AT&T 3b1/Unix PC Floppy controller help

tony duell ard at
Sun Mar 15 13:12:26 CDT 2015

> 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.


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