In part 5, upgrading the software on my 16500A’s CPU board to version 00.02 resulted in a self-test failure, which wasn’t fixed by downgrading back to 00.00. I’ve just had a look to see why that was happening.
The front panel is connected to the backplane by a 10-way connector. I traced out the pinout of the connector:
- +5V power
- +12V power
- +5V power
- +12V power
- On/off switch, ground to switch system off
There was no activity at all on pins 6 and 8, the data pins, even during self-test. I traced them through to the CPU board via pins 59 and 60 of the CPU board’s backplane connector, and thence to U90 pins 18 and 19. This is an HP custom chip. There seemed to be activity on many of its pins, but nothing on those two. Poking around, I found that two of its pins are connected to a nearby ceramic resonator which clearly wasn’t resonating. Aha! Got it. One of the pins of the resonator had broken. I soldered it back together and now the self-tests pass!
But there’s a fly in the ointment. V00.02 software doesn’t seem to be able to access the floppy disc drives, simply reporting ‘No Disc’ for both of them even when they’ve got discs in.
Looking at the floppy disc connector with a scope, the ‘drive select’ pins are definitely active. It seems that the pinout of Sony 3.5″ floppy disc drives changed at some point. The details are here:
Pin# Signal (Old Sony) Signal (New Sony) Pin# Signal (Old Sony) Signal (New Sony) 1 Disc Change Reset NC 2 Disc Change Indicator NC 3 +5V Key 4 Density Bit NC 5 +5V Ground 6 Drive Select NC 7 +5V Ground 8 Index Pulse Index 9 +5V Ground 10 Drive Select 0 NC 11 +5V Ground 12 Drive Select 1 Drive Select 1 13 Ground Ground 14 Drive Select 2 NC 15 Ground Ground 16 Motor On Motor On 17 Ground Ground 18 Direction Select Direction 19 Ground Ground 20 Step Step 21 Ground Ground 22 Write Data Write Data 23 Ground Ground 24 Write Gate Write Gate 25 Ground Ground 26 Track 0 Indicator Track 00 27 Ground Ground 28 Write Protect Indicator Write Protect 29 +12V Ground 30 Read Data Read Data 31 +12V Ground 32 Head Select Head 1 Select 33 +12V Ground 34 Ready Disc Change
It seems that the ‘disc change indicator’ has moved at some point in the history of Sony disc drives. Maybe the new software is expecting to see the ‘disc change’ indicator before it tries to switch the drive motors on? Grounding pin 34 doesn’t seem to make any difference. The disc controller is an FDC9793. Looking up the data sheet on it, it doesn’t have any means of indicating whether a disc is in or not. Without a schematic, it’s going to be very hard to figure out how the software determines it.