I made an incorrect assumption about the way the shift registers work.  I
was :hoping: that the Qout was open collector, so shift data lines could be
shared.  From what I can tell, they are actually push-pull CMOS outputs, so
I'll have to MUX the outputs together, or use separate input pins.  Thanks
for all of the help.

----------
> From: Andrew Kalman <aek@NETCOM.COM>
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: MM74C165 shift registers
> Date: Tuesday, December 02, 1997 8:57 PM
>
> Hi Justin.
>
> Re:
> >I've been experiencing peculiar problems with MM74C165 parallel-in
> >serial-out shift registers.  I am using the clock inhibit line to select
a
> >set of shift registers, and clocking in the data.  This works
beautifully
> >UNLESS the shift register has some unused parallel-in bits.  I've tried
> >everything... tying them high, tying them low, tying them via a resistor
> >bank to Vcc or Gnd, but the only way they seem to work is if I let them
> >float.  Otherwise the chip outputs a '1' when not selected.  Does anyone
> >have any ideas to explain this?
> Hmmm ... I use '165s as the "input side" to long serial shift-register
> chains where '595s form the "output side". Both part types are
ubiquitous,
> inexpensive and fast. But that doesn't answer your question ...
>
> Is it possible that your chip isn't getting power from VCC? I ask because
> floating those pins would be fundamentally different from the other
> configurations you mention. Remember that CMOS is happy to "pull" power
in
> at any of its inputs if it's not getting power at the correct place (i.e.
> VCC). CMOS does very strange things when it doesn't get power like it
wants.
>
> By tying SDI (pin 10) low you could rule that out as a source for the "1"
> data you're seeing at the output.
>
> I would also look at your clock inputs (pins 2 & 15), and the serial
input
> (pin 10) -- perhaps you're getting more clock pulses than you think?  If
> you have access to a Logic Analyzer, you should be able to figure it out
> pretty quickly.
>
> The other option is that somewhere along the line you're making an
> incorrect assumption, either about the chip, or about your circutry to
it.
> Perhaps your datasheets are cryptic or something -- I personally prefer
the
> Motorola datasheets over all the others.  I recently had a terrible
timing
> figuring out what I was doing wrong on a long serial chain, until I
> realized that I had read the datasheet wrong and was trying to clock 16
> bits into what was actually an 8-bit device that required _seperate_,
> _consecutive_ bytes to complete its 16-bit control word format.  IOW, my
> _assumption_ was wrong. All my circuitry worked fine, yet I kept looking
> for electrical problems when it was, in fact, just the way I was clocking
> the data and running the Strobe signal.
>
> (PS. I've never used the '165's clock inhibit pin.)
>
> HTH,
>
>
>
>  ___________________________________________
> | Andrew E. Kalman, Ph.D.   aek@netcom.com  |
> |        standard disclaimers apply         |
> |___________________________________________|