More on my HP7245A problem

[Tony Duell]

> If you could point me where to look, it might have the parts. 
> 

I would be very suprised, alas if the same chips were used in any other 
HP product (put it this way, the 7245B is different according to the 
service manual).

The 7245 is a somewhat odd product (AFAIK the idea was never carried on). 
It's a thermal printer -- there's a srocket-feed mechanism for thermal 
paper driven by one stepper motor and a second stepper that moves an 
12-dot printhead across the paper to print chracterers.

What makes is unusual is that there's a thirteenth element in the head 
and the paper mechanism is designed to run both ways. It will act as a 
pen plotter, rolling the paper back and forth and runing the head across 
to draw lines, etc.

Mechanically it's farily simple. Electronically, it's not. Thre are 5 
boards of logic: 

Processor, using an HP custom 16 bit microprocessor, which was also used
in some other printersd of the time. There are self-test swtiches and 
LEDs on this board, and enough ROM to get the thing started. In fact you 
can run the first test with just the processor board (and PSU) plugged 
into the backplane.

ROM/RAM

I/O (printhead drivers, beeper, switch/LED interface, etc)

Motor control. Very complicated, it's got a 4*4 register file chip (input 
data), the output of which is sign-extended to 8 bits (!), then fed into 
a full adder + accumulator regiters circuit. The output of that is fed to 
a couple of PROMs that seem to be waveform lookup tables, then to a DAC. 
The output of that DAC goes to 4 sample-and-hold circuits, then to active
filters, then to the driver transistors on the PSU chassis. The clocks 
for all that lot come from a counter chain, decoded by another PROM.

HPIB Interface. Remarkably hardware intensive, for example, HPIB commands 
are hendled in hardware, not passed to the processor. Only data is passed 
to the processor).

These all plug into a backplane that fits in the bottom of the case. 
There is an SMPSU that fits in on the right hand side with another 5 PCBs 
in it, and a mains rectifier/smoothing capacitor board at the back.

Mine doesn't work. From fiddling with the self-test switches on the 
processor board (based on the instructions for the 7245B), checking 
signals, etc, I am pretty sure the first problem is the chip U10 on the 
ROM/RAM PCB. This is an 82S123 ROM (32*8 bits), I have no idea what it 
should contain.  And I susepct the 7245A was the only device to use this 
PROM programmed this way.

-tony