Yet Another Black Screen (YABS)

[namaman]

It's this motherboard, but with the 40010 gate array in the upper socket:

MC0002B Motherboard

[McArti0]

It isnt good pattern.

Now show by Scope READY/WAIT signal. 1MHz duty 25%

Now add ground to READY line. This will stop the processor.
Is the image degrading itself?

Without ground to READY.

Show by scope READY & nCAS
READY & nRAS
READY & RAMRD
Trigger to READY.

~500ns/div

[namaman]

This video shows:
- Machine turning on and starts the Diag ROM
- ROMDIS connected high (random noise)
- CPU reset briefly to GND

The vertical bars appear but everything is constantly moving as you can see in the video.
If I then ground the READY line the movement will stop until I remove the jumper. At some point the system freezes anyway and all movement stops.

I'll do those other scope tests asap. (Maybe tomorrow as its getting late here now)

[namaman]

Yellow - CAS
Red - READY
500ns

[namaman]

Yellow - RAS
Red - READY
500ns

[namaman]

Yellow - RAMRD
Red - READY
500ns, 5us, 5ms (three seperate captures)
(please keep in mind that this is a pretty cheap scope)

[McArti0]

Yellow - CAS
Red - READY
500ns

RAS 

Yellow - RAS
Red - READY
500ns

CAS 

 Screenshot 2025-04-03 at 1.21.20 PM.png

What is this?

Try repeat this and write about setting.

[namaman]

...Thats what I saw on:
Yellow - RAMRD
Red - READY

I can have another look on an analog scope...may be better.



*Oops...I was sure I didn't get RAS and CAS the wrong way around...oh well!!!!

[McArti0]

How many channels do you have in your oscilloscopes?

Btw. https://bread80.com/2021/06/03/understanding-the-amstrad-cpc-video-ram-and-gate-array-subsystem/

[namaman]

Two, but I have a logic analyser somewhere....but I only have one IC clip for it.

Quick question, I'm assuming the RAMRD signal is actually the nRAMRD from pin 29 of the 40010 Gate array?

[namaman]

so here is a video...

500ns/div
Tested with the Diagnostic ROM installed.
Red channel is the READY signal of the Z80.
Yellow channel is the nRAMRD signal from Pin 29 of the 40010 Gate array. nRAMRD stops doing anything once the dashed lines are displayed on screen.


Have I done something wrong?

[McArti0]

Yes.

https://www.cpcwiki.eu/index.php/File:CPC664_Schematic.png

Look at the diagram. 664 is also on 40010 so it is the most similar.

If you confirm this noise on RAMRD then check if Z80 pin21 /RD has a good connection.

[namaman]

I'm still not sure what I have done wrong. On the 664 schematic you sent, the nRAMRD is on pin 29 as I have tested.

I will check Z80 pin 21
*UPDATE Pin 21 connects to IC112 Pin 5...good

Thanks

[McArti0]

Yellow - RAMRD
Red - READY
500ns, 5us, 5ms (three seperate captures)

The noise on the second one is very bad.

[namaman]

hummm...yes.
I'm using a switching power supply, and just tried my bench supply...no change.

[McArti0]

Yes or no...

[namaman]

Yes I can repeat this, but I'm not sure if it's just some artefact from the cheap digital scope. Weird.

Here is the same thing from an analog scope:

[McArti0]

Then we are to assume we didn't see it.

Then permanently Lo on resistor R124. And test pin1 of 74LS373

[namaman]

let me think of a better way to investigate that

[namaman]

Yeah...Possibly we could just ignore that :-/

[namaman]

Then permanently Lo on resistor R124. And test pin1 of 74LS373

Lo on R124 makes no difference.

Here is pin 1 of the 74ls373

[McArti0]

RAMRD is so rare that you have to set the trigger on it itself. Analog scope may have a window that is too small.

Now I would check all the multiplexer control signals.

nCPU, CASADDR. Basically everything will be fine here but maybe GA is not mistaken.
Then multiplexers at the end - the verdict - 8x RAM.

[namaman]

nCPU, CASADDR. Basically everything will be fine here but maybe GA is not mistaken.
Then multiplexers at the end - the verdict - 8x RAM.

Sorry, I'm not sure what you mean here.

[namaman]

All RAM removed and socketed.
Diag ROM now just shows RED screen and then goes to the dashed lines.

[Audronic]

@namaman 

Ok That's good news.

For a quick test Just fit 4 Ram chips at Bit0  to Bit3 and retest

and you should get as per Photo

You are nearly there

Keep Safe

Ray