Sinclair ZX80 / ZX81 / Z88 Forums

Does anyone know what this bit of hardware is?

I'm thinking.. maybe wrongly, Memory and hi res graphics in one unit??? There are no markings of any kind on the outside or inside.

http://pasteboard.co/1s8HslR4.jpg

Looks like a combined RAM and ROM expansion board. At the back is an EPROM (the chip with the window). I suggest you stick a sticker (better still, some black insulation tape) over the window. Over time, the UV light from the Sun or from man made light sources will partly erase the memory.

Some of the chips on the left look like RAM chips. I can't read the numbers on my phone, can you post the chip numbers that start D Followed by 4 numbers please.

Mark

As viewed looking as i would in the pic :-

The 8 chips that are most likely ram chips are D2118-4 x8
Above those are two SN74LS257AN
to the right is the eprom 2516JL
top right of that is an N82S129S
and three bottom right (two under the ribbon cable in the pic) from left to right are SN74LS373N, SN74LS04N and SN74LS32N

hope that makes things a little clearer

Yep, the D2118-4 chips are 16k bit DRAM chips, so eight gives you 16k bytes of RAM.
The EPROM is a 2k byte type.
The two SN74LS257AN chips are used to multiplex the CPU address lines for the DRAM chips.
N82S129S is a 1k bit TTL bipolar PROM, most likely used for address decoding.
The SN74LS373N is a 8 bit latch (stores one byte, which it outputs as logic level outputs on eight pins). Not sure what it is doing on this board. If there are no other connectors (printer port maybe) then it may be being used as a bank memory register, but why this would be needed for the amount of memory on this board is a mystery...
SN74LS04N and SN74LS32N are "glue" logic used for address decoding and control.

So this board gives you 2k bytes of ROM and 16k bytes of RAM Nice

Mark

Thanks for the information.. it does seem a bit odd.. I'm still thinking it could be for Hi Res graphics

The Memotech HRG addon had a 2k eprom so i've read...

Anyone know for sure what this is?

The Memotech HRG is much more complex, using (by remapping) the ZX81 internal 1K RAM so to "patch" the ROM with its own HRG routines. It however doesn't include any RAM expansion like your card does. I'd suspect that the EPROM (if not yet totally erased because of the lack of any protection on its UV erase window) contains some firmware, mapped somewhere between addresses 8182 and 16383: it's easy to find out by PEEKing at those adresses (try 8192, 10240, 12288, 14336 to see which address reports a non-255 byte and a byte different from the ZX81 ROM's shadow corresponding byte, and you'll have found the EPROM start address).

The casing looks similar to one of these made by Basicare Micro System.

Thanks for the help

I do have 3 of the Basicare modules in my collection as well as a few other odds and ends.. One day I will get my website with all my tapes, books and hardware online.. slow progress though... so many other things to do.. and Real Life too..

The ZX81 was my first computer.. bought a kit with my saved pocket money.. soldered it all together.. and... for some strange reason it worked....

Now I collect all things ZX81 (tapes, books and hardware) still have my original zx81 in working order