Date   : Thu, 12 Apr 1984 12:00:19-PST (Thu)
From   : hplabs!hao!seismo!ut-sally!mordor!arp@Ucb-Vax.ARPA
Subject: Xerox 820 ADD. INFO.

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Before anyone starts cutting traces on their boards it should be noted
that the 820 used a TTL monitor ie. seperate HS,VS,and VIDEO. If you do
a bit of reading you will find that TTL monitors have no horizontal 
oscillator and require a 25-30 microsecond HS pulse to sync them while
COMPOSITE monitors, having their own oscillator, require only a 5-8 micro-
second HS pulse to trigger the oscillator. If you use a circuit to combine
the sync and video signals to drive a composite monitor you should indeed
use the shorter HS signal. This does not require any trace cuts since there
are unused pins on J7 and an unused IC position on the board next to U34.
I used that IC position for my combiner circuit and brought out the composite
signal to an unused pin on J7. This gives both TTL AND COMPOSITE video output
so you can use either type of monitor. I prefer the TTL type since it has a
greater bandwith thereby giving a sharper display. The circuit I used appeared
in the Oct. 6, 1982 Electronics Magazine page 113.



                                               Al Pierce L-276
                                               P.O. Box 808
                                               Livermore, Ca 94550