Hi Jeff
I don't have a PDF editor so I am going to have to refer you to the full document:
http://electrickery.xs4all.nl/comp/p200 ... Manual.pdfI really would have preferred to paste the schematics into a new PDF!
Anyway, the main board schematics start at page 134 and the FDC description including detail of PLL, FDC and timing starts on page 87.
The diagram on page 134 shows the board layout; the PLL and FDC connector are at bottom right on the bit of the PCB sticking out. The full schematics are on pages 87-90, and the full FDC circuit is on page 89 (with PLL bottom left).
I am discussing this on two other forums right now, so I can say that I have checked all parts of the read circuit with a scope and it's working, apart from VCO SYNC from the FDC, which stays low all the time. It's supposed to go high during reads, to switch the PLL from the 250khz signal to the RDD signal from the drive. The result bytes coming back from the drive are as follows
Code:
C0 00 00 00 10 00 FF
..which seem to be saying (per upd765 data sheet)
C0 : SR0: abnormal termination because disk ready signal changed state (D6 / D7 = 1)
00 : SR1 - nothing
00 : SR2 - nothing
00 : SR3 - nothing
10 : ?
00 : ?
FF : ?
So I checked the RDY line on the FDC (pin 35) and it is held high all the time via a resistor to +5v (marked as 4709 on the schematic).
I'm now thinking the FDC is encountering an error condition prior to initiating the read, which might explain why I am never seeing VCO SYNC go high.
Since this thread was started to try to get an understanding of the generation of VCO SYNC, there is no context, which I will now provide. The machine is a Philips P2000C, a portable CP/M computer with a 9" screen and twin Teac FD55F drives. They are DSDD. I have boot disks that are readable on a PC using one of the FD55Fs and special software. The machine has a monitor which allows you to issue commands that test the drive (read track, write track, format track, etc) plus a number of memory view and edit operations. To get the result set above, I am asking it to read the content of track 0 on side 1 of the disk. It never completes the operation. And I now think it is because something else, prior to reading, is failing. In the FDC data sheet it explains how a read should be conducted. You issue recalibrate command (moves head to drive zero), then seek to the track to read, then read. So I think that, although it is seeking and recalibrating (head moves), there is an error or missing signal somewhere that prevents the read command being called by the monitor.
Regards
JonB