ZX81 16K / 32K 62256 didn't work.. advice?

Discussions about Sinclair ZX80 and ZX81 Hardware
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rmzalbar
Posts: 12
Joined: Mon Jun 22, 2020 5:07 am

ZX81 16K / 32K 62256 didn't work.. advice?

Post by rmzalbar »

Hi, this is my second post, my first post was introductory where I described how I repaired a found ZX81 by replacing the ROM with an EPROM.

I also attempted to upgrade the internal RAM to 16k without success, and am looking for advice.

The motherboard itself already had a 28-pin socket loaded with a 4118 with LK1 set, for a 1k configuration.

I decided to go about this in stages and confirm operation at each point. I first installed a 6116 and switched LK 1 to LK2 to confirm 2K is working - and it is.

I then followed the steps in this guide to install a 62256 I had on hand in the 16k configuration, which I would confirm working before moving forward to 32k.

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

Unfortunately, I ended up with a white screen with no prompt. If I wait 30 seconds or so, I may randomly see single-frame flashes of grid patterns on the screen, but mainly just a blank white screen.

I buzzed out and scoped the address and data lines and they all seem active. The SRAM I used is a Hitachi HM62256LP-12. I tested it in my TL866 MiniPro beforehand, and it reports it's working. I am using an Atari 2600 9VDC power supply that is rated for 500ma. I see 4.98 volts at the 5v regulator. The only other mod is a 2N3904 transistor as a video output buffer, and I've disconnected the RF modulator from power.

Here's a picture of the SRAM with the connections I made. I tried it both with and without the pullup on pin 27 (optional for hi-res mode) and I also tried adding a 100pf capacitor to /RAMEN after reading some threads here, but while this stretched the signal on that pin a small amount it did not change anything.

In the meantime I've gone back to the 6116 2K configuration which is still working. Thank you in advance for any advice. -Rob
20200622_094318.jpg
zx-heinz
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Re: ZX81 16K / 32K 62256 didn't work.. advice?

Post by zx-heinz »

Pin27 has to be connected with /WR. If you just use a pull up resistor, then a writing to the RAM is impossible. My board looks as follows (no further wires on the component side, but a cut for pin 26. Pin26 has to be isolated from +5V and connected with D3 or D5 on your board. (You can permute D1, D3, D5 as you like). And I don't use a 100pF cap.
my_board#7.jpg
rmzalbar
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Joined: Mon Jun 22, 2020 5:07 am

Re: ZX81 16K / 32K 62256 didn't work.. advice?

Post by rmzalbar »

Thank you for your reply. I left pin 27 in the socket, so it is still attached to both /WR and the pullup. I've also tried removing the pullup as it's optional if you don't want to use the hires scheme. Pins 1,2,22,23, and 26 are all isolated by bending out the pin, with the connections made by wire to GND or the diodes as indicated. Does that seem correct?

The capacitor I added later, to see if it would help. I also have another SRAM I could try of a different manufacturer.
rmzalbar
Posts: 12
Joined: Mon Jun 22, 2020 5:07 am

Re: ZX81 16K / 32K 62256 didn't work.. advice?

Post by rmzalbar »

Success! It worked with a 84256 SRAM. For some reason, the 62256 just wasn't happy.
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1024MAK
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Re: ZX81 16K / 32K 62256 didn't work.. advice?

Post by 1024MAK »

Alas, I’m rather busy with work and real life at the moment, so I will just point you to this post where you will find various links to a number of different ways of expanding the internal RAM to 16k bytes ;-)

Mark
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ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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rmzalbar
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Re: ZX81 16K / 32K 62256 didn't work.. advice?

Post by rmzalbar »

Thank you. I managed to get the method on the German page to work, and even the 32k using an AND gate. The issue seems to have been with the 62256 I initially used. The SRAM may have been bad, or at least didn't want to play nicely in the ZX's environment, even though the TL866 tested it as good. I tried again with a 100ns 84256 that I had on hand and it worked fine.
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