HP9815 (was Re : prn plotters)

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. 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. 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 -tony