22 Feb
2007
22 Feb
'07
1:11 p.m.
Inspired by all that talk of plotters last week, I lugged the box containing
the HP9872A upstairs and opened it for the first time in 6 years. A few
repairs later, an evening in a comfy chair with the manuals (it's nice to
Not the Comfy Chair!
have manuals for a change), and some doodling on the
9815 calculator which
drives it, and the plotter is zip'in and zoom'in about. Yup, it's pretty
neat,
esp. with the built-in character generation with scaling.
The weak point in this scenario however, is the torture of programming the
9815 and problems with the tape cartridges and drive of the 9815. Those
What's wrong with programming the 9815? I thogut it was a fairly standard
4-level stack RPN machine.
cartridges and drive may have been OK in the day, but
today with the gooey
wheel issue and failing elastic bands in the cartridges they're nothing one
wants to rely on.
That I'll grant you...
Given that the plotter uses HP-GL as the control language, I began to think
along the same lines as Chuck wrote about a day ago: building a simple
parallel interface to the plotter HP-IB port that does just the data transfer
(minus all the HP-IB device selection, etc. functions) to connect it up to a
Well, the 'device selection' just means you need to be able to assert the
ATN line so you can send a command (it's sent over the 8 data lines with
the normal handshake sequence).
more flexible/modern source/controller machine.
A couple of thoughts.
1) I don't know what the 'dedicated' 9815 interface on the 9872 plotter
is, but there are manuals for the ploter and interface over on
http://www.hpmusuem.net, which contain scheamtics. I think a little work
with a 'scope or logic analyser would decipher said interface (I will bet
the actual data sent is ascii-encoded HPGL, just as it is on the HPIB
interface)
2) The HPIB (which I beluieve you have), GPIO (8 bit parallel) and RS232
interfaqces for the 9815 all allw you to load or save a program over said
interace. You could keep the 9815 and use its HPIB interface to
communcate with some more modern machine that stored its programs, thus
avoiding the use of the tape drive and cartridges.
Incidentally, according to the manuals, the serial interface and GPIO
interface have the same firmware ROM. The latter has a box of electronics
that's basically aserial <->parallel converter (it contains the standard
40 pin UART, etc). The only difference on the ROM PCB is that links are
rearranged to provide a 5V output on a normally-unused pin (it can be
used for all sorts of other things too) to power said converter.
-tony