15 Mar
2015
15 Mar
'15
6:12 p.m.
I'm working on one of these systems, and have run across a floppy
controller issue. Everything else in the system is functioning fine,
including booting from HDD. However, I have a problem where the floppy
drive has power, physically works, and connected properly etc, but what
I find is the drive/controller will recognize a disk is put in, click,
then nothing. Just due to the way I'm working on this, I haven't been
able to test any signals at the controller IC (WD2797), but I have
completely swapped the IC with a different one just to check.
Now, the question is, without looking at signals, what is the
probability the lack of any action is could be a problem with the input
to the IC (write, ready, track0, index, write protect, density signals)
or with the output signals (step, direction, DRQ IRQ)? There are
tri-state buffers (74LS244-input side, 74LS240-output side) on these
signals, and I'm thinking one of these are misbehaving, but trying to
determine which. Or could it be something completely different? Or any
good suggestions on troubleshooting this?
Yes : Why do you insist on doing it blind?
Seriously I do not see how you cna troubleshoot any part of a classic computer without
observing signals. The problem could be any one of a number of things (which I will
come to in a moment), unless you start making measurements there is no real way to
determine what is going on. Troubleshooting _should_ consist of knowing what the unit
should be doing, determining (by measurement) what it is doing and working out what
could cause the differences.
At this point, do you know if the problem is the controller or the floppy drive? In the
case of
the latter, have you tried to format, write and read a scratch disk? If it will do that,
it points to
head alignment problems. If not, the fault could still be in the drive. I assume the disk
is rotating
but have you checked this? Do the heads move at all? If you start with the heads near the
spindle
do they restore to cylinder 0 when you try to boot from the disk? The problem might be a
fault in the head or read amplifier -- even something as simple as a dirty head
On the controller side, given it's a 2797, I'd try asserting the test input and
seeing what the
setup waveforms look like. This will detect a serious failure of the data separator part
of the
IC or associated components. At this stage don't adjust anything though.
Does the CPU try to access the FDC chip (is CS/ being asserted)? Is there activity on the
other bus lines (maybe a bus buffer has failed and there is no data bus at the FDC chip).
Does
the FDC ever assert DRQ or IRQ? Does the appropriate bit of the system respond? Are the
floppy drive signals (in particular Read Data) getting to the chip?
Without more information as to what it is actually doing I don't think you have a
chance to
logically troubleshoot this.
-tony