DRAM 128K system has stopped working

[CheshireNoir]

Right. I was looking at IC11 on the Core board not the Main board. My mistake.

I'll go have another look tonight. That'll teach me to rush things :-)

Time to break out my trusty risers.

Chesh

[someone]

Not necessarily.
Burn your own test ROM and see if IC11 is actually working or not.
It's a simple task (made even easier if you had a ROM emulator or battery backed RAM)

Get the video working first & work on the POJ circuit later.

[CheshireNoir]

Hello someone,
The lead time on me successfully developing my own ROM is going to be a long one. (Most likely months or even years).While I consider it a worthwhile endeavour, I don't want to be without my shiny new 128K 'bee in the meantime :-)

Chesh

[CheshireNoir]

Hey everyone (and someone). I made some diagnosis progress today.
OK, so at an off list suggestion, I tried reverting my main board back to SRAM and dropping back on my PC85b core board on and all the glitches and video issues immediately disappear. I was happily doing BASIC apps and playing games without a hitch.
I also tried the RAM test rom and guess what? It flickers anyway! (I am also now confident about the RAM on my SRAM board. :-D )
So that gives us another point of information. It would suggest that it's either something confined to the 128k coreboard or it's something on the main board that's only used by the DRAM configuration.

I'm going to go and revert this main board to a DRAM configuration, as I figure at this point there won't be anything I can test without it being in DRAM setup.

Cheers!

Chesh

Ps. this post would have been sooner but I got distracted AGAIN by Kilopede.

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Wouldn't you know it? I reverted it to being a DRAM main board and now the symptoms have changed. Still can't boot from disk, but now I can run the RAM test, which is passing happily for all 4 banks.
The RAM test comes right up and a quick reset later and it's scanning everything.
The "proper"† BN55 still boots to no sync. I can get into monitor but can't type anything, by pressing RESET while holding down the M key. Just pressing the RESET key a whole bunch of times gets me to what looks like a cursor but it won't read any tracks. (Oddly every now and again, it'll read a single track when I do this but only the one)

Cheers,

Chesh

†If anything about this monstrosity can be said to be "proper".

[Ernest]

Hi Chesh,
Good progress, looks like the RESET is now working fine and you have now proven the RAM is all OK.

A) I have never used the BN55 myself and have no idea if that maybe a contributing factor.
Personally I would stick with either BN54 or BN56 for this system.
Is your BN55 boot ROM a CMOS variety? I have had some issues with the CMOS ones on this Coreboard in the past some makes are more of a problem than others. If it is a CMOS one, try burning the into a none CMOS EPROM and see if that works? I have a spare BN56 if you want to give that a go.

If you can’t jump into the Monitor with the M key held down during boot-up then it seams like the initialisation routines for the 6545 is not happening so there is no keyboard scanning.
Try getting into the monitor you could try removing your FDC 2793 chip and the system should then boot straight into the Monitor. But that may not work either if the 6545 is not getting initialised at all to see any screen output.

At this point I would be putting my money on you having a CMOS EPROM problem.
Does the Coreboard have a set of resistors in series with the Data lines coming in on Pins 18 - 25 of X1?
I have done this MOD on all my MB8342 Coreboards with SMD resistors.

Ernest

[CheshireNoir]

Thanks Ernest.
I don't think BN56 works with non Premium systems? I can try it. There's quite a few versions of the BN54 ROM in the repository and I'm not sure what the difference is between them. Do I want the bn5443 ROM or a different ROM?
The EPROMs I'm using are TMS 2764JL-30 ROMs salvaged from upgrades to the PC85 coreboard. I think they're TTL not CMOS? (Hard to find a data sheet)
While there's a spot for a resistor network (RN1) over near X1, there's only a single resistor on it. There are no other resistors in the area. Is there documentation on the resistor mod? ECN 2-090487 would seem the appropriate mod?

Cheers!

Chesh

[Ernest]

Ok, that theory has been dispelled.
The ROM you are using is definitely not a CMOS one.
The BN 56 will work. But I’m now a little puzzled.
The Test ROM passed the Memory test Ok.
But it looks like the normal boot ROM is unable to even initialise the 6545 OK.
I guess it is possible that the salvaged boot ROM may have failed.
Can you run a validation check of the EPOMs contents on your burner?

Ernest

[CheshireNoir]

Well, now my EPROM programmer has decided to stop playing. (I was trying to burn a BN56 image)

Grr argh. I know someone with a better programmer than mine so I may need to ask them a favour...

Chesh

[someone]

Someone's had troubles posting on the site.

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Someone forgot you've been modding away & something went awry along the way.

[CheshireNoir]

Well, I haven't actually modded it since I got it working as a 128k DRAM board, someone.
I was working fine for a few days and it's stopped since. Everything since then has been me trying to get it back on its feet. I'll try the BN54 ROM as soon as I can get a reliable ROM burner. (I think I will have to bite the bullet and see if I can find something a bit more old fashioned that supports the older ROMs I use.)

BTW I checked and I did try programming this with the BN56 EPROM and it didn't work. This isn't a premium baseboard. Anyone know which file in the Repository is the "proper" BN54 ROM? There's 3 different candidates in there :-)

Cheers!

Chesh
Ps. Tests and fixes will be slow this week. I'm work from home which means my entire workbench is set up with my WFH setup instead of all my test gear. OTOH, work from home is so nice!