ZENITH Z-19CN VIDEO TERMINAL
3 MHz Modification
(9/25/82)
The Z-19 terminal is capable of running at baud rates to
19.2K standard (38.4K with Super-19 rom) but the standard clock
frequency of 2MHz may be too slow to operate satisfactorily at
these transmission rates. In particular, I experienced occasional
buffer overrun at 19.2K baud when the cursor was near the bottom
of the CRT and the computer sent a screen-full of characters.
Inspection of the Z-19CN scematic reveals that the ACE runs
at 3.072 MHz, while the CPU, CRT controller, etc. run at 2.048
MHz. It is a simple matter to change the clocks to 3.072 MHz for
all of these chips; it is my understanding that as a rule, no
chip replacement is necessary (this was the case for my
terminal.)
To make this modification, remove the Z19 PC board and cut
the trace between U429 pins 8 and 14 and U426 pins 1 and 3. Now
run a jumper from U427 pin 13 to U426 pins 1 and 3. This is all
that is required to convert to 3 MHz operation.
I have run my terminal for about a week under this configu-
ration and have experienced no ill effects. The buffer overrun
problems have vanished.
