HP9815 (was Re : prn plotters)

Tony Duell ard at p850ug1.demon.co.uk
Sat Feb 17 17:21:06 CST 2007

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. 


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