Sinclair ZX80 / ZX81 / Z88 Forums

BASIC always uses its own ROM routines to read the keyboard. At the lowest level, this is the Z80 processor using the Z80 machine code IN instruction. Each keyboard half row has its own I/O address. The data returned is one byte, but only five bits are keyboard data. Hence it takes eight IN instruct...

I was looking at the schematic and wondering how external keyboards work via the edge connector if there is no access to the five KBD lines that go to the ULA. Is the ROM checking for keypresses? I haven't yet tripped over how this actually works yet. That's an entirely new subject... The simple an...

Moggy wrote: ↑Thu Mar 14, 2024 4:36 pm I do Have a Kempston but this I seem to remember, unlike what Mark describes, has the extra tangs as used on a Spectrum interface.

As far as I am aware, all the later Kempston versions did have the longer ZX Spectrum edge-connector sockets.

Mark

msknight wrote: ↑Thu Mar 14, 2024 4:45 pm I had read that, but Kelly's, "last active" came up with just a dash, so I concluded hadn't been around for a while.

Still active, has visited this week.

Mark

There was no particular joystick interface that was popular for the ZX81, hence there is no real "standard". Just various interfaces that don't have much in the way of software support, most of which were forgotten about long ago. Kempston produced joystick interfaces for the ZX Spectrum, but used t...

These days, it's whatever you can get hold of that work. And yes, you can mix and match. Sinclair used various makes/brands of 4116 DRAM or equivalents. Alternately, use 4164 type and modify the board to suit these. Whatever you use, they do need to have an access time of 150ns or faster. Slower par...

ULAs seem harder to come by. Original 'spare' ULA chips ran out years ago. Only used ULAs come up occasionally. However, there are replacement ULA modules. But they are in a limited supply, and are not cheap. Nebula - Scroll down on this page . vLA82 - link From my understanding I'd need a clear vi...

The MCM21L14P30 is a low power version of the MCM2114. The MCM21L14P30 has a maximum access time of 300ns NOT 30ns. The MCM21L14P45 has a maximum access time of 450ns NOT 45ns. A data sheet for these is available here Any 2114 SRAM chip with a maximum access time of 300ns or quicker should work incl...

If you hear the key click sounds and BASIC appears to be running (try SAVE "P" ), then it's more likely to be a ULA fault rather than a DRAM fault. But without a test/diagnostic ROM or another ULA to swap with, it's hard to be certain.

Mark

In answer to the chips in sockets, your machine was sold as a 16K RAM model. The chips in sockets have been added later. It was normal for Sinclair to fit sockets for the extra chips needed for the 48K RAM option. Does the border always turn white at every power on? Do this (power cycle waiting 30 s...