I bought an LCD screen on eBay about three weeks ago. It said it was UK stock, being shipped from Leicestershire — just down the road from Coventry, so it should have arrived really quickly. Unfortunately, it was actually shipped from Hong Kong, so it only just turned up today. So I had something fun to play with this evening:

However, that's for my homebuilt Z80 project (it's going to connect to a Raspberry PI Zero to make a serial terminal), so I've had to put it to the side for now. Instead, I fixed my PCW's memory expansion.

It took a while. I improved my Z80 test programme, to copy things around a bit more and give me a good idea of what works and what doesn't. As I'd been informed, when I write to a high RAM page it does actually write to a low one instead — it has absolutely no idea that my memory is there. To start with, there was some corruption in the text I was copying though. After connecting up my logic analyser I realised that I'd managed to get the OE and WE pins the wrong way round. I swapped them, and hey presto — still no extra memory. :(

As far as I could tell, the PCW had no interest at all in my memory, connected to the external connector. And then I suddenly remembered that the PCW's memory is connected to a separate set of connections on the gate array, and that there's a mysterious signal on the edge connector called MDIS. Perhaps this disables the internal memory and lets the external memory be used? I connected the RAM_CE signal to MDIS and hey presto — the memory was finally recognised, and my test programme worked.

Now I'm feeling quite pleased with myself! Although my breadboard is looking a bit of a mess: