It's a strange desing using a terminal to interface with a CP/M computer.
A bit like an H89/Z90 :-)
I was looking at the HP120 design at a lower level. There are, as you
said, 2 processors. They communciate by a simple 1-byte parallel
interface, and AFAIK you can't run user code on the 'terminal' processor.
The terminal side uses that strange video chip I asked about a couple of
weeks back. It interrupts the terminal Z80 on every character line, the
Z80 then loads the start address of that line into the processor and the
apporpriate attributes into a latch. The video chip then does DMA and
fetches 80 byes from memory into an octal 80-bit shift register chip.
This is sent to the character generator and then to the CRT.
Note I mentioned that attirbutes are loaded per line. On each line, a
character is either 'normal' or has the atributes applies (some
combination of underlined, blinking, intensified, etc). Note you can't
have some underlined (but normal brightness) and some intesified (but not
underlined) characters on the same line. There's also no grpahics
capability. This would be reasonable if memory was limited, but that
terminal processor has 16K RAM and 32K ROM hung off it. The serial ports
are on the terminal processor bus
Now the user processor has 64K RAM, and an 8K ROM that can be paged out.
It also has the HPIB itnerface on its bus. And from what I can tell, the
HPIB interface is polled, there are no interrupts.
I mentioend the lack of any form of graphics. I also can't find any way
for a program on the user processor to configure the serial ports. You do
that from the setup menu, which is code running on the terminal
processor.
And while there's an exteded CP/M BIOS function to send a byte to an HPIB
device, there doesn't seem to be one to read from an HPIB device. Yes,
OK, since the HPIB controller is in the user processor I/O space it's
possible to get round that, but since the disk drives are hung off that
HPIB port, you could easily get the thing into a state where it couldn't
access the disks, or worse managed to corrupt them.
I find the sales pitch for this series of machines (on
hpmusuem.net) to
be worring due to the number of misconceptions and downright lies it
spreads about comepeting machines. I would have though an HP machine
should be able to stand on its own merits (most, IMHO, can!).
-tony