🚀 go-pugleaf

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

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)