djo7613@blake.acs.washington.edu (Dick O'Connor) (01/03/90)
What happens when you plug a VGA monitor (ZCM-1492) and an EGA monitor (ZCM-1380) into a Z449 video card at the same time? Can they co-exist? Can they take turns being the main display, depending on whether EGA or VGA signals are sent? No, I won't Try This At Home until I know what I'm doing. But I sure am curious... "Moby" Dick O'Connor ** DISCLAIMER: It would Washington Department of Fisheries ** surprise me if the Olympia, Washington 98504 ** rest of the Department Internet Mail: djo7613@blake.u.washington.edu ** agreed with any of this!
fayette@halibut.nosc.mil (Daniel F. Fayette) (01/03/90)
In article <5200@blake.acs.washington.edu> djo7613@blake.acs.washington.edu (Dick O'Connor) writes: > >What happens when you plug a VGA monitor (ZCM-1492) and an EGA monitor >(ZCM-1380) into a Z449 video card at the same time? Can they co-exist? >Can they take turns being the main display, depending on whether EGA or >VGA signals are sent? > My Z449 guide strongly warns _against_ doing this. It suggests that damage will be done to the Z449
joe@junkyard.UUCP (Joseph Sarkes) (01/04/90)
In article <5200@blake.acs.washington.edu>, djo7613@blake.acs.washington.edu (Dick O'Connor) writes: > > What happens when you plug a VGA monitor (ZCM-1492) and an EGA monitor > (ZCM-1380) into a Z449 video card at the same time? Can they co-exist? > Can they take turns being the main display, depending on whether EGA or > VGA signals are sent? Normally the sync and video signals would be terminated with 75 ohms at the monitor. If you parallel two monitors, the signal levels would drop by a factor of 2 to each monitor, since the card would have a source impedance of 75 ohms also. Since the ega monitor uses ttl level signals, this would lead you to believe that it might work. However the sync signals are probably common between the two plugs. Also, the 449 card looks at the type of monitor that is plugged into the ega port during boot (as I recall) and saves this info for when you latter change video modes. The switch also is read for what type of monitor is connected and what mode you want the card to default to. With two monitors connected, the auto sensing would be thrown off, and who knows what you'd end up with. Another problem is that the scan speed of a vga monitor is fixed for the horizontal rate (also as I recall) and varies with an enhanced monitor or the latter multisync monitors. This is another source of incompatibilities. Another thing is that the mode for the monitor to automatically select itself to (vertical rate) is varied by the sync signal pulse polarity, and also possibly by frequency. Is there anything common between a vga type monitor and ega type monitor in this? Probably, if the card thought that it was connected to a vga, a multisync type monitor could be paralleled with it and have a possibility of working. The best way to do this would be to have multiple monitors of the same type, and use buffer amplifiers on the signals so that the card sees a 75 ohm load, and each monitor is driven by a 75 ohm source on each line. (sync, and each video signal) This would give the proper signal levels, and prevent echos in the connecting lines. The buffers would be different for each type of monitor. Good luck. If anyone knows of a product already out there to do it already, speak up. I seem to have a DIM memory of something like that having been advertised at one time. Joseph Sarkes (junkyard!joe)