three, Four.

To use a diskette drive on the Model-I, or to add an extra disk to the Model III or 4, you needed one (or two) of these TRS-80 "MiniDisk" drives.

The Model-I also requires a "expansion" box which contains the diskette controller.

Here is a view of the back.

1984 catalog accessory page.

Homebuilt S-100 TRS-80s

The most unique TRS-80s I've ever seen - click the photo on the left to visit my page on two homebuilt TRS-80s on S-100 cards!

These things really run, and perform exactly like factory TRS-80s!

TRS-80 Model II Operation Manual (3.2M PDF)
TRS-80 Model II Technical Manual (9.2M PDF)
TRS-80 Model 4 Hardware Technical Manual (9M PDF)
TRS-80 Model 4 Software Technical Manual (5.5M PDF)
TRS-80 Model 4 Memory Expansion Information (90k PDF)
1982 Advertisement with Isaac Asimov (160k JPG)
TRS80 Owners Report - Byte Apr78 (1.2M PDF)
An Introduction to your Disk System TRS-80 Model 4 (1.1M PDF).
Z80 CPU Technical Manual (5M PDF)
Simulator for TRS-80 Model 1
CP/M version 2.2 Manual (0.5M PDF)
Z80-CP/M emulator
Z80-CP/M simulator - boots CP/M

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Copyright 2004-2005 Dave Dunfield.