According to the museum site (
www.hpmuseum.net) the electronics of the HP125
and HP120 are mostly the same.
Well, the physical layout is different -- the HP125 ahs one large PCB
with all the digital stuff on it, the HP120 has a pair of PCBs, one with
most of the terminal circuitry on it, the other with all the user
processor stuff, the serial ports and the interprocessor communication
latches, etc. But, yes, AFAIK the circuity is very much the same.
The firmware ROMs are the same (at least between a version C HP125 (uses
64K DRAMs like an HP120) and an HP120). The only major component to be
different is the keyboard interface microcontroller. And that's mainly
because hte keyboard interface is different. The HP125 has the DA15
keyboard connecotr, the keyswitch scan is generated by, AFAIK, that
microcontroller. The HP120 has the 6 pin RJ11 socket, carrying clock and
reset lines fro a counter in the keyboard to scan the keys, these signals
coming from the microcontroller.
It seems the housing and screen size are the biggest
difference between
these machines.
I think the PSU and video monitor PCBs are different, but the latter at
least might be similar in design.
Of course the HP125 can take an internal thermal printer, the HP120 is
not documented as being able to do so. But the interface (for an HP2674)
seems to be there, at the same I/O addresses as the interface on an
HP125. And since the terminal firmware is the same, it should work.
Sometime I'll try the cabling from an HP150 and see what hapopens (yes,
I'll check it carefully first...)
I didn't checked my HP125 out just tested and
fixed (bad video ram 4116) it
when I got it, and ran some progs CP/M on it.
I like the looks of it, not the internal design solutions.
YEs, the 'ET head' styling of the HP125 is rather unusual...
The HP120 looks very like an HP9816 (or an HP15 (without the space for
the printer)
Playing with a 200 or 300 series is more fun.
I don;t like the 9000/300 too much, they've got too much custom silicon
in them (or at least the ones I've seen inside do). But I do like the
HP9000/200 machines. Very clean designs...
-tony