HP9815 (was Re : prn plotters)

I've been looking at some of the manuals... You're right. The plotter interface's ROM contains commands to drive the plotter. The HPIB interface ROM contains the commands to send arbitrary bytes to an HPIB device -- to drive the plotter with the latter would be a lot of owrk. On the other hand the HPIB interface looks quite interesting (it's one I will look out for :-)). The are commands to send/receive the user program ('in internal code' -- presumably the key codes, etc) to an HPIB device. COuld be interesting. Actually, I think there's a similar command in the GPIO (8 bit TTL level parallel interface) ROM, and that one I do have. It appears there was a standard PCB to plug into the calculator I/O slot. It contained the ROM, address decoder to select said ROM, and logic to provide 8 bit input and output ports iwth control lines. This board was used on its own in the GPIO interface (and plotter interface?), and was linked to an extenal box containing 1 or 2 PCBs for the BCD, HPIB and serial itnerfaces. Obviously the ROM contents varied between the interfaces, and in some cases the input latch chip on the board that plugged into the calculator was not fitted, it was bypassed with links. I also looked at the 9872 service manual (www.hpmuseum.net has some interesting stuff). The electronics looked very familiar. The motor driver circuitry, processor circuit, self-test controls, etc are all much the same (but with a different layout, of course) to those in the 7245 that I've worked on. That motor control circuit is ingenious, I spent a long time looking at logic analyser traces, etc, before I figured out what was going on. -tony