Odd FDC problem (Compupro) - test-drb@ccmp.vtda.org

19 Jul 2005

Hi Jules,

...
 I probably won't be at Bletchley until this
weekend, but we got a
Compupro 8/16 a few weeks ago with a *lot* of documentation. I could
have a flick through that assuming nobody's taken it home with them... 
Thanks - I do have the technical manual for the 8086/8087 CPU board,
the DISK1A (FDC) and the Interfacer 4 (I/O) - I could use information
on the RAM16, Selector Channel and DISK2 - the latter two are for the
HD system which I have pulled at the moment. Info on the RAM16 would
be especially useful.

If you happen to have a listing for the 8086 boot, that would be REALLY
helpful.

...
   The odd thing
is that if you put a disk (any disk) in the drive, it immediately stops
 selecting it - it's as if getting the index pulses from the disk prevents the FDC
from
 proceeding ... 
If you're seeing different behaviour occasionally, then this can't be
it, but I remember our machine did something very similar to this if the
drive cabling was back to front; the drive LED would pulse but then as
soon as a disk was put in it'd do nothing. 
What do you mean by "back to front" - do you mean the connector reversed?
When I first connected the drives, I put the connector on backwards, which
resulted on both drives coming ON permanently (grounded selects).

...
   Sometimes,
usually after taking a board out to check something, it will actually try
 and boot from the disk - If you put Disk-2 of the Compupro CP/M 86 distribution in the
 drive, you can hear it boot, it seeks a bit and obviously reads a track or two - after
 the first seek, you can see the RTS+DTR lights come on on all three serial ports, which
 suggests that it has come up enough to init the serial ports ... but I do not get any
 output on any of the three ports. 
Are the disks known to be good? It does sound like a faulty connection
though - does moving the boards around to different slots make any
difference? 
I'm pretty sure the disks are go - I've tried most of the slots, as well
as cleaning the sockets and connectors, as well as reseating and cleaning
up any socketed IC's that look at all suspect.

When I first put the system together, it would try to boot fairly consistantly,
but while trying to figure out why the console was not working, I had several
boards in and out, and all of a sudden it stopped accessing the drive when there
is a disk inserted (but still showing the correct attempted accesses when the
disk is out). I thought for a while that I had fried something, but later, after
a bit more shuffling, it began working again ... but 90+% of the time it does
not select the drive if a diskette is is inserted.

Time to pull out the extender cards and scope.

If any one has any ideas as to what would cause this: The system selects the
drive at approx. 1 second intervals when attempting to boot with no disk 
inserted (this is exactly as described in the docs), but never loads the head
and stops selecting the drive for as long as any diskette is inserted.
Please let me know - I really can't think of a possible cause which makes it
tough to figure out where to start!

Regards,
Dave
-- 
dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Collector of vintage computing equipment:
                http://www.parse.com/~ddunfield/museum/index.html