Category Archives: TRS-80

More TRS-80

A little bit of progress with the TRS-80.

I went ahead and hooked a floppy disk drive (modified for use) on a modified cable to the Tandy.. wrote a disk with the Compaq.

trs-80 and compaq

The write was successful..

trsdos 1.3

Booted into TRS-DOS 1.3!


Got a few disks written but they don’t read properly. I need to get the manual for the operating system. I have a few other avenues to go, writing disks with an emulator. That’s for another day. I need to find a few more compatible drives and get some holders to go in the slot, rather than using an old SCSI tape drive frame. Tried writing a couple things with the 386, but no dice.

Chips, chips, chips

Back to the computers again, after the post office took a week to deliver a small envelope sized jiffy bag from California. Anyhow, it did arrive, so ultimately all is not lost.
The TRS-80 took a road trip and came home. It was stripped down (lid off, motherboard shield off) in this picture.

trs 80 motherboard

That has to come undone next, and sorta flips up out of the way. Kinda.

floppy and serial cards

Taking care not to break the ribbon cables (again), the floppy drive board (left) and serial card (right) are accessible. The new chip looks a lot like the old one:

floppy chips

It now asks a question it never has before..


This now makes the heads seek to and fro, making a noise akin to a PC on POST, hunting the floppy drive (DRZZ-DRZZ-ddddrrrt).

BASIC listing

The thing still doesn’t boot and I need to research more to see if it’s possible to get it to display what it reads off the disks on the screen. Progress though.

Attention then turned to the Compaq.

I ordered these two beasts on Friday. They arrived here today.


NEC V20 and an Intel 8087. V20 has some 186 instructions and is faster than the 8088 whilst being 99% compatible. Been a long while since I’ve had a V20 powered machine, and the 8087 helps out on programs that use it.

To begin, we have the info screens in CheckIt showing the computer’s stats and benchmark:

8088 stats
8088 benchmark

Shut it down and threw the two new chips in, remembering to slip the DIP switch for the co-processor:

processors in situ

Power it back up and run CheckIt again:

V20 stats
V20 benchmark

Oh yes. that’ll do nicely. Believe it or not, it makes a noticeable difference to the computer.
It’s slowly on the road to being a hotrod!

Well, because

Set up a bit of BASIC code to read the serial data off the P3 terminal server under my desk. The TRS-80 L2 ROM BASIC interpreter is not exactly fast… it kinda manages to do a little bit of if/then at 300 baud. Mostly.

OK, so it doesn’t all fit on the screen at once.

More retro page

…so I tried LPRINT which sends to the printer instead of the screen, but the code for that is even slower:


Fun times 😉

TRS-80 Model III

This machine, and a couple others are really the starting point and inspiration for this site. I figure, in that case, I ought to give them a mention.

Starting with this, one of three machines left over that were in good enough shape to recover from years of damp storage. Wikipedia has a page about it (TRS-80 Model III).

I say that about this site because I came by this via a work colleague. His brother in law was clearing a house he had for sale and these were in storage inside, no climate control (ever read the operational/storage parameters for electronics? 120F/80% rH non-condensing – Storage) in the wonderful swampland of south-east Louisiana. He asked if anyone was known who’d be interested in taking them, rather than it all going straight in the dumpster. My name was mentioned. The rest, as they say, is history.

Anyhow, this is the beast in question, after a rather hasty scrub-up to remove about a decade of grime from storage and then operational gunge.

TRS-80 Model III first sighting

48kb of RAM.
2 MHz of Zilog’s finest 8-bit processing power.
Incompatible text display.
Floppy drives.

In the true spirit of things, and considering I was at work, the first thing that happened was it got pulled apart.
This one is a fairly advanced model, having a pair of floppy drives in. That means there are two Astec power supplies inside. One runs the screen (12V, it’s basically a portable TV without the tuner), the serial card and motherboard, the other runs the floppy drives and controller card.

I’ve worked with those Astec power supplies before. They were used widely in Acorn machines, notably the BBC Microcomputer. They have a tendency to go bang when powered up, and were notorious even when new.
Therefore, in went a slightly modified Dell desktop power supply, rated at about double both the Astecs put together. Dismantled the brightness and contrast pots because they had seized solid, and presto:

TRS-80 first power up

You can see the PSU there on the left. I ended up mounting it so the fan draws in cool air from underneath the floppy drives and vents out through the grille in the top. As it forms a chimney, the fan never runs at anything other than idle and is nearly silent due to the convective nature of the box.
Works a charm there, booted up into BASIC.

It migrated around for a while, at home being poked and prodded:

TRS-80 troubleshooting

In so doing I sat down and went through the entire motherboard, chip by chip to find out how it worked.
It’s from an era when you can tell there were one, maybe two people involved in the overall architecture, and you can get inside their heads a little when you look at the design:

TRS-80 chip layout -Motherboard

Got it running properly, with the obligatory “Hello, World!”:

Hello, World!

…and as the PSU is no longer switched by the main power switch on and off, the soft-on is controlled instead. Hooked up an LED into the reset button (top right of keyboard) to show power’s applied, using the clock keep-alive rail of the PSU:

Power light

Decided to run it flat-out as a test…

TRS-80 failing at pi.

Gave BASIC some floating-point recursive calculation to do. Calculate pi to as many places as possible. It nearly got there.

Buttoned it all up and powered up the other bonus haul for it- the daisy-wheel printer that you could purchase for $1995. Gave it a thorough clean out and oil up, at the same time. It has the optional fanfold paper tractor on the back also.

Desktop computer, circa 1983

It is noisy and slow, and with a change of cable will even talk to an IBM-PC, despite adding extra line-feeds in where they need not be. The print quality isn’t bad either, considering it’s just a glorified typewriter.

Printer test

As it turns out, the controller chip on the floppy board has gone bad. It no longer outputs the signal to hunt the heads one track over- applying 5V to the pin causes the drive to hunt, so new chip needs to be obtained. Unfortunately, Western Digital’s popular FD1793 chip now commands a high price on most popular bidding sites and I honestly don’t have the patience to be ripped off for one ($45 at current market prices).

TRS-80 Floppy drive controller board

Typically also the ribbon cables fell to pieces so several hours rebuilding them meant that I was no further forward with the floppy drives than I was to begin with. However, the optional add-in RS-232 serial port card is fully functional.
With a little bit of BASIC programming, 300 baud serial communication is possible. Here it is, attached to my terminal server, displaying the output from an apt-get upgrade action on Ubuntu.

RSS feeds on TRS-80

That’s about as far as I’ve gotten with the thing so far but there is light on the horizon for this one. I do plan on getting another controller chip for the floppy drives. Those 5.25″ ones are going, to be replaced by 3.5″ ones and possibly a hard-disk emulator board that runs off Compact Flash.

Fun piece of equipment. I can see why it was America’s best selling computer of the early eighties.