gjh@otter.hpl.hp.com (Graham Higgins) (03/14/89)
Apologies for this not being a systems topic but it's doing my crunch! I have just taken delivery of a brand-new 1040STFM -- the mob for which I work (Hewlett-Packard) have, stored in the equipment attic, some "old" (~1985) colour monitors (CGAs). I have managed to get one on semi-permanent loan, which makes the ST purchase quite a bargain. However ... (isn't there *always* a "however"?) ... the monitor inputs are RGB BNC connectors and it uses a composite signal on green for the H & V syncs. Here's the spec ... Type: Analog RGB composite video, sync on green Polarity: Negative sync Level: 0.71 Vp-p for red and blue 1.0 Vp-p composite signal for green (0.71 Vp-p for video and 0.29 Vp-p for sync) Input Impedance: 75 ohms Video Bandwidth: 35 MHz. Scan Frequency: Horizontal 24.90-25.8 KHz Frame Rate: Vertical 60 Hz (16.67 ms) When connecting the ST's RGB outputs alone to the RGB inputs , I do get the merest hint of a picture, the colouration seems accurate, but obviously, it's not synced properly, the picture is ripped horizontally (I can make out the desktop icons and mouse cursor, though). If I add the H & V syncs, I lose the green and it still doesn't sync. O.K. the ST outgoing H & V syncs should be inverted -- (I think, or rather that's what I have been told), so I tried that, using a 7405. Still no joy. I have checked the signal levels on the H & V sync lines from the ST, using a scope and it seems that the signals are somewhere in the region of 5 to 7 Vp-p. Now I haven't got a very good model of video signal technology, but it occurs to me that if there's supposed to be an analog signal on the green line at about 0.71 Vp-p, then shoving a 5V TTL output on that will swamp the video signal completely --- at least that's my current theory to explain why it doesn't work. I have some information on the ST outputs, i.e. that the H & V syncs are "active low" but this info comes from the "rbgtocom" instructions for early STs on how to generate a composite video o/p from the RGB, the factual accuracy of that info may now be incorrect (I have the STFM option). Anyway how does "active low" map to "Polarity: Negative" -- is it that the line is normally high and is brought low for a signal --- that seems to map with "active low", but what is "Polarity: negative"?, normally low, going high for a signal? Whichever way I try it, I can't seem to get the d**n thing to work. Do the H & V sync signals need attenuating and if so, what's the best way to do it? I would be grateful for any suggestions. Yours in hope Cheers, Graham ====== ------------------------------------------------------------------ Graham Higgins @ HP Labs | Phone: (0272) 799910 x 24060 Information Systems Centre | gray@hpl.hp.co.uk Bristol | gray%hplb.uucp@ukc.ac.uk U.K. | gray@hplb.hpl.hp.com