vLA81 and 32k internal RAM - a little confused...

[gwpt]

I have a zx81 which I had done the 16k RAM upgrade on, according to this guide (I had only done step 1, not the 32k steps 2):

https://www.zx81.de/english/32k-rame.htm

I am about to get a vLA81 ULA and wanted to upgrade the RAM to 32k.

I've mapped the difference between what I have and what I found in the docs for vLA81:

There appear to only be 2 differences, pin 1 and 22

* Pin 1: replace the ground wire to Pin 1 and replace it with a link to A14 (Diode D7). That seems fine.

* Pin 22: replace the ground wire to Pin 22 and link it to Pin 20.
- Question: If I were to not use vLA81 and use my normal ULA, would that still work (i.e. 20 + 22 on RAMCS? Or should I put 22 back to ground?

* In the vLA81 manual it states: "Note that if the original ULA is installed then A13 will need to be
removed from ULA-pin 35."
- Question: Currently I have A13 connected as part of the old mod and it seems fine. And looking at the schematic, ULA pin 35 seems to be a clock input and nothing to do with A13. Am I missing something here?

Thanks!

[1024MAK]

There appear to only be 2 differences, pin 1 and 22

* Pin 1: replace the ground wire to Pin 1 and replace it with a link to A14 (Diode D7). That seems fine.

* Pin 22: replace the ground wire to Pin 22 and link it to Pin 20.
- Question: If I were to not use vLA81 and use my normal ULA, would that still work (i.e. 20 + 22 on RAMCS? Or should I put 22 back to ground?

Pin 1 indeed needs to be connected to the Z80 address line A14.
Pin 22 should indeed be connected to Pin 20. These pins were originally connected together via a PCB track. And yes this connection would work with a "normal" ULA. However, read on as the next part means that a simple swap is not possible.

* In the vLA81 manual it states: "Note that if the original ULA is installed then A13 will need to be
removed from ULA-pin 35."
- Question: Currently I have A13 connected as part of the old mod and it seems fine. And looking at the schematic, ULA pin 35 seems to be a clock input and nothing to do with A13. Am I missing something here?

Yes, you are missing something very important.

In order for the vLA81 to control 32K bytes of RAM, and to be able to change the memory map under software control, the vLA81 has to control A13 of the RAM chip. The vLA81 does not use the ceramic oscillator circuitry on the main board. Hence the designer of the vLA81 has reused pin 35 for the A13 address line output to the RAM chip. So pin 35 needs to be wired to pin 26 (A13) on the RAM chip. This is why the vLA81 instructions that you have quoted are there.

Mark

[gwpt]

Thanks Mark, argh that makes sense now.

So i need to rewire a13 from diode 5, to pin 35.

Is it ok to leave the ceramic oscillator circuitry?

[1024MAK]

Is it ok to leave the ceramic oscillator circuitry?

Yes.

Mark