AT&T 3b1/Unix PC Floppy controller help

Michael Lee mikelee at
Sun Mar 15 16:55:53 CDT 2015

On 3/15/2015 1:12 PM, tony duell wrote:
> Yes : Why do you insist on doing it blind?
Just a limitation of where and how I'm working on it.  I figure I'd try 
to start with the basics.  The machine is a pain to work on without a 
lot of space or dismantling it to a point I could get that accomplished 
properly.  Also, limits of what test equipment I have to work with at 
the moment, as in a multimeter at best.
> Seriously I do not see how you cna troubleshoot any part of a classic computer without
> observing signals.
   I agree it's not the best approach to it.  It is going blind at it, 
but at the same time, I want to make some educated guesses.

At this point, do you know if the problem is the controller or the floppy drive?

I've swapped the controller IC itself, and also the floppy drive with 
known working ones, and same behavior, which leads me to believe it's 
something just slightly upstream or downstream from the IC.  The disk 
does rotate, seems to recognize a disk in the drive, but no seeking or 
other movement.

> 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.
As far as I can tell actually the IC itself is operating properly, which 
leads me to believe it's either not receiving the proper signals into 
it, or able to send the proper signals out through the tri-state 
buffers, leading me to this discussion.

> 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).
The general data bus and address bus appears to be fine, since the hard 
drive portion is working, which again leads me to think it's the signal 
line buffers leading in or out.  Obviously if the signal to move the 
drive isn't working, then the drive wouldn't move.  At the same time, if 
it never receives a signal to operate, then it'll also not move.

> 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?

This might be easy enough to test, and I'll try that.  Now, thinking about it, if I can test the input and output of the buffers at the same time, I could see if the right things pass, and that could also be doable.

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