Mega ST4 - no video output at all

[asapreta]

Badwolf was suggesting testing the RAM chips' temperature using your finger to see if any are getting overly warm.

When the screen showed black with white border, bad RAM was my first insight.
I did try to see if any RAM was warmer than other but none was.

Answering your last question, it'll need one bank of 16 chips. I can't remember what the layout is for that board but I know it should run with half the RAM banks as that was how it would have been sold as a Mega2 ST (2MB).

My board is C100167-001 REV 5.
It has 4MB of RAM (32 ICs).
TC511000P 1Mbit DRAM

So if I unpopplate the upper two MB banks I will have a 2MB machine?

[ijor]

I captured again the both CAS0 H/L and CAS1 H/L, as below:
...
Also I tried to use the Logic Analyzer and I don't know if it makes sense what I captured, still learning:

I am confused about these CAS captures. In first place, please be precise because it is important. To avoid any ambiguity tell us at exact at which pin you are probing CAS1.

You said there was no activity on CAS1, but now there seems to be something and it is not clear if it is noise or what. Your CAS1 scope capture shows a couple of pulses, why you just two pulses? The Logic analyzer trace also shows a lot of CAS1 activity. Can you please zoom on that trace so that we could see individual pulses. If it's noise, I am wondering if that pin is not floating, and of course it should not.

Is this what you asked for, please?
7 PRIMEIROS_.png (1009.18 KiB) Viewed 53 times

Why such a short trace? I understand this streams to the PC. That would mean you can perform very long captures. Captures the whole bus activity since reset until the CPU is halted. Once you have that "full" capture, measure the HALT pulse. That would give us a good idea at what point the system is halted. Also zoom and post the last cycles. Can the software export the capture to some kind of file standard?

I would include CAS in the trace. Because, again, what happens with the CAS signals might be critical. May be sacrifice DTACK that is not so important at this point and add CAS0 and/or CAS1.

[asapreta]

I captured again the both CAS0 H/L and CAS1 H/L, as below:
...
Also I tried to use the Logic Analyzer and I don't know if it makes sense what I captured, still learning:

I am confused about these CAS captures. In first place, please be precise because it is important. To avoid any ambiguity tell us at exact at which pin you are probing CAS1.

You said there was no activity on CAS1, but now there seems to be something and it is not clear if it is noise or what. Your CAS1 scope capture shows a couple of pulses, why you just two pulses? The Logic analyzer trace also shows a lot of CAS1 activity. Can you please zoom on that trace so that we could see individual pulses. If it's noise, I am wondering if that pin is not floating, and of course it should not.

When I captured that "noise" CAS I left the probe in the CAS pin of the DRAM while was adjusting the oscilloscope, so the computer was on for a while and it wasn't captured during a reset, as I did the other pictures which I filmed the oscilloscope then edited to get the pulse frame.

Is this what you asked for, please?
7 PRIMEIROS_.png (1009.18 KiB) Viewed 53 times

Why such a short trace? I understand this streams to the PC. That would mean you can perform very long captures. Captures the whole bus activity since reset until the CPU is halted. Once you have that "full" capture, measure the HALT pulse. That would give us a good idea at what point the system is halted. Also zoom and post the last cycles. Can the software export the capture to some kind of file standard?

I would include CAS in the trace. Because, again, what happens with the CAS signals might be critical. May be sacrifice DTACK that is not so important at this point and add CAS0 and/or CAS1.

It is the first time using a Logic Analyzer.

When I got the "full capture" to see where the HALT starts, it is too compact to show the pulses:

INICIO-FIM.png

I have exported this capture as a .CSV file, let me see how to add it here.

INICIO-FIM.csv

[stephen_usher]

You should be able to zoom in using the mouse wheel or similar.

You can also trigger on /HALT going high so the capture starts then. (Click on the upward slanting icon to the right of the text "/HALT".)

[ijor]

When I captured that "noise" CAS I left the probe in the CAS pin of the DRAM while was adjusting the oscilloscope, so the computer was on for a while and it wasn't captured during a reset, as I did the other pictures which I filmed the oscilloscope then edited to get the pulse frame.

So you were scoping the DRAM chips and not the MMU pins? Exactly which CAS pins you probed with the logic analyzer. Again, if you are not precise it would be very difficult to help.

When I got the "full capture" to see where the HALT starts, it is too compact to show the pulses:

You can of course zoom and pan the capture. I am not familiar with that software to tell you exactly how. But it shouldn't be too difficult to find out. Probably you can just zoom with the mouse, otherwise there should be a menu option to zoom in and zoom out.

I have exported this capture as a .CSV file, let me see how to add it here.
INICIO-FIM.csv

Thanks. But CSV are usually a PITA to process. Does it have other options, like VCD?

But again, anyway, zoom to to the end of the capture and post a screen dump. And again, try to perform another trace now with the CAS0 and/or CAS1. You can remove DTACK that doesn't add too much information here.

[asapreta]

When I captured that "noise" CAS I left the probe in the CAS pin of the DRAM while was adjusting the oscilloscope, so the computer was on for a while and it wasn't captured during a reset, as I did the other pictures which I filmed the oscilloscope then edited to get the pulse frame.

So you were scoping the DRAM chips and not the MMU pins? Exactly which CAS pins you probed with the logic analyzer. Again, if you are not precise it would be very difficult to help.

Only when I got that noise which I thought was a signal.
The other CAS captures were in MMU's pin.

The one I got with the Logic Analyzer was CAS0 and 1 LOW in MMU's pins, the one in the previous page,

The data I capture with multiple signals I used the expansion header next to CPU.


I will capture the signals you mentioned.
Thank you.

[asapreta]

You should be able to zoom in using the mouse wheel or similar.

You can also trigger on /HALT going high so the capture starts then. (Click on the upward slanting icon to the right of the text "/HALT".)

Will trigger that and yes the zoom is available but if I zoom I lost a lot information to the sides.

[ijor]

The one I got with the Logic Analyzer was CAS0 and 1 LOW in MMU's pins, the one in the previous page,

I see. So zoom any part of that capture and post it. I find that CAS1 capture very strange.

[ijor]

Question, you said you tested the MMU chip in another computer and it worked fine? But you tested the other way around, the other MMU chip in this Mega?

[rubber_jonnie]

Just going back through this thread and I think it would be useful for you to post what you're currently seeing on screen when you boot as the last image was all the way back at the beginning.

Are you still getting a black screen with white border? Or is it something else?

When threads get long a summary of what is happening now and what you've done to this point are helpful for new people joining the thread.

Also, you mentioned at the beginning that the previous owner could get it running by slapping it, which suggests a mechanical issue. Do you know where it was slapped? on the side, on the top? a location might allow you to home in on a specific IC, or at least area of the mainboard.

One of my Mega's had a super fragile PCB, so don't assume that something you've already looked at has not since failed if you've been handling the board a lot.

Obviously the other folks are providing some great technical help, so I'm sure it will live again :)