HP 9845 C with 280 option startup failure

I don't know all the option codes for the 9845. I assume at least this has a 98770 colour monitor nad the high-speed (bit-slice) language processor option. Firstly, try contacting the HPCC secretary, Dave Colver (I think the details are on the HPCC web site, http://www.hpcc.org/ , if not, ask me). If you ask him nicely, he might send you a reverse-engineered schematic for the 9845. This is for a 9845B with the high-speed lagnuage processor and the 98780 enhanced mono monitor, so it's not directly applicable to your machine, but it might be a start. Be warned that it's over 200 pages long (that is _just_ a schematic). The HP service manual is on http://www.hpmuseum.net, along with a manual for the colour monitor. These do not seem to cover the high-speed language processor at all, and they are boardswapper guides with no schematics or even pinouts. But you probably should read those too. I would love to see inside the colour monitor (that is, I'd like to pull one apart and really examine the boards. I am told there are some 2901's in there for the video processor. The 98780 mono monitor has an HP 64 pin ASIC for this. Hmmm... Probably toally irrelevant, but there's a signal from the monitor to the I/O processor -- basically a frame-rate interrupt, that IIRC has to be present for the bus arbitration to start up properly. If you run the machine without a monitor there's a turn-on fixture that you have to plug into the LH monitor connector (on the video interface PCB) to provide this. Maybe... Maybe not... How much do you know about the buses of this machine? It's not at all simple. There are 2 processors -- the PPU (Peripheral Processor Unit) on the left side and the LPU (Language Processor Unit) on the right side. Each processor has buffers/latches on the board that connect it to one of 2 buses (I think HP called them X and Y, I call them L and P for obvious reasons). The L bus carries all the memory on the LH boards, and the ROMs in the RH drawer. The P bus carries all the memroy on the RH board and the ROMs in the LH drawer (I think I've remembered all that correctly). Also, the test side of the video display does DMA from main memory via the buffers on the PPU board, it transfers a line of text at a time to buffer storage on the interface PCB and thnece to the text board in the monitor. What I am wondering is that if there's a bus problem, or the arbitration logic (which is spread between the 2 processor boards) is playing up, you could get some really odd failures. Alas not, and I don't think there's anything in the service manual. The schematic would at least relate addresses and bits to actual chips. I haven't done yet. What I can tell you is that these are not totally standard ROMs, they have internal address latchs (the buses on the 9845, like that of the 9825, being multipexed address and data). You ahve to deassert and reassert the chip select line after changing the address to latch the new address in. I hope the above is a start. I'll carry on thinking about it. -tony