I've borked it

[1024MAK]

A couple of points before I go through your most recent posts.

A0 to A6 address lines should always be changing state even if the processor is halted, as these pins also carry the refresh address (designed to refresh DRAM chips).

The data lines will only be at a valid logic level when being actually driven. In between instruction cycle fetches, or reads/writes to memory or I/O devices, the voltage will decay upwards or downwards (depending on a number of factors).

The important thing is not so much the actual signal. But looking to see if the highs are greater than 2V and the lows are less than 0.8V. And that they are changing state,

A14 and A15 determine if ROM or RAM are being addressed:

Code: Select all

A15 A14
 0   0  ROM
 0   1  RAM
 1   0  ROM
 1   1  RAM  (when drawing the screen)

If you have a 'scope with four channels, you can compare with the /ROMCS and /RAMCS lines.

With the power off, you can test on resistance to test that all nodes (points/chips/components) are connected as per the schematic. Just keep in mind that some of the lines to the RAM are swapped around.

The normal electronic bench mats are what I use. They may be described as anti-static, but their resistance is so high, it won't affect a Zeddy. Alternatively use several layers of thin cardboard on top of a magazine or newspaper. Just make sure it's flat.

Mark

[1024MAK]

It's also worthwhile visually checking each and every socket contact using a good light and a magnifying glass. Compare each contact against its neighbours to check that it's not distorted or bent differently.

Mark

[1024MAK]

The most important address lines between the processor and the ULA are A14 and A15.

Looking at the signals on the pins in your post at 12:25, the processor is clearly not correctly running the ROM code. But it is executing something. Could be a ROM, RAM or socket or PCB fault. Not likely to be a ULA fault if you have tried another.

For A7, A8, A14 and A15, can you please test that there are no open circuit faults due to dry joints or dodgy sockets.

Mark

[msknight]

All chips and sockets check. They all tone out from top of the pin to bottom of the socket. I've also checked for shorts as far as I can.

One interesting thing is that ram select is never going high, either on the ram chip, or the pin on the ULA.

I have tried tracing, but the direct resistance between them is 700k ohms, so it doesn't tone out. I'm not sure if that is supposed to be the case, or whether I've got a break somewhere. However, that trace goes under the processor and gets lost in some resistor packs, so I'm not sure where to pick it up for further toning out.

[msknight]

I found it. It goes via R2 which is nearly 700k ohms, so that's fine.

EDIT - I toned that out as well, and it checks.

[msknight]

But this is how close some of the data lines are...

[1024MAK]

R2 should be 680Ω not "nearly 700k ohms".

If the ROM code can't access the RAM, the ROM code program will crash.

And yes, that waveform in your last post is not good to put it mildly...

Mark

[msknight]

Apparently there was a . in front of the number of k ohms, but no 0 in front of it. I just didn't see the . in the light.

[msknight]

Took another shot of the data lines. They're fairly much the same for processor, ULA, ROM and RAM.

The 470 ohm resistors are fine, but when I measure between the 5v pin and the output pins on resistor pack one, I'm only getting about 5.7K instead of the 10K listed... although I'm aware that this could be affected by it being in-circuit. I can't see an entire resistor pack being affected like that?

[msknight]

By contrast, the address line signals are a lot better.