ZX81 Hi-Res

[britain4]

I've just upgraded my ZX81 to 16k internal RAM with a 62256 chip as described here: http://blog.tynemouthsoftware.co.uk/201 ... m.html?m=1

However when I tried to run the 25thanni demo the Hi-Res parts were corrupted. To be expected I thought as I hadn't followed the instructions to enable Hi-res so as described here I tried connecting pin 27 with 5v via a resistor

The machine won't boot at all with this configuration and only works when I restore the original connection from pin 27 to the ZX81 PCB.

At this point I am completely lost so can anyone point me in the right direction to enable Hi-res on it? Unless I'm missing something pin 27 is at 5v all the time anyway so I can't see what effect it would have

Thanks!

[PokeMon]

For HRG graphics you need to enable RAM read during refresh phase of Z80, too.
Usually OR'ed with two diodes using schottky or germanium types with low forward voltage (0.3V).
See here for more information:
http://www.z88dk.org/wiki/doku.php?id=platform:zx81

[britain4]

Thanks for that. I might just modify my external RAM pack then as I can't get my head around how to apply that mod to the internal RAM!

[RetroTechie]

For HRG graphics you need to enable RAM read during refresh phase of Z80, too.
Usually OR'ed with two diodes using schottky or germanium types with low forward voltage (0.3V).

No need for those with internal SRAM. ZX81 (true) hi-res works with the internal RAM in factory condition, and (if done right!) expanding that 1 or 2 KB to 16 KB doesn't change that. Adding diodes or logic gate(s) is only needed for DRAM-based external RAM packs. Or going beyond 16K...

@britain4: could you post exactly what signals are connected to each pin of the SRAM, as you have it wired up now?

Also: try running a 1K hi-res demo, some regular 16K software, and some (16K) pseudo hi-res software like Booster or Forty Niner. The latter games are hi-res but using graphic data from the ROM so should run fine even if true hi-res doesn't work. Perhaps these tests will provide more clues as to what exactly is or isn't working.

And of course there's always the possibility of a defective SRAM chip. Did you use/test it beforehand in other equipment? If you have another specimen to try: please do!

[britain4]

For HRG graphics you need to enable RAM read during refresh phase of Z80, too.
Usually OR'ed with two diodes using schottky or germanium types with low forward voltage (0.3V).

No need for those with internal SRAM. ZX81 (true) hi-res works with the internal RAM in factory condition, and (if done right!) expanding that 1 or 2 KB to 16 KB doesn't change that. Adding diodes or logic gate(s) is only needed for DRAM-based external RAM packs. Or going beyond 16K...

@britain4: could you post exactly what signals are connected to each pin of the SRAM, as you have it wired up now?

Also: try running a 1K hi-res demo, some regular 16K software, and some (16K) pseudo hi-res software like Booster or Forty Niner. The latter games are hi-res but using graphic data from the ROM so should run fine even if true hi-res doesn't work. Perhaps these tests will provide more clues as to what exactly is or isn't working.

And of course there's always the possibility of a defective SRAM chip. Did you use/test it beforehand in other equipment? If you have another specimen to try: please do!

Hi

Thanks for the reply - that makes more sense now!

With the internal 16k mod I have tried normal 16k software which works fine, I have tried 16k pseudo hi-res Pac-Man which works fine (although I'll test the ones you mentioned later). I haven't tried any 1k Hi-Res stuff yet.

The address lines are connected in exactly the same way as the first link I posted, in the same order, but I opted to bend the legs of the chip out rather than modify the PCB.

The hi-res images are "there" and visible but with a square of garbage around each screen element (I will post a photo to show what I mean).

I have not tried the chip in anything else, I bought it new fresh off eBay and tried the print poke routine in the ZX81 which have the proper value for 16k RAM.

Thanks!

[dr beep]

With the internal 16k mod I have tried normal 16k software which works fine, I have tried 16k pseudo hi-res Pac-Man which works fine (although I'll test the ones you mentioned later). I haven't tried any 1k Hi-Res stuff yet.

Since 1K hires doesn't use extra RAM it should work as expected.

[britain4]

With the internal 16k mod I have tried normal 16k software which works fine, I have tried 16k pseudo hi-res Pac-Man which works fine (although I'll test the ones you mentioned later). I haven't tried any 1k Hi-Res stuff yet.

Since 1K hires doesn't use extra RAM it should work as expected.

I have just tried 1k hi res software on it - it gives the same result. Testing a different RAM chip would mean me ordering another one off eBay (I thought i had bought 2 but only 1 came). It's a brand new CMOS chip. maybe a timing issue or something?

I've tried the same 1k hi-res software on another Z and it works perfectly fine.

External (unmodified) RAM pack gives a solid black rectangle in place of the hi-res graphics.

The Hi-Res stuff with the internal 16k works and is visible but has garbage around it like I say.

The ZX81 has a switching regulator, new caps, and a ZX8-CCB board fitted, none of which should make any difference but thought I'd mention it.



I tried a different Z80 in it (from the ZX81 I successfully tried with the 1k hi red software) and the garbage was lessened but still there

[1024MAK]

Please post a photo of the RAM chip.
I showed how I did an issue 1 board here
Note that my mod is designed to allow a further modification to 32k bytes. Hence the cutting of the +5V track, then fitting a wire link across it.

RetroTechie's site shows how to do the mod on an issue 3 PCB. See the archived web page here.

Mark

[britain4]

There's the identical one in the eBay listing but I will post a picture of the actual one when I get home.

Cheers for the link, I'll have a browse of that, I might even order some different chips remove everything I've done and start again.

[britain4]

Here's the chip



It's messy but I intend to tidy it up or redo the whole thing when I get it working as I want it