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Tandy Radio Shack computers and compatibles

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Dick Smith System 80

Posted on by Graham

The System 80 is an attempt, largely successful, to create a machine compatible with the TRS-80 Model I.

Terry Stewart has done a wonderful job of documenting the System 80. Everything that you could ever want to know about it is there. I’ll just focus on my own experience.

I bought this machine from a colleague, Scott, at the Adelaide Retro Computing Group back in 2020. My recollection is that he had collected a number of computers from a seller. This one was outside his interests, so he offered it for sale. I was happy to take it; i never expected to have such an opportunity.

The machine came with an expansion unit and a floppy disk drive. The expansion unit, X4010, has a three slot S-100 backplane. It provides 32k of memory, a disk controller, RS232 port and printer port.

The machine can run any of the myriad of operating systems that were created for the TRS-80 Model 1. Similarly, it can load programs from cassette, just as the TRS-80 Model I can.

This particular machine is an early version with no tape level meter. It has a keyboard modification that replaces one of the shift keys with backspace and tab keys.

It came with a soft plastic dust cover. Instead of protecting the unit, it caused a lot of unsightly “melts” in the plastic. There were also several other issues to work through.

The computer is badged as a “third generation computer” but, as far as i can work out, it would normally be considered a fourth generation computer. Perhaps the usage of the term changed. Or perhaps it is the only example of “underselling” in the computer industry.

System 80 Inspection & Power Up

Posted on by Graham

The system, as it arrived, looked like this:

I set the monitor aside as a separate project. It had very crude and broken mod that held a sheet of coloured perspex over the screen. I will eventually add that back.

The main unit was in good shape. I just reseated the main RAM.

To judge from the date codes, it was made in 1981 when I was still at high school.

This machine uses an old school linear supply. I inspected it and reformed the power supply capacitors by bring up the supply voltage slowly with a variac.

Two new keys where the second shift key was. The sideways A is the tab key.

Switch added at the top of the pic. Shift changed to backspace. Backspace to clear.

With power (and a kludge for the video cable) it came to life.

The plastic had melted against the soft plastic dust cover. It looked pretty awful.

I had to make some compromises. I used a combination of acetone and wet & dry paper to get it back to an acceptable finish. It may be possible to do better.

I crafted, and then glued on, a replacement door latch. It failed, so i had a second go and that is still holding several years later. To be fair, i don’t use the cassette deck very much.

The tape unit itself was gummed up, but after a clean and lube, it seemed like it might work.

System 80 Expansion Unit

Posted on by Graham

There were two different types of expansion units for the System 80. This is the early one with the S-100 bus.

The first card connects to the System 80 via a 50 pin interface. It also provides the floppy, parallel and RS232 ports. I particularly like the termination networks that have been added to several of the ICs.

The second card provides 32k of RAM. There was one faulty RAM chip.

The power supply unit is the same as the computer. I reformed the caps with a variac.

There is one spare slot for an additional card but the case is not great for adding additional I/O and there’s no obvious mechanism for adding more memory beyond the current RAM/ROM.

System 80 Floppy Drive Unit

Posted on by Graham

The system came with a matching Floppy Drive Unit that connects to the expansion unit. It uses an MPI B51 40 track single sided drive.

I gave it the routine clean and lube, but it would not read disks. Drive speed was good.

Configuration was preset:

I located a service manual on the net and that was very helpful for fault-finding.

I convinced myself that the differential amplifier (CA3054) in the read circuit was cactus and replaced it. With this, the system booted. Alas, it wouldn’t format disks.

Breaking out the CRO and checking the signals on the drive interface showed there were no index pulses. In an unexpected twist, it turned out that the disks i was using were not sufficiently opaque. Is that even possible?

It appears so from the traces; the voltage in the top picture never gets high enough to flip the comparator, whereas in the second picture, with a different disk, it does.

I checked over and over. It’s fine with five other brands, so I’ll call it good enough.

It lasted about a week before it stopped reading from disks.  Perhaps it had an intermittent problem and the original differential amp didn’t fix it. 

With the scope, I was able to see differential signals going into the LM311 comparator, but nothing coming out.  I replaced the comparator and i was back in business.

I made up a few disks and had a play!

I also had no trouble setting up a gotek as a drive. I just powered it off the real drive and connected with a custom cable. It has to be set to drive 1.

System 80 Monitor

Posted on by Graham

The System 80 came with a Thorn 7433 black & white TV that was used as a monitor. It had had a couple of homebrew additions.

The first was a composite video input, so modulation/demodulation is redundant.

The second was a green screen, which was appropriately agricultural. The mount has broken and in any case is too rough even for my low standards.

It seems to be working fine.

I have removed the mount, and rubbed back the glued areas. It’s currently awaiting paint and then i’ll attempt to mount some coloured perspex that looks a little more appealing.

Tandy TRS-80 Model I

Posted on by Graham

Generously given to me by Michael from the ARC Group.

The Model I was originally released in 1977, but this one was made in about 1980. It has 16k of memory and has been highly modified.

It also came with an expansion unit with RS232 interface and 32k of additional memory.  There was no disk drive, but that was only a minor obstacle.

It also came with an original monitor (probably not the ideal one to use with the expansion box), buffered cable to connect the computer to the expansion unit, a box which turned out to be a high resolution graphics mod, a joystick, and dust covers.

