7 May
2001
7 May
'01
9:49 a.m.
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
--
Collector of Classic Microcomputers and Video Game Systems:
Home of the TRS-80 Model 2000 FAQ File
http://www.geocities.com/siliconvalley/lakes/6757