I had a little funny that took me an unpleasant while to find.

     If there's a little vibration or electrical noise, the count may creep a
     little without the encoder moving a full click.  I had about 5 inches of
     cable, and a 512 count encoder.  Seemed simple enough, so I just ran the
     two channels into the pins on the processor with 2.2k ohm pullups.

     After awhile, I found that I could move the encoder until I could see
     channel A change state, and rock it back and forth (without channel B
     moving).  This shouldn't have incremented the position counter, but it was.

     I fixed it with a .01uF cap on each channel input at the encoder connector,
     and a lot smarter algorithem to move the position counters.  (The code is
     in a PLD not a PIC... but the story would have been the same)

     I had the time and budget to get away without the HP interface chip.  If I
     had better things to do, or production quantities were low enough I would
     have definatly dropped in a decoder chip.

     Joe

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     >Phil,
     >
     >I would suggest that you don't do the quadrature decoding in the >PIC, but
     rather use on of the decoder chips from either Texas or >Hewlett Packard.


     I have a little circuit that transforms the quadrature signals into up and
     down pulses (just a pair of NAND gate if memory serves).  Then I was going
     to drive the PIC interrupts with these up down (and reset) signals.

     Phil
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