I looked up my VGA documentation and I find that IBM did not use RS-170
signal levels for VGA video. The VGA spec is 0 volts for black and 0.7
volts for Color maximum.
So for Sync on Green, the Black level needs to be shifted up to 0.3
Volts. The Philips DAC does this since it is designed for Sync on
Green, but the converse is true, when not using Sync on Green, the Green
level needs to be lowered from 0.3 Volts to 0 Volts. This is
accomplished by having the sync input always active (Low).
Note on TV that Sync is a positive pulse, but on TV the video signal is
inverted so with the raw video that Sync is a negative going pulse.
VGA sync is TTL level Negative pulse. The spec says that the sync
drivers must be able to sink 20 mA, if that matters.
An interesting historical note. The original VGA board only had 18 bit
wide palate RAM.
--
JRT
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