Hung machine

[Albuzz]

Post Sun Apr 30, 2023 11:44 am

Hi,

I originally posted this as a dead keyboard issue but following some helpful advice, I now realise that there is an issue with all I/O and not just the keyboard. Here’s a potted history:

I replaced the keyboard membrane and I’m doing so dislodged one of the led wires the tinned end of which shorted out somewhere after the keyboard was refitted.

On power on the keyboard was not responsive and the machine sat on the F1/F2 prompt.

I swapped in my minerva Rom to see if it would default to F2 and try to boot from Mdv1.. it didn’t

I the replaced the ZX8302 and IC24 to no avail. I assumed that since the system got as far as completion of the memory test and successfully displayed the F2 prompt the the COU must be ok.

Can anyone advise if that is correct or could there yet be an I/O related issue with the CPU?

Any pointers would be greatly appreciated

Thanks
Alan

[XorA]

Minerva will lock up at that point if it either 1) is flooded with IRQs or 2) has no IRQs.

Next step is probably checking the 8049 chip which is in the area you said you lost a wire.

68000 has no IO space, its all MMIO so you cant have one working and not the other!

[Albuzz]

Thanks for the advice

[Albuzz]

One step forward but !!

I was convinced it was the 8049 so eventually got hold of one (well 2 actually) and replaced it, expecting/hoping thing to at least improve, but with the replacement 8049 in place, it no longer gets as fas as displaying the memory test screen let alone reach F1/F2!! Instead the screen shows no sign of life. Sadly my scope gave up so I’m waiting for a new digital one to arrive (birthday present). I’ve tried both replacement 8049’s and get the same with both. I’m assuming that the 8049 was faulty and the new status is due to a masked fault somewhere downstream affecting the data/address bus or IRQ? Any ideas anyone?

I’ve now subbed all of the major IC’s so I’m down to only 2 or 3 suspects and/or transistors/diodes.

Btw, reinstalling the original 8049 returns to the F1/F2 screen.

Thanks

[jobdone]

Where did you source the 8049s and are you sure they are programmed with the QL software?

[martyn_hill]

Hi Alun

Quick question that may prove relevant...

What version of the main ZX8301 ULA is fitted? There are two variants (2310 and 2345) and I've found that in some versions of the motherboards (Issue 5, in particular) with the original ULA (2310), the Minerva boot screen also hangs immediately after the F1/F2 prompt appears.

Although the Iss5 boards were matched with the 2310 version ULA, some slight timing differences seem to exist between those 2 main ULA versions that appear to trip up the comms with the secondary ULA (ZX8302) which is accessed by Minerva to read the 'real-time' clock in order to show the date/time in the boot screen. It's also about then in the Minerva boot cycle that interrupts are enabled for the first time, which are managed by the secondary ULA...

When I replace the ZX8301 with the later version (2345), it all works as expected following the boot screen.

In any case, likely as not it is the comms with the secondary ULA that is breaking.

I'm sure it's all fixable, though having access to both variants of the main ULA would be a great help here... Check out RWAP's SMR site to see if he has a 2345 version ZX8301 for sale...

[Albuzz]

We’ll thanks for the replies..

The 8049’s are definitely good but it isn’t the problem.. I’d bent a pin when fitting so once that was sorted I’m back where I was.. stuck on the F1/F2 screen.

Thanks for your comments Martyn, I’m on the former version of the ZX8301 and I can’t seem to locate the later release, so nonetheless I’ve ordered a replacement (same release) just in case as the ZX8301 is the only major chip I’ve not been able to substitute.

Presumably if it was down to timing issue with the Minerva then if I swap out the Minerva back to the original then it might work?

Thanks
Alan

[martyn_hill]

Hi Alan!

Given what you've found with the bent pin on the 8049, my earlier comments are probably not relevant to this case afterall

However, if the ZX8301 version/timing is still relevant, it would still impact older QDOS versions, but perhaps at a slightly different point in the boot cycle or show slightly different symptoms during boot.

Anyhow, I'll think some more, given that you've gotten back to the F1/F2 lock-up...

[Albuzz]

Thanks to everyone for your comments.. well if only I’d changed the ZX8301 first I’d have saved myself a few pounds and everybody’s time!

Swapped out the duff 8301 and popped in the replacement and hey presto, she’s up and running again.

Now I’ve just got to do a proper Kobe of those keyboard led connectors.

Thanks again
Alan

[Albuzz]

For Kobe read job