Homemade 64K RAM pack

[Erm]

This is the inside of a rather attractive homemade 64k Memory pack that I got recently. Whoever made it, was a very skilled person and seemed to have stock of some late 1970s chips when they made it.

As you can see, it consists of 8x Mostek RAM chips, 7x 74X logic chips and an 82S123N PROM, which had a sticker on it labelled 'Orig 64k Sinclair'. When plugged in to a ZX81 it shows up and continuous Ram from the end of the ROM all the way to the edge of the ZX81's little universe.

I haven't actually loaded anything other than SysTest (to show me where the Ram was) yet, but I assume this will not work with any kind of HR stuff? And out of interest I also wondered what the 'Orig. 64k Sinclair' chip is all about, as I don't think there was ever any kind of original Sinclair 64k Ram add-on.

[avrovulcan]

That's a very nice piece of kit you have there. Certainly someone's labour of love. They obviously took a great deal of care and pride in it when they built it. Nice to see kit like this ending up in a good home.
As regards the Prom chip, perhaps it was originally from something like a Memotec 64K RAM pack or something similar??

[siggi]

Since it has a Sinclair prom: maybe that is a prototype, made by the Sinclair developers, which never lead to a product?

Siggi

[Erm]

Since it has a Sinclair prom: maybe that is a prototype, made by the Sinclair developers, which never lead to a product?

Siggi

It certainly is an amazing bit of kit. Externally is also nicely done. Simple and stylish, and it's length fits perfectly behind the whole width of the ZX81.

It was just a white sticker on the PROM, so who knows, it could be a copy of something from a commercially available 64k mem pack as Avrovulcan suggests. I'm going to eventually try it with HRG, but am not expecting that to work.

Anyone got a commercial 64k mem pack? Do they contain a 82S123N PROM I wonder?

[1024MAK]

The Memotech 16K, 32K and 64K RAM packs all use a fast PROM for address decoding.

A hand drawn schematic for the control circuitry for the 16K version is in this thread

Note that since posting that, I have become aware that later versions of the 16K unit used a different board and used HM4816AP-7 DRAM chips instead of 4116 DRAM chips. These use a N82S123 PROM.

Mark

[siggi]

This is a commercial ram pack, built into a new case. It does not have a prom, but additional hardware to drive a relay (8 bit output, e.g. to control a cassette recorder).

Siggi

[Wattles]

Was a prom left out deliberately, Siggi?

[1024MAK]

The board that Siggi posted photos of, uses a Texas Instruments DRAM chip. This particular DRAM chip needs a 8 bit address (256 cycle) refresh address. So the board includes additional circuitry to handle this. It uses conventional 74 series chips for the address decoding and control circuitry.

There were basically two types of 64K bit DRAM chips. Those that need a 8 bit address (256 cycle) refresh address and those that were made to be compatible with systems/technology that use a 7 bit address (128 cycle) refresh address system. The Z80 microprocessor only generates a 7 bit (128 cycle) refresh address, so most Z80 systems use DRAM chips that are designed for a 7 bit address (128 cycle) refresh address.

Also, in the early days, it was also thought that due to design of the ZX81 system (which uses the Z80s refresh system for other purposes) would result in improper refresh of dynamic RAM (DRAM). Sinclair’s own RAM packs included extra refresh circuitry. But somewhere, sometime along the way, RAM packs dropped the extra refresh circuitry. This includes the later versions of Sinclair’s own RAM packs.

Hence the homemade board shown in the first post uses MOSTEK MK4564 DRAM (which are 7 bit address (128 cycle) refresh address type) chips.

Boards that use a fast PROM use it to reduce the chip count, as fast PROM chips are useful in address decoding circuits.

However, I don’t know why there are so many 74LS244 tri-state line driver/buffer chips.

Mark

[Erm]

I have a couple of questions on the 64k RAM pack (mine, and in general) after having a play around with it today.

1. When I use SysInfo it shows the 4k from 2000h upwards as being filled with FF's and when I peek e.g. 8193 I get the expected result 255 and am unable to poke it with any other value. Is that normal?

2. I can raise RAMtop with Poke 16389,255 and get access to 47/48k RAM for BASIC. I thought the maximum for BASIC was 56k, but obviously I can't poke higher than 255. Is there a trick or is 47/48k the max available for BASIC and the remaining 8k is for storage of variables/other stuff?

Thanks.

[1024MAK]

The area 2000h to 3FFF in an unexpanded ZX81 / TS1000 is normally seen by the Z80 as ROM due to the partial address decoding.
The RAM pack should be disabling the ROM and enabling its own RAM in this area. This only works if all the address lines to the RAM pack are good and the ROMCS signal back to the computer is good.

Have you made sure that the edge connector is clean and that all connections look good (no damage to any contacts on either the computer or the RAM pack)?

Also, if this is a Memotech RAM pack, are the DIP switches set correctly? The instructions are available on this web site.

If it’s the homemade RAM pack featured in the first post, it very much depends on the circuitry used and what that PROM was programmed with.

Mark