exxos's Atari, Amiga & retro forum

Pin 1 isn't connected as that would be for A8 on a 41256 chip.

Tying pin 9 (A7) high means that the address space is a quarter of the original, so 16Kx1 as the /RAS and /CAS multiplex the address, giving two sets of 6 address lines. There's no way that the chips can give more than 16K in this scenario.

stephen_usher wrote: 30 Nov 2023 21:16 Pin 1 isn't connected as that would be for A8 on a 41256 chip.

Tying pin 9 (A7) high means that the address space is a quarter of the original, so 16Kx1 as the /RAS and /CAS multiplex the address, giving two sets of 6 address lines. There's no way that the chips can give more than 16K in this scenario.

So much as I thought then. Pin 1 is irrelevant and with pin 9 tied to 5v I should see only 16kb.

Why then does the TRS-80 still show 48k of RAM available. Very weird, but I've tested it with both versions of BASIC I have in ROM and both do the same.

I also tried the RAM from my Speccy that's been fixed to 16kb to replace the 4116s and the result is the same.

Ok, so I think I've figured it out using the memory map shown in the TRS-80 Level II basic book. Pin 9 (A7) on all the RAM IC's is lifted and tied to +5v.

If you look at the image below:


You can see that 'end' of RAM in a 16K machine shows as 32767 in the book. So I poked a value into 32767, and it was fine.

I then tried to poke 32768 with a value and got an OV (Overflow) error. I also tried to PEEK 32768 and also got an overflow error, as do I trying to access any RAM above 32767. This is telling me I have 16KB of RAM, and I can't use anything beyond that limit.

Why the '? MEM' command is telling me I have 48K, I don't really know, but it's lying:


Bug maybe? I don't know, but when the Quinnterface eventually arrives, I should have an extra 32K and will test accessing that memory.

A bit strange, but hey ho.

Woo hoo, the Quinnterface is in the air on the way from Australia :)


:girldance:

Sighs...

So it looks like in order to fully revert the TRS-80 to fully revert to 16K I will have to undo some of the wiring changes for the 64KB upgrade.

I can leave the changes that modify the sockets so they have the correct voltages for the 4164's rather than 4116's, but there are RAS and addressing changes I'll need to undo.

I'll have to print out the instructions and work back through, which is annoying as I made a very neat job of the upgrade :(.

Thinking positively, the QI will give me the 32KB back and a hard disk, so on balance I'll be up at the end of the day.

Does sound a tad annoying.

stephen_usher wrote: 02 Dec 2023 11:50 Does sound a tad annoying.

Sadly yes.

Having spoken to the provider of the QI, he's told me that I'll have to in order for it to correctly revert to 16k and so it will work with the QI.

The original EI was easy, just a few small mods and it's good to go with a 64kb base machine.

Ooooh.

The QI has arrived in the UK, I'd better get on with reverting the TR-80!!!

:girldance: :girldance: :girldance: :girldance: :girldance:

stephen_usher wrote: 03 Dec 2023 13:37 :girldance: :girldance: :girldance: :girldance: :girldance:

Celebrations aside, it's time to undo my 64KB upgrade....

16KB here we come.