HP 9845 C with 280 option startup failure

[Tony Duell]

> 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.

> signals are identified, but there are some minor differences between the 
> B and C models, which have to be solved. You should always know what you 
> do before getting serious...

'My' schemaitcs do show pinouts of all the connectors. OK, my names, not 
HP, but at least it's a start.

-tony