7 Nov
2005
7 Nov
'05
10:30 p.m.
Tony Duell wrote:
Yes, and they implemented the whole control logic in TTL (ROM based
state machine). The state machine is one of the more complex works,
since it uses both ROM tables and 16-bit multiplexer and a couple of
gates/counters/latches. Maybe the multiplexer is used primarily for the
access control of the PPU bus. I'm currently looking for the signal
lines between the board and the CRT. Besides the 15 character
code/character attribute lines and the power lines there are up to 7
lines which are dedicated for status/control information. You say there
are 3 lines which are not connected to the 98780 monitor backplane, can
you tell me which they are or anything about the estimated purpose of
the lines you found?
--Ansgar
Slowly but
steadily. I'm currently figuring out some bus & connector
pinouts, especially around the A34 alpha control logic assembly. Most
I am not sure what that board is (I have my own names for all the PCBs,
since I worked it out long before I saw the boardswapper guide).
What I call the 'video interface PCB' -- the board in the left hand
mainframe cardcage that connects to the monitor is supposed to be the
same for both the 98770 colour monitor and 98780 enhanced mono monitor.
Certainly the one in my machine sends 15 bits per character up to the
monitor (7 ASCII code bits, 1 chracter set select bit, 4 attibute bits, 3
colour bits, the last 3 are not connected on the monitor backplane PCB in
my machine).
I asusme you have some idea what that board contains. There are 2
80-character (15 bits per char) buffer memories. One is read out to the
text display board in the monitor, while the other is loaded from the
mainframe's memory. There are 2 address counters on the PCB, one for
rerading, the other for writing (it is not one per memory). And of course
cotnrol logic. The board connects to an internal bus on the PPU card --
basically the address/data bus of the PPU hybrid -- and therefore talks
to memeory via the buffers on the PPU card.