HP9815 (was Re : prn plotters)

Tony Duell ard at p850ug1.demon.co.uk
Fri Feb 16 17:27:49 CST 2007

> I opened it up and the 7 ROM chips are pretty obvious, I must have
> seen them before. The 6800 is marked only with HP's house number, so I'm
> guessing I read something somewhere that told me it was a 6800 and left me 

It certainly is a 6800... The other 40 pin chip, BTW, is a 6821 (or maybe 
a 6820, a PIA anyhow).

The 16 port lines of that PIA are used as a 12 bit output-only data bus 
and 4 select lines. The latter are decoded by the '154 on the 
keyboard/display PCB and select things like the tape control circuit, 
printer, display, keyboard, output ports (for the peripheral slots) and 
so on. Input from the peripherlas (internal and external) is via an 8 bit 
input data bus (which also carries the data from ROMs in the I/O modules) 
to a cvouple of /257 muxes on the CPU board.

The memoery map is a little odd in that the 6821 is mapped as the first 4 
locaionss, then RAM (and obviously ROM at the top of the memory space, to 
get the reset vector, etc). Address line A15 from the CPU is not used, 
the memory space is effectively 32K bytes. There is some logic in the 
fact that the PIA is at the bottom of the memory map (rather than in the 
spcace ebtween ROM and RAM) in that said PIA can then be accessed using 
the page 0 addressing instrucitons, which are shorter and faster than 
ones specifying the full 16 bit addess. The PIA is, of course, used a lot 
in the I/O routines.

> > 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....
> On the unit I have ~ 9 of every 10 of those tiny screws is instead a push-pin
> inserted into the plastic, so the keyboard isn't about to come apart without a
> high likelihood of breaking things. I'd rather have the screws, for cleaning
> and potential repairs.

All the ones I've seen (and the 'chicklet' style 9825 keyboard [1]) have had 
all screws, fortunately.

[1] My 9825 has the full-travel keyboard, but my 9831 (electrically the 
same machine, but with BASIC, not HPL, in ROM) has the chicklet keyboard.

> > 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...
> (.. or power it up, press the keys and find out what they do..)

Err, it's a 9815. It has those conenctor blocks sandwiched between the 
PCBs, so you can't conenct things together (easily) without almost fully 
assembling the machine. Something I'd rather no have to keep on doing...

I learnt my lesson. I now make a diagram of the key locations before 
removing any screws


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