Epson PF10 problems

[Tony Duell]

I am currently working on an Epson PX8 system. I bought this on E-bay, so 
the history is essentially unknown, but I was told it was 'untested' 
which I took to mean non-working.

The PX8 had suffered a minor burn-up in the PSU area, which I have cured 
(one new transsitor, resistor and zener diode. The computer now seems to 
work find (and it's a very nice toy).

With it, I got a PF10 floppy drive. This is a single 3.5" unit, with a 
38400 baud serial interdace back to the PX8. I had to make up my own 
cable, but I have checked and double-checked it, and anyway it works fine 
to link a PX8 to a PX4 via the RS232 interfaces (essentially the same 
pinout as the 'high speed' serial interface to the disk drive). In fact 
what I did was make up a cable from the 8 pin nini-DIN plug to fit the 
PX8 to a DB25 plug,, wired as a DTE and a second cable with a DB25 socket 
to a 8 pin mini-DIN, wired to do the right swaps and jumpers.

OK, a bit of background on the PF10 internals. There's a 6303X 
microcontroller (in an 80 pin package...) linked to a 27C64 EPROM 
(socketed) and a 6116 SRAM (SMD again). Address decoding is mainly done 
by a '138, with a few other TTL chips. 

Linked to that is a 765 disk controller, drivers for the head stepper 
motor, a data seperator chip, read/write chain, etc (note, there is no 
seperate logi board on the drive mechanism, the heads, stepper, etc plug 
straight into the controller board). 

There are several power supply lines, most of them under the control of 
the microcontroller. In particular, there's an always-on 5V line, a 
switcehd 5V Line (the disk controller, etc, is powered down when the unit 
is idle), +/- supplies for the RS232 drivers, a +12V line for  the 
analogue circuitry.

OK, at power-on, the 'power' LED comes on (I have connected 5V from my 
bench supply in place of the NiCd battery pack). The always-on 5V line is 
at 4.95V. None of the othher supples are present (this could be right at 
this point) 

I connected it to the PX8, DIR D: gives 'BDOS ERROR ON D: SELECT'. A 
breakout box connected between the computer and the drive (RS232 levels, 
rememeber) shows that when I run that command, the computer's serial port 
is turned on, something comes on on the TxD line, but the disk drive 
never turns on its port or sends anything back. 

More worrying, reducing the voltage from by bench supply never turns on 
the low battery LED on the drive unit. This is controlled by the 
microcontroller, BTW.

I pulled it apart, of course. The serial data from the PX8 does get to 
the appropriate pin on the microcontroller. The microcontroller is 
clocking (1.23MHz on the Eclk pin), it's accessing the ROM, but it also 
appears to be wandering around the memory map (outputs on the '138 
decoder that are not used for anything are being asserted at times). The 
address lines look odd to me, with pulses narrower than the Eclk on some 
of them.

Since the ROM is socketed, I pulled it and read it out. It looks sane. 
Certainly no data bit is stuck high or stuck low, and all the address 
lines do something. Assuming it's like a 68xx processor, with reset and 
interrupt vectors at the top of memory, that looks sane too.

I suspect the microocontroller or the RAM. Alas both are SMD (and 
therefore a pain to remove), I have spares for neither and no way to get 
them.

Any comments, suggestions, or things to check?

(Yes, I know about PC-based emulators for these drives, but I'd like to 
get the real unit working too).

-tony