HP 9915A (industrial version of HP 85A)

From: Eric Smith <eric_at_brouhaha.com>
Date: Sun Oct 13 22:30:01 2002

gil smith <gil_at_vauxelectronics.com> wrote about HP-8x ROMs:
> The roms have +12V, +6V, and -5V
> power. There is an 8-bit bi-directional bus, and four non-overlapping
> 12V clock signals. There is a "load-memory-address" line, a "power-on"
> line, "read" and "read-control" lines, and even a "write" line (I don't
> know why write is available on a rom).
> This all leads me to believe the roms are pretty specialized.

Yup, custom for the HP-8x, and CMOS versions for the HP-75. AFAIK they
aren't used in anything else, although sometimes Corvallis division
custom parts wound up in instruments, like the HP-35 chipset (with
different ROM code) in the 1722A oscilliscope.

> Since
> there are no lines to enable a specific rom socket, I think the 85 must
> poll for roms using fixed address ranges or something -- this implies
> that the roms contain address qualification circuitry of some sort.

They all map to the same address range, but have unique IDs that enable
the software to select one and deselect the others.

> I'm just
> speculating, but it would make the roms very difficult to duplicate,
> since this is not a standard address-bus/data-bus (or even a typical
> multiplexed addr/data bus).

Yes. I'd be interested in hearing how your buddy did it. I suppose
one could write an assembly-language program to dump ROM data, but
otherwise one would have to build some specialized hardware.
Received on Sun Oct 13 2002 - 22:30:01 BST