Sorry I've left everyone hanging so long.  I actually did send a message
with the answer, but found out later it got hung up in a new mail server
without even giving me a bounce message.  I replied to a few other messages
during that time too, but by now I've forgotten what they were about.

Anyway, after trying all the things I listed in my original post, I started
thinking about edge glitches.  I thought this was a long shot since 20
inches of cable really isn't that much, but wanted to make sure it wasn't
the problem.  The first try was adding schottky diodes on the PGC line to
Vdd and ground.  The thought was that these would clip any overshoots or
undershoots.  To my surprise, it worked and I could program the target chip
normally.

After some more fiddling I found that it would work when 100pF capacitance
was added between either PGC or PGD and ground.  Given that, I tried to see
what I could do at the programmer end to deal with the problem.  Just adding
100pF to PGC and PGD at the programmer end of the cable didn't help.  Then I
added two schottky diodes on both PGC and PGD to Vdd and ground.  That
didn't help either.  Just trying to get something to work, I added 1nF to
both PGC and PGD to ground.  Surprisingly, that didn't help either.

PGD and PGC are now driven thru 2Kohm resistors followed by 1.1nF to ground
and schottky diodes to Vdd and ground.  The 2Kohms and 1.1nF form a low pass
filter with 72KHz rolloff.  I can't imagine how 20 inches of wire makes a
difference at these frequencies.

In the end I've decided that my programmer is not suitable for in circuit
programming, and will stop trying to make it do that.  It was designed to
program a chip in its own ZIF socket, and it does that very well.  Perhaps
I'll make a different programmer with different drive characteristics that
is meant for in circuit use.  In some ways that's easier since there is no
need to deal with the multiple PIC programming pinouts.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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