I have a situation at my client office where we must provide testers for 
some sophisticated
PCBs. These PCB's have heavy relays, NiMH battery chargers, and 
sensