Quest for NEC CM-2791

[Endymion]

Well those of you who have been keeping up with this will remember the NEC CM-2791 from a couple of weeks back. This monitor has 4 BNCs, R, G, B, and Composite sync, which I can always do if I can't find the pinout for the DB9--but lack of extra tubing and the general pita of four connectors and I would really rather go with the DB9 if I can. Trouble is, I can find absolutely zero specs for this anywhere on the web, including at NEC.com.

The closest that I can come to a pinout for this specific monitor I found here via google with the search parameters "nec db9 pinout." It mentions a seemingly generic DB9 pinout for some kind of VGA, and mentions that NEC Multisync monitors used it. I am really wondering about this bit (as it relates to the CM-2791) because the BNC does not have H+V sync, just composite. The seller also mentioned he was unable to use this monitor with his computers. The "Note" is interesting (does anybody know anything about the "IBM PGC?") and gives me some hope but I'm a little apprehensive. What are the chances I could blow something if I just wired it up and prayed for the best? RGB return? What is that, ground? :unsure:

As for the monitor itself, it is frickin' great! Even the composite is very nice and sharp--there is no S-video but so far I prefer this to my Sony PVM2530. (It has a small topcurl which, while not severe, is really annoying to me.) The screen is the same size (27") and the screen itself is in generally better condition. I am honestly thinking about contacting the seller to buy two more, the only thing stopping me is that I'm uncertain where I would keep them for a while.

[Endymion]

Well once again, gamesx is the best reference, teach me for not looking there sooner. (Twice--right grey?) At least I was right about the ground thing.

Unless anybody has any objections or caveats I guess I'll take my monitor's health in my  hands and try wiring that up soon.

[Endymion]

I did a little more digging on this and came across this info over on our friendly monitorworld.com. It looks like the 9-pin VGA is identical to an older standard, a 9-pin "PGA," in every respect except for its use of composite sync (pin 4) instead of separate (4 & 5).

I went ahead and wired an Xbox SCART to this, skipping the #5 pin, and grounded to the housing of the DB9. What I got was a generally unusable picture (wavy/slanted image even when not rolling), removing the ground from the housing turned this into a rolling unusable picture.

Not to be set back, I tried tying the ground to the sync pin and had the same results. Noticing that this setup seems to call for ground to all three colours as well as a separate ground (pins 6 though 9) I tried soldering a small bit of metal across those four pins, and it isn't any happier with that.

So naturally you would figure that I would move straight to BNC. Of course I would, only now, with only four pins instead of five (RGB+Sync+Ground) or eight (RGB+Sync+Ground for all four of those) I have no idea where to tie the ground, and without it, it looks like I have the same unstable picture. Surely it has to be grounded, and I would really rather use the DB9 to have a simple, single, easy plug. The SCART just didn't have a separate ground pin for them all. Can anybody toss me a clue on this one?

[NFG]

All the pins that are labelled Ground should be grounded.  IF there are four grounds, ground them all.  Typically these are only for shielding (to prevent RF interference) and for convenience when wiring up a connector and connecting one is the same as connecting them all, but sometimes - as with the Japanese RGB connector and an XRGB - not wiring up all the grounds causes trouble.

You cannot have any electric circuit without ground.  If you use a multimeter you'll probably find the outside of the DB9 socket IS grounded and connected to the monitor chassis internally.  It sounds to me like your problem may not be a simple ground issue.  Have you tried a console with a known composite sync output?  The Xbox outputs, IIRC, composite video not sync.

[Endymion]

D'oh! Thanks, I completely forgot about this bit! The Sony PVM can take the sync out of a video signal so it was always very easy to make/mod cables for. I have a bunch of LM1881s (free samples from National Semiconductor ) that I have yet to do anything with, I'll wire one into a SCART switchbox and see what happens.

[Aidan]

The RGB lines usually have seperate grounds so you don't get cross channel interference. That way cable inductance and cross talk are minimised.

Generally, if you get a rolling picture, the display can't lock onto the VSync signal. If you get a wavy signal, the display can't lock onto the HSync signal. If you get both, then it's not syncing at all!

If you fail to ground anything, you'll often get a picture, but display will end up using the RGB lines as a return. The result is that dark pictures may result in a sync, and bright pictures will throw the sync out, due to the voltage difference between the lines.

Identifying the colour pins is usually fairly easy, as you only need to connect one at a time. Running seperate V and H sync can help you find the sync signals too. A multimeter (Unlike my dead one ) should help locate the grounds, as often they're connected together. That's a good hint as to which pins should be grounded.