Page 1 of 1

Another "keys not working" issue

Posted: Thu Dec 27, 2018 4:10 pm
by DaveR
I have been getting my (kit built by me) ZX81 working again in the last couple of months.
It has a new sockets for the chips (except the 2114 SRAMs - planning on upgrading to a 16k or 32k SRAM at some point), new Andy ULA and freshly programmed EEPROM. Also a DKtronics replacement keyboard. All buzzed out fine before inserting the chips. Not sure if I have got the NMOS or CMOS Z80 in at the moment.

Using a scope, the video signal is a bit noisy but usable at the moment. Power up gets the K cursor (big relief as well), all the keys work except for some of the shifted keys. For example, all of 1 to 0 keys work but shifted, only 1 and 0 work. Some other shifted keys don't work but there doesn't seem to be a consistent pattern on each row. The display still flickers when say shift 2 or shift 9 (or other combinations) is pressed, just nothing else happens.

The cable between the keyboard and computer might be an issue. It is about 3 metres long so that the computer and power supply can be at the TV and the keyboard be a bit more remote - I don't have a small "portable" TV any more - the smallest being 28" and wall mounted.

Anyone come across anything similar before? Anything to try? I don't really want a shorter keyboard cable but will if nothing else seems to be an option if the cable has too much load depending on what else is shared on the address bus. The scope is an office one, no access until the new year.

I'll cover the video issues when this is one sorted - one step at a time!


Re: Another "keys not working" issue

Posted: Thu Dec 27, 2018 5:30 pm
by 1024MAK
I would say that 3 metres is far too long!

Remember that the output lines from the ZX81 are effectively half of the Z80 address bus (the 8 way connector). The input lines (five lines) are inputs to the ULA.

If you do want a long cable, like 3 metres long, I suggest that you use eight transistors to “isolate” and buffer the address bus signals. They need to go at the ZX81 end.

Also you may need to buffer the five lines going to the ULA. A 74LSxxx chip should be used for this and needs to go at the keyboard end.

But first, I suggest that you try your set-up with a cable that is no more than 35cm long.


Re: Another "keys not working" issue

Posted: Sun Dec 30, 2018 6:42 pm
by DaveR
:oops: so shortening the cable did indeed get all of the keys working - thanks for the suggestion. I didn't fancy stripping 13 fiddly wires again but at least it was a nice simple fix. As you say, I'll need to buffer the signals if I want a longer cable.

Next the strange video. I'm thinking of starting with more decoupling across the chips. It looks like the new ULA has a voltage regulator on the underside, so how effective is adding a cap across the ULA if the FPGA, I presume, is hidden behind the regulator?
I'll start a new topic when I've taken some more pictures.