I got no clue but do like your monitor !! reminds me of the one I got with the 5160/3270 many decades ago, but it was CGA (looked the same iirc)

You can use a XOR circuit seen in VGA to SCART things to convert polarity to whatever is needed automatically. The general idea is that you feed direct sync to one input and RC delayed one to other. The capacitor on the delay will charge up to max or discharges to minimum depening on sync polarity, and it causes the XOR output to stay in a specific configuration once the capacitor reaches a steady state. You can use remaining XOR gates to flip polarity to what you need and also generate CSYNC out of H and V sync.

Horun wrote on 2023-03-05, 02:12:

I got no clue but do like your monitor !! reminds me of the one I got with the 5160/3270 many decades ago, but it was CGA (looked the same iirc)

+1

What we're looking at is a proto-VGA monitor, essentially.

The IBM PGA (or PGC) graphics hardware used full VGA resolution (640x480 pels, 31.5KHz 60Hz), but with 256 colours on screen .

In one way or another, this one is the very original VGA monitor. Very interesting to see it in action! 💙

JonF wrote on 2023-03-05, 00:14:

Hi all, I have an IBM 5175 PGA display. It is a neat little CRT. I've read it was a common mod in the past to convert these to […]
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Hi all, I have an IBM 5175 PGA display. It is a neat little CRT. I've read it was a common mod in the past to convert these to have a standard 15 pin VGA cable. I am having trouble figuring out how this mod would deal with different sync polarities at resolutions other than 640x480.

So, I think the 5175 internal sync processing is setup to take combined H&V sync. It then splits the H & V, and runs it through a CD4093B IC before sending to the analog board. I have only been able to get the monitor to sync to negative sync polarities. Either with composite or using clip leads to inject H and V separately on the video board.

Video cards are setup to run different polarities for sync at different resolutions, presumably to help multisync monitors ID the signal.

Horizonal Dots 640 640 640
Vertical Scan Lines 350 400 480
Horiz. Sync Polarity POS NEG NEG
Vert. Sync Polarity NEG POS NEG

So for 640x480 everything is great. Monitor will sync OK without an Extron box in the mix. However for other resolutions ie during DOS boot, command line, etc, one of the sync lines is positive and the monitor won't sync. Unless I use an Extron box to force composite sync(which is negative) or negative HV sync.

So, any ideas on how to make this monitor work without lugging around an Extron box or trying to wire one up inside the monitor case? Any one out there have an already modded 5175 they can share pictures of the insides?

Thanks!

My honest advice is to just keep using the extron. Sync combination is non-trivial to implement correctly and there is no good reason to reinvent the wheel here.

Tiido wrote on 2023-03-05, 02:36:

You can use a XOR circuit seen in VGA to SCART things to convert polarity to whatever is needed automatically. The general idea is that you feed direct sync to one input and RC delayed one to other. The capacitor on the delay will charge up to max or discharges to minimum depening on sync polarity, and it causes the XOR output to stay in a specific configuration once the capacitor reaches a steady state.

That's the standard way to approach the problem, which is built into monitors, too. I wouldn't call it "RC delayed", though, but "low-pass filtered". Combining a low-pass filtered sync signal with the unfiltered sync signal compares the "standard" level to the "current" level. This causes HIGH output while the sync pulse is active, and LOW output when there is no sync pulse, i.e. converts positive or negative sync into positive sync. This is the opposite of the requirement of the OP, but as you say, you can use two of the XOR gates as "sync detector" and the other two XOR gates as inverters to generate "always negative" sync.