Woo hoo - Collection Completed!! TRS-80 inbound

[rubber_jonnie]

I finally got round to finishing off the TRS-80 memory upgrade today, and it's been a happy day in TRS-80 land :)

It is quite time consuming as you need to both re-wire the RAM sockets to remove 12v and move 5v to pin 8 on all the sockets, then add a new address bus to pin 9 on all the RAM sockets, and then mess with the RAS line and some other logic to get the addressing correctly hooked up, but as long as you take your time and follow the wiring schedule carefully, it will all run just fine.

So, it's finally sporting 64KB of RAM, with about 48KB available for BASIC:

PXL_20221008_142627515.jpg

It's interesting, as once you hit ENTER at the MEM SIZE? prompt, it clearly takes a fair bit longer to initialise the RAM.

Here is the bodging all done:

PXL_20221008_145748317.jpg

As you can see it is almost all under the board with just one cut track up top, the green wires being my upgrade and the orange already present, I think it's part of the lower case upgrade.

To neaten things up, after cleaning up with IPA, I have put some solder mask on areas that have cut tracks to reduce the risks of corrosion later as it is highly unlikely that I will ever want to revert it to original 16K spec.

I've added a few spots of hot glue too to prevent the longer cables from moving. One last thing to do is the tweak to the expansion port that prevents the RAM from being enabled in the expansion unit if you have one.

I'll also see about a cassette cable for storage until I find something more modern like an SD card system.

Overall I'm very happy with the results, but I may do an update to Errol Rosser's original write up to clear up the detail around the pinouts on Z73 and make it overall a bit clearer for those wanting to do the upgrade.

[stephen_usher]

That's a load of bodging!

[rubber_jonnie]

That's a load of bodging!

But not as much as Brett's STE :)

I'm pretty pleased with the pin 9 bus, I managed it with a single piece of wire, no breaks.

[rubber_jonnie]

So, the TRS-80 is proving to be a formidable opponent!!

I did the 64KB upgrade, but was getting some weird issues, and I think it was due to the use of 41256 RAM (I have a TON of this lying around) and the fact that pin 1(A8) was held at -5v and not 0v. The upgrade doesn't call for anything to be done to the -5v pin since the 4164 pin 1 is not connected, so it's irrelevant when using those ICs.

I can't be 100% sure, that's a bit of an assumption, as it could be to do with the refresh cycle, but I can get 4164s easily enough, so why not fit the proper stuff.

So I ordered some 4164 ICs as was originally specified in the upgrade and fitted them this morning, ran a small program to test and went to breakfast.

Came back and it had hung...

One out of the 8 4164 ICs that came had died when it was put under stress. Meh!

I'll have to contact the seller to see if they'll send a replacement, though to be fair I ran them all through the tester before fitting and they did all work, so it's clearly just died to the shock of going into a machine and being used!!

Ho hum...

[rubber_jonnie]

So, the seller has agreed to send out a replacement 4164 today :)

For short term use and so I can test the rest of the RAM, I've put a 41256 into the 8th bit slot (If I use a 41256 lower down, like bit 1, it makes stuff go weird again after a short while) with pin 1 lifted and connected to GND.

It's been running since about 10:30 with no problems, so I expect once the replacement 4164 arrives it'll be good to go, and in the meantime whilst it's apart I will do the last part of the 64KB mod to disable the address lines for the expansion box, since it won't be needed as a) I don't have an expansion box at the moment. b) If I do get an expansion box, I don't want the RAM in it to be enabled. :) c) I might as well whilst it's apart again.

This has been a really interesting machine to play with I have to say.

[stephen_usher]

I never trust the N/C for pins, especially if there's a strange voltage on the pin.

For 41256s in 4116 sockets I'd always wrap pin 1 over and solder to GND and pin 9(?) over to VCC. For 4164s I'd cut off pin 1 entirely.

[rubber_jonnie]

I never trust the N/C for pins, especially if there's a strange voltage on the pin.

For 41256s in 4116 sockets I'd always wrap pin 1 over and solder to GND and pin 9(?) over to VCC. For 4164s I'd cut off pin 1 entirely.

Yeah, the 41256's definitely don't like it and with pin 1 lifted and connected to ground, it seems way more stable.

Pin 9 is rewired as an address line (A7) anyway, and pin 8 is now VCC to match the 4164s, it's just pin 1 that still has -5v from the 4116's originally installed. I'll disconnect, then rewire all pin 1's to GND for completeness.

In theory the 4164 shouldn't need it as pin 1 is NC, but having it all wired to GND would be a more sensible option just to be sure.

[rubber_jonnie]

And having just downloaded the service manual for the TRS-80 Model 1 expansion interface, I now realise that it's the expansion interface, not the computer, that needs modding to prevent the RAM in it being accessed when the Model 1 has 64KB of RAM, so no more mods to do.

[stephen_usher]

So, it's "complete" then. :-)

[rubber_jonnie]

So, it's "complete" then. :-)

I'll get back to you on that...