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IBM Professional Graphics Controller not working

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dkarguth

Experienced Member
Joined
Jan 26, 2019
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Location
Texas, USA
I recently acquired an IBM Professional Graphics Controller that appears to be brand new in the box. However, when I plugged it into my 5175 monitor, all it seems to display is these little vertical lines.
CQx77rPh.jpg

I then ran the IBM diagnostics disk that came with the PGC, and it gave me the error code 3980, which is listed as a "Graphics controller RAM timing error" on this website: http://www.bioscentral.com/misc/ibmdiag.htm
The CGA emulation works just fine, but CGA emulation mode is completely functionally separate from native PGC mode.

I have located the Technical Reference, which is hosted on minuszerodegrees. I'll link it here: http://www.minuszerodegrees.net/oa/OA - IBM Professional Graphics Controller.pdf

From what I can tell, there are three types of RAM on the PGC. The first is the framebuffer, which is made up of a whole bunch of DRAM on the first layer. I don't believe that this is the problem, as there are specific error codes listed for each DRAM chip. Also, the DRAM is referred to as "Adapter RAM" in the list of error codes, and the error I get says "controller RAM".
The second type of ram consists of 3 AM9122-25 SRAM chips. I am not entirely sure what it is used for, but I'm sure it is explained further in the technical reference.
The third type of RAM is what I think is the system RAM for the card. For those of you that don't know, the PGC is basically a stripped-down PC on a card. It has its own 8088, RAM, and onboard system bus, and can actually execute full 8088 instructions. Due to the wording of the error, stating "controller RAM", I initially suspected this RAM was the issue. It is a 6116 SRAM, (U42 in the schematics of the Technical Reference) and is also on the processor card. I went ahead and replaced it, and there is no change in the operation of the PGC. It still gives the same 3980 error.

In the image below, you can see all three types of RAM (but not clearly)
The framebuffer is on the middle card, it consists of a bunch of socketed 16-pin DIP DRAM chips.
The three mystery RAM chips are on the processor card (the top one) and are the 3 22-pin DIPs that are located between the right side of the pin headers.
The system memory is the 24 pin DIP that is to the left of the two socketed ROM chips on the top card.
pgcbits.jpg

I would appreciate any help that I can get in the matter, as it would be extremely interesting to get this card working.
 
The 6116 is the "communications" ram. It was faulty on mine, but my symptoms were different than yours.
 
AM9122 is a 256x4 1k SRAM. Since there are three of them (12 bits) I guess it's something to do with the CLUT.
 
Ah, that makes sense. I had suspected it was a color RAM, but I wasn't sure. The main thing I'm perplexed about is the "timing error". What exactly does that mean? If a RAM were bad, I would expect that it would say "memory error" rather than "timing error". I wish their error codes were a little more specific. At the very least, I wish I could see some more detailed results from the diagnostics, like what exactly prompted the error.
 
In that case I'd be looking closely at the part of the board represented in the Tech Ref by the state counters/decoders/muxers referenced on page 8, or maaaaybe "Timing and Control".
 
today I received my single-chip DRAM tester in the mail, and tested all the dram on the PGC. Out of 40 DRAM chips, there were 8 faulty chips. The diagnostics probably didn't know how to deal with that many errors, and gave me the timing error. I'll be sourcing some replacement DRAM chips and reporting back my progress.
 
This does not relate specifically to your cards, but, if a RAM IC fails in the video ram storage area (or fails to produce an output), it will produce a geometric array where the pixels can be turned on (or possibly stuck off). The pattern I think would depend on the way the hardware storage was arranged.

I have attached a photo of an example where just one video RAM IC was removed from a bank of 16 IC's in a video card with a 256 x 256 pixel array (this is Matrox's first S-100 graphics card) , so its outputs are interpreted as logic high. As you can see it produced 16 lit lines as one might expect. The boxes or checkerboard are plotted as 20 pixels x 20 lines as a reference from a simple program I wrote to test the card. And it is somewhat reminiscent of your screen image. So clearly you have some faulty RAM and it looks like you have found them already.
 

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Good work, congratulations! :)

My PGC has only one memory chip faulted and after replace is working fine. I dont have IBM 5175 monitor, so I am using VGA monitor or TVM color monitor, which have digital/analog switch and support for PGC mode.

By the way, from where you have that nice color palette? I would like to try it on my PGC.
 
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