Sinclair ZX80 / ZX81 / Z88 Forums

Would anyone have the wiring diagram and parts list to replace the 1H035E ULA in the Sinclair 16K Ram Pack issue 3?

acsi wrote: ↑Fri Dec 06, 2024 6:51 am Would anyone have the wiring diagram and parts list to replace the 1H035E ULA in the Sinclair 16K Ram Pack issue 3?

Issue 3 wiring diagram



parts list is:

• 1H035D Ferranti ULA

An alternative to the ULA is to use several ICs. Which is what Issue 1 and Issue 2 did:


The downside was: to fit inside the pack it used an entirely separate board:

In another topic on here, a member worked out that the ULA does not in fact fully replicate the whole counter and multiplexer circuitry shown in the issue 1 and 2 boards. Instead it only provides a subset, the Z80 providing the remaining address bits for refresh. In the issue 1 and 2 design, the counter is providing a guaranteed refresh address for the DRAM chips.

Another point is that most third party RAM packs completely dispensed with a refresh address counter. It having been found that it was not necessary. They just have a single logic chip for address decoding and DRAM control lines, two multiplexer chips for the 14 bit to 7 bit switching, and of course the eight DRAM chips. This part of the reason that they could undercut Sinclair with a lower price.

Mark

i was delighted to find those old magazines online by clicking on the schematics. i love old magazines, i often spend hours reading everyday electronics magazines from the 70s and 80s online.

Yes, I get "sidetracked" when researching some ZX81 technology or software, and end up examining a totally different article from the magazine ...