stuart@tellab1.tellabs.com (Rick Stuart) (02/26/91)
VGA VGA VGA VGA VGA VGA VGA VGA VGA VGA VGA VGA VGA VGA Gee... It's been a while since I asked about my VGA problem, and I did say I'd post a follow up, So.... ====================================================================== I said... >>I have a (I was told this my the manufacturer) CGA, EGA and PGA >>multi sync monitor. Remember PGA (I don't), IBM's first try at >>analog high-res monitors? Just so happens that VGA freq's (from >>what I can tell) closely match that of PGA. Also, this monitor >>appears to accept separate OR combined sync's (so I don't need that >>nifty sync combiner in the last issue of Circuit Cellar mag! Bob Duckworth said... >I haven't seen that article. I designed a circuit to do that and >sold a bunch of them with PGA monitors. My circuit was a single >4077 some resistor packs some caps and two transistors. This was >the 5th of a number of schemes to make these things work. Having >converted 800 PGA monitors, I can speak with authority. The IBM PGA >will not accept separate syncs. I said... >>My problem is impedance mismatch (I think). There's a lot of >>smearing going on! Maybe it's the cable, I'll try that tonight. >>Or, maybe it's the wrong impedance at the monitor... Bob Duckworth said... >Try terminating your lines. 75 ohm should work just fine. I got >away with 10K on my adapter board. There was a little ringing but >not enough to notice. The reason for the high impedance was... >a) it worked >b) a 'match' would have required more components or larger caps and > caps cost more the larger they are. I asked... >>So, once and for all, what is the expected impedance at the monitor >>for VGA (does any one know?) -and- does VGA use "balanced twisted >>pair" (for each of the three colors (red, green, blue)) or >>individual shielded wires which reference off a "common" ground? Bob Duckworth said... >The PGA used twisted pair, The VGA uses coaxial wires. The IBM 8513 >I have on the bench has 120ohm terminators. I don't have the bridge >out but I suspect it would look like 75 ohm or so in the circuit. Steve Gaarder said... >The vga video outputs are single-ended, and should be terminated in >75 ohms. Even though they are single-ended, there is a ground wire >for each color, and the monitor ground should be connected to at >least one of these. I hooked up an analog monitor to an ati vga >wonder by cutting one end off of a 15-pin extension cable. For some >bizarre reason, it has only 2 of the three video ground wires, >plus, of course the "digital" (sync) ground. I connected all of >these to ground at my monitor and everything seems fine. The boys/gals at Western Digital/Paradise said... "The VGA paradise card expects to see 75 ohms from the monitor." (They can be reached at 1-800-832-4778.) I said... >>I'd be interested in anyone's comments and/or problems who have >>hooked up a PGA monitor to a VGA card. Bob Duckworth said... >The biggest problem with the PGA is component value changes in the >E-W circuits cause pin cushion. It can be fixed. It's just a pain >and you reach the point at which a new VGA at $375 is a better deal. >BTW I am using Professional Graphics display right now. (What are E-W circuits?) Bob Duckworth also adds... >Oh! If you are going to post my comments to the net, I have some >nifty 16" 64KHz monitors for hacking. They are new old stock built >in 1987. Analog RGB. 3 BNC They are in modern cabinets and look >kind of like Hitachi or Mitsubishi. I have about 100 of these at >$500/each if anyone wants one. (Keep in mind I received bob's mail December 4, 1990) Evidently Bob can be reached at: 404-874-5051 ====================================================================== In the end, I used a shorter cable between my monitor and video card. This eliminated just about all of the smearing. My problem now is a bad Xtal on my video card. After about ten minutes, the frequency of the Xtal just about doubles! So... Wanted: One KYOCERA 25.175 MHz four pin Osc/Xtal. (From a broken VGA card maybe?!) Anyone have an extra? ...Rick Stuart ...uunet!tellab5!stuart Tellabs Lisle, IL