Model I Inspection and Power On

Posted on by Graham

Inspection didn’t show any particular problems.  The power supplies are quite simple and don’t seem to have any of the usual EMI filtering.

Everything fired up without smoke.  The rails looked clean, and I adjusted the supplies on both the computer and the expansion unit.

There are a lot of modifications including:

  1. High resolution graphics
  2. Lower case mod
  3. High speed mod
  4. Joystick
  5. Termination of control signals on the expansion unit
  6. Cassette modification
  7. Reset switch

This machine was obviously owned by an enthusiast.

I only briefly got clear text on the monitor, and then it would not sync.  This was traced to Z5 in the sync generator, which is a common failure point.  This is a CMOS part and I replaced it with an 74HC02.

Some keys were generating strange character sequences.  This was caused by a broken wire ground on the keyboard cable which was somewhat fragile. 

The machine uses an ALPS keyboard and the key switches seem to be working well.

I replaced the keyboard cable using the instructions here:

TRS-80 Model I 48K Level II System – Bring up and repair

I found the keyboard ic positions too close to the connector so I had to bend the pins up at 45°.  I also added some earth lines to try to provide some shielding. The keyboard is mounted using some rubber mounts which had deteriorated.  I replaced them with rubber tubing.

The amount of RAM reported by the ROM basic (PRINT MEM) was variable.  I had four spare 16k RAM chips, so I kept on swapping the mainboard RAM chips until I got the full 15572.  Then I repeated the exercise on the expansion interface.  I found 5 dubious RAM chips which were replaced.

Expansion Interface 48k topFFFF-1
Expansion Interface 48k bottomC000-16384
Expansion Interface 32k topBFFF-16385
Expansion Interface 32k bottom8000-32768
16k top7FFF32767
16k bottom400016384
 3FFF16383
 00000

The screen showed artefacts on the two bottom lines and occasionally on the two lines above.  These changed as new text was displayed over them but the text was incorrect.  Looking at the text it was apparent that the LSB was always set.

This becomes very annoying because the bottom lines are copied up as the text scrolls.  Eventually the whole screen is filled with corrupt text.  For now, I can keep clearing the screen, but it needs to be fixed.  This was due to a faulty video RAM chip 2102 at Z48.  It was replaced.

With this last fix the machine appeared to be working.

Model I High Speed Mod

Posted on by Graham

The machine included a modification that allowed either the original clock (1.7MHz) or a slightly faster clock (2.6MHz) to be routed to the processor.  The high speed was enabled at power-up, although that seemed to be a matter of luck as there was no reset on the register.  I’ve added a system reset so that it defaults to the normal speed.  I suspect that this might be more reliable.

The mod includes a register in I/O space to enable / disable the high speed clock.

OUT 254, 1 --> Changes to the high speed clock

OUT 254, 0 --> Changes to the normal clock

The mod includes a LED to indicate whether “turbo” mode is activated.  I’ve added a series connector so that the lid can be separated.

Model I Joystick Mod

Posted on by Graham

The joystick mod was incomplete – there are some cut wires and an unused DPDT switch nearby that is unused.  It’s possible that the mod was completed or stopped working and was disabled.

Commonly, a joystick was added by using the switches to tug down on D[4:0] using the joystick switches.   Apparently, sometimes this wasn’t even gated with an address, but it looks like this mod had some address decoding.

Reverse engineering it shows three signals ANDed together.  One of these would have been IN_L and the other two would have been address lines (both 0).  This doesn’t fully decode the I/O space but does restrict it to a quarter of the space.  This at least gets it away from the cassette interface and the high-res graphics at 255 and 254.

I found an instruction sheet for the “Alpha” joystick.  It has the same data line assignments as the mod:

D0 UP

D1 DOWN

D2 LEFT

D3 RIGHT

D4 FIRE

The instructions indicate the address is 0 which is again consistent.

It notes that some games expect FIRE to be indicated by D0 and D1 being low simultaneously (STICK-80 vs TRISSTICK).  I expect that the toggle switch was used to configure the interface accordingly.

I wired it all up. Being something of a tangled mess of flying leads makes it very easy to break wires, so I’ve added an inline connector.

I used the alpha test program:

Alas – no joy.  The scope showed that there was no IN strobe and this was because of a dwarf pulse at Z23-8 (74LS32).  I replaced the IC and it came good.

Scarfman played ok!

Model I Smaller Mods

Posted on by Graham

Lower Case Modification

I have not tested this yet.  An additional character RAM has been added at Z46.  The machine seems really designed to have this IC because the modification seems quite simple. This is covered by Technical bulletin I:35.

The lower case version of scripsit “SCRIPSIT/LC” demonstrates it.

Signal Terminations

As per Technical Bulletin I:12 (source unknown):

Tandy Technical Bulletin – Model I No 12 (1983)(Tandy)Download

Cassette Modification

XRX-111 modification as per Technical Bulletin I:15 (source unknown):

Tandy Technical Bulletin – Model I No 15 (1983)(Tandy)Download

Reset Switch

The reset switch just parallels the existing reset switch.  I added a series connector so that the lid could be separated.

Top cover connections

The mods add an LED (turbo), reset button, and joystick mod.  This makes it impossible to remove the cover from the main board.  I’ve added in-line connectors so it can be detached.