AT&T 3b1/Unix PC Floppy controller help

Michael Lee mikelee at
Sun Mar 15 00:23:09 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?

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