8 May
2001
8 May
'01
9:44 a.m.
I have never had an particular interest in EGA or CGA monitors, but (1) every
IBM CGA (which is the name of the adapter card) has a composite output, and
every CGA monitor I've seen has been a severely lobotomized (as Tony Duell
described) TV set. Back in the '70's, I modified a small portable TV set to
make a small monitor I could package together with a physically small
microcomputer system in a rack-width desktop box about 6" tall. That was the
only time I personally did that sort of thing, but everyone I knew to be
involved in home computing at the time was able to offer plenty of useful advice
on how best to effect this mod, suggesting that it was widely practiced.
I have attached many dozens of "EGA" monitors to VGA adapters, and have yet to
need to modify the monitor in any way not associated with its sweep rates. This
suggests to me that they happily accept the 1V or so analog signal amplitude
that the VGA adapter requires. Of course, I normally used the commercially
available hardware adapters, consisting of two connectors and cabling ranging in
legth between 7' and 1.25", depending on whether it was a cable or just an
adapter. I'd suggest that if you have something that works differently and it
seems that your schematic does, you may have to proceed differently than I did,
but the fact that such a large percentage (100%) of the EGA monitors I've dealt
with had no difficulty with the VGA video signals, suggests you'll have no
trouble finding EGA monitors that deal with VGA output just fine. I did find
some that required a capacitor be changed in order to make the sweep circuit
operate at the appropriate rate. At least one case, it required, in fact (this
was a NEC Multisync-1) that the vertical hold signal be tweaked (from the knob
in the top of the cabinet) when switching from text mode to 1024x768 graphics
(Windows) mode.
The fact that NEC provided an adapter to go between its monitor and the typical
VGA card suggests that this is not an unusual application for their products.
Dick
----- Original Message -----
From: "Jeff Hellige" <jhellige(a)earthlink.net>
To: <classiccmp(a)classiccmp.org>
Sent: Monday, May 07, 2001 3:49 PM
Subject: Re: Converting TTL monitor to Analog
The video input
stage of the EGA monitor I've just turned up schematics
for has the following stages.
Incoming video signals (6 wires, MSB and LSB for each colour) are
terminated by resistor networks and fed to the inputs of a 74LS244
(enables always asserted). The outputs of that TTL buffer go to a 28L42
PROM, programmed to produce the correct colour drives for each of the 64
input 'colours' in EGA mode and the 16 input 'colours' in CGA mode. 6
outputs of that PROM (2 per colour, R, G, B) are buffered by 74LS05 O/C
inverters, then go to a transitor network that produces the CRT drive signal.
To me, the inputs to that circuit are most certainly TTL level _digital_
signals.
EGA, like CGA, was a digital standard, which is why many of
the early multisync-style monitors have both analog and digital
inputs. VGA was the first of the IBM PC standards to use an analog
video signal. Unfortunately for Amiga owners, VGA syncs at twice the
NTSC rate.
Jeff
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