HP9815 (was Re : prn plotters)

Tony Duell ard at p850ug1.demon.co.uk
Thu Feb 15 12:54:02 CST 2007

> > I'm probably mis-remembering the numbers, but isn't the 98135 the HPIB
> > interface for the 9815 calculator? I thought hte plotter had HPIB as
> > standard.
> > 
> You are correct, I noticed after sending the message that my phrasing was
> ambiguous or misleading.
> The plotter has two ports, one for HP-IB and one specifically for a connection
> from the 98130A interface in the 9815.
> For some reason the package I received came with both the 98130 interface
> (9815->9872) as well as the 98135 interface (9815->HP-IB). Given the HPI-IB
> interface already on the plotter, this would seem to be redundant, as I don't
> think this 9815 was used with any other HP-IB devices. Going by the doc though,

Well, the 9815 wasn't used with HPIB disk drives, and I am not sure if 
you could simply print to an HPIB printer (you could certainly send 
characters to one with the 98135, but you may not have been able to, say, 
list your program to such a printer). But it certainly could work with 
HPIB lab instruments. It appears that HP designed their desktop 
calculators as instrument controllers -- BCD interfaces (capable of 
taking the BCD data sent ot the display drivers in many older digital 
instruments) exist for the 98x0, 9815 and, IIRC, 98x5 machines.

> it appears that the firmware in the 98130 may make for much easier programming
> of the 9815 to control the plotter, compared to going through the generic HP-IB
> interface.

I am almost sure it does.

> > I don;t know how much you know about the 9815 internals, but anyway. It's
> > a 6800-based design -- apart from the firmware ROMs, all the chips are
> I don't have the schematic and haven't RE'd it, but am aware of the 6800.

If you can put up with my handwriting, you can download my RE'ed 
schematics from http://www,hpmuseum.net/ . They're on the documantation 
page for the 9815. 

> Having written some assembler code for the 6800 and being somewhat aware of
> object sizes for such, I think when I looked inside the 9815 I was somewhat
> surprised that they managed to fit all the firmware code into (IIRC) 2KB of
> ROM, considering all the math functions, programming capability, tape-drive
> control, printer control, etc. that would be in there. Either that or I
> missed seeing a ROM chip somewhere.

No, you've nis-interpretted the ROM size.

There are 2 versions of the HP9815 processor board. The original one uses 
2112 RAMs (256*4), and 2Kbyte ROM chips. It's laid out for 8 such chips, 
only 7 are fitted (so 14K of ROM). This board can take a RAM expansion 
board that mounts on the track side (top) of the PCB at the rear edge. 

The later version of the CPU board uses 2114 RAMs (and has a total of 4K 
RAM that can't be further expanded). THis board has 2 8K byte ROM chips, 
but only 6K of the second ROM is normally accessible (so the sake size -- 
14K -- of ROM). You can solder a link on the CPU board (it's shown on the 
schematic) to enable that last 2K. I don't know if it does anything 
useful, but it is structured as a peripheal ROM, the system will find it, 
and it does have sensible command names.

I've had my 9815 totally apart, of course. If you're tempted to do the 
same, can I recmend against dismantling the keyboard. For one thing the 
PCB is held down by around 100 timy screws. And when you get the PCB off, 
the keycpas fall out and are a right pain to get back. Don't ask.... 
Suffice it to day I had to send a panic e-mail to a couple of lists 
asking for the order of keycaps in part of the keyboard...


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