An upgrade too far?

[Trevor_B]

I had to fit a socket - was previously 2x socketed 2114's. Sockets have been removed/desoldered.

I followed the upgrade instructions to the letter - and yesterday went off-piste to connect A13 to pin 35 of the ULA. That's when my "challenges" started!

Trev

[1024MAK]

So if you think (or know) that you are not getting the correct amount of functional RAM, it may be worthwhile testing all the address lines between the Z80 and the relevant pin on the SRAM chip (continuity or resistance). Just in case there is a high resistance or broken connection somewhere.

The loss of one address line will half the amount of useable RAM at best. Depending on what voltage level a disconnected address input pin on a CMOS SRAM chip floats to. If it floats to an intermediate voltage between logic low (logic zero) and logic high (logic one), the SRAM chip may flip between different memory cells, that may cause the program to crash.

You may also want to test to check that there are no short circuits between adjacent pins.

Mark

[Trevor_B]

I'm back at 16k - moved the A13 wire back to D5 and it looks to be giving me the full 16k. I did the simple DIM A(3000) - no problem. Before swapping the wire back I ran out of memory at DIM A (1500) - hence my "8k" estimate.

Once I'd done this a few times - just to make sure - I refitted the ZX81x2 ROM just to see what would happen.
That's working fine now too.

I've decided to stop there - for the moment - while it's working.

Trev

[Moggy]

That's great news Trevor got there in the end.

[1024MAK]

By D5 do you mean diode D5?

Mark

[Trevor_B]

Yes - diode D5, where it was originally before I tried to get the full 32k.

[1024MAK]

So, in order to help you, can you please list where the following RAM chip pins connect to:
Pins 1, 2, 21, 23, 26

I'm assuming pins 3 to 10, 24 and 25 go directly to the socket pins or PCB pads.

Mark

[Trevor_B]

So...
The RAM chip is fitted into the socket soldered to the motherboard.
All pins EXCEPT pins 1, 21,23 and 26 are in the socket; these pins have been bent out and wires soldered as follows:

Pin 1 - wired to Diode 1
Pin 21 - wired to Diode 2
Pin 23 - wired to Diode 3
Pin 26 - wired to Diode 5

This follows the information here:- https://web.archive.org/web/20200929054 ... rsion.html

After following these instructions, I had a verified 16k available.

My "32k" mod was to take the wire from pin 26 (A13) to pin 35 on the VLA81, which I was assured was all that was required as the VLA81 has the necessary additional logic to handle the 32k addressing. This resulted in the "8k" rather than 32k, so this has now been reverted back to Diode 5, and it's back to 16k again.

Trevor

[1024MAK]

So in your case, maybe pin 2 of the RAM chip needs bending out and connecting to the vLA81. I need to check in this. Back later...

Mark