ZX21 Project ZX81 Replacement PCB

[fordp2002]

OK, I have been working on getting the YMZ284 sound going today. Well after quite a bit of RTFM I found a few issues with my design. I only roughed it out on this first version as I was pretty sure it was not going to work as this was a new design.

Well, the first hint is that for IO decoding there is no point in looking at A0 - A2 as for every add-on it will always be 111. This is actually very cool as you only need to look at 5 address lines to fully decode the add-on addresses. It is not technically fully decoding but it will not matter. I will change that on the next issue. I can easily use the debug connectors to work around this issue. The next thing having checked is that although I connected A7 to the sound chip that won't work as it is the wrong way up. I can cut the track and use spare pins on the GAL16V8 to work around this without even adding a passive. I will have to add my first bodge wire of course.

[fordp2002]

I didn't notice the DB9 connector until I looked at the recent picture. Is that a standard RS232 port?

Yes like Mark says it is a Joystick port. A serial port on a ZX81 is much rarer than a joystick port and hence has less software out there that can use one.

There is some mention of using WESPI to talk to the outside world beyond loading and saving but there is no support for that yet. This is clearly possible as tape loading and saving is technically a type of serial port.