RAM fault rabbit hole

[gbejniet]

Hi @jobdone, I just mean that I have to press F2 more than once sometimes to get to Basic. This is probably just because of the keyboard reading fault. Minerva is booting 128K ok, I mentioned the 64K thing to illustrate that while I was working through the bad upper ram I didn't have this issue with the keyboard, so it's a recent (iatrogenic ) thing I believe.

[M68008]

It could be the ZX8302

[gbejniet]

I am keeping an open mind here, as it gets slightly weirder. When I boot into F1, the problem is much reduced - I get the occasional wrong keystroke but it's good enough to both (a) type and run a Basic program and (b) go to MODE 8 and observe that the problem doesn't get worse.

Whereas from F2 it is unusable, and prevents me from going to MODE 4 and proving the reverse.

So how might this be, if F1 versus F2 are purely in software? (Unless I'm mistaken.)

And F2 did give me monitor mode once, which I first put down to bad key reading, but thanks @jobdone and @M68008 who suspect the ZX8302. It's an interesting one for sure.

[Martin_Head]

Have you actually tried another ZX8302, 8049, and keyboard membrane?

There are a bunch of diodes and pull up/down resistors connected to the keyboard connectors. Are they all OK.

The 8049 has its own crystal. Is that running at the right speed. If not then the timing of the communication between the 8049 and ZX8302 would be wrong.

Well after over two years and much butchering of traces

Is there any track problems you have missed.

[gbejniet]

Thanks @Martin_Head - I haven't yet tried replacing ICs, I am sleeping on it before I look at all of the above. The membrane is recent so I'd be surprised, especially given the behaviour, but otherwise it could be anything. (The F1/F2 thing is so odd, given neither 8302 nor 8049 are deeply involved with the display, AIUI.)

I'll give it a few weeks of brow-furrowing, then I'm in the market for some of these rare chips (I've had a kind offer to lend some, but honestly given my soldering I wouldn't lend me anything)

[Pr0f]

One of the QL's I've resurrected recently had a membrane fault - that's quite easy to diagnose - in this case visibly too - as there was evident 'cracking' on the ribbon tails, and on testing - one whole row was unresponsive.

But, interestingly - the ZX8302 was also faulty - and this is harder to diagnose - not only will it be responsible for handling the keyboard input from the 8049 IPC chip, it also acts as the interrupt controller for the machine - and this is where I think the 'randomy' type behaviours come in. Why it should change depending on the video mode - not sure on that, it is certainly a conundrum.

[gbejniet]

Just one more point I realised while I've been thinking this over. One of the characters that appears rather than 'a' is '@', which isn't an unshifted key - in my mind this points closer to the 8302 as I can't imagine a contact issue at the keyboard end causing that.