Newbie - My Hardware

[EGGS=58]

Hi All,

Just found & joined this forum after getting nostalgic about my old ZX81...! I thought I'd start by listing what hardware etc I've dug out from my 'archives'. None of it's for sale, but I do now plan to do some tinkering with it.

Couple of ZX81s (dead issue 1 PCB & at least one fully working unit)
Couple of ZX80s (both working)
Memotech 16K RAM for ZX81, complete with original box, instructions etc (even the price ticket on the box!)
Audio Computers 16K RAM for ZX81
ZX81 ROM upgrade for ZX80
ZX81 keyboard overlay for ZX80
Maplin Talk-Back speech synthesiser board (uses SP-0256-AL2 chip) with original magazine article
William Stewart Systems interface board with AY-3-8910 I/O & stereo sound (with schematics & instructions I think)
Spare chips (couple of AY-3-8910 & SP-0256-AL2 I think)
Currah MicroSpeech board for Spectrum

Many other Maplin, Your Computer, Sinclair & other 80s magazines with ZX-related articles. If anyone's interested I'll list the hardware projects etc that are in the magazines and can create PDFs of them for you to read.


Plans...
Well I guess the main plan for now is to design & build my own small ZX81 using SMD components, replacing the ULA with a Xilinx FPGA (or CPLD). I've wanted to do this for ages, but keep getting side-tracked. I guess it'll consist of a Z80, RAM & ROM, plus the Xilinx chip. Obviously if anyone has any suggestions as to what else should or could be added, just let me know.
I have (and regularly use at work) all the tools needed to design the schematic, PCB layout & FPGA/CPLD code. If it works in prototype I might make them available for anyone who's interested. I usually get Eurocircuits to supply my bare PCBs as the price & quality is pretty good.

Cheers for now.
EGGS

[yerzmyey]

Hi there & welcome!

[sirmorris]

Hiya!

Welcome aboard. As you can probably tell we like our add-on projects here Expertise is always valued

[XavSnap]

Welcome !

[zx81jens]

hi and welcome!

Plans...
Well I guess the main plan for now is to design & build my own small ZX81 using SMD components, replacing the ULA with a Xilinx FPGA (or CPLD).

EGGS

Bodo Wenzel did that, but not with SMD... you´ll find information here:

http://zxgate.sourceforge.net/


many greetings
jens

[Citabria]

Hi, all,

This is my first post. When I saw a BYTE ad for the ZX81 kit in early 1982, I ordered one right away and had lots of fun building it up and programming it. I was impressed with how much it could do with only 1K RAM. I already had an Atari 400, so the membrane keyboard did not bother me. Over the next two years I added SRAM inside. Firstly, piggy-backed 2114s plus gates for 2K. Then a Z6132 in the 28-pin socket for 4K. Next a 6264 for 8K. Finally, piggy-backed 6264s and a gate for the full 16K.

I dug it out recently and opened it up:

[Citabria]

How many of you have done internal SRAM upgrades to avoid the wobbly DRAM pack, or just to free up the connector for more interesting uses? Which RAMs have been used?

I recently found some MCM6290 16Kx4 SRAMs on an old '486 motherboard and I noticed they are very nearly pin-compatible with the old 2114s used in original ZX80s and ZX81s. They share 15 pins in common. So there's a straightforward internal upgrade without piggy-backing. Has anyone done that?

[sirmorris]

Hi!

I certainly have - though I used a 32k S-RAM. There's a few schematics hanging around. http://www.zx81.de/english/32k-rame.htm and (the one I used) http://8bit.yarek.pl/computer/z80.zx81/ ... ecoder.gif

You've probably got all this worked out already, but from the looks of it you'd seat these 16k chips with 1,2 and 24,23 hanging, 10 bent up. Patch wires from the socket pins 7 and 15 to the chip's hanging address lines. The 2114 uses the /WE line as read/not write. You'd have to wire an inverter to the write line then on to /G. /E would be tied low. Finally a patch wire for VCC, and all the remaining address lines would need to be tied to appropriate points on the motherboard.

And you'd have to do this for both chips You could share the inverter logic between the two RAMs, but that's not such a big deal.

All in all I think I'd go for one of the 32k options. Arguably it's not much more work but you would have to source a fairly common 32k S-RAM. You'd gain the ability to run all hires programs because you'd have 8k in the 8-16k area. Oh and you could run Xav's 32k graphic adventures too! Win!

[yerzmyey]

Hi there.

[Citabria]

Thanks for the welcomes and for the schematics. I agree that the 32K RAM is more useful but I think these 6290s can be wired in without any inverters or gates or diodes. Plus they fit into the ZX80's 2114 sockets without cutting any traces on those valuable machines (although you might have to move the heat sink). Of course, they also fit the ZX81's and TS1000's 2114 sockets.

You would bend up pins 1, 2, 3, 10, 21, 22, 23, and 24. Connect pin 10 to the same signal as pin 11, -RAMCS, since the truth table (below) shows that -E and -G can be connected together. Pin 13, -W, is just fine where it is, in pin 10 of the socket, the Z80's -WR signal. Then, as you state, connect pin 24 to Vcc and the other pins to the remaining address lines. I haven't tried it but if there's any interest then I will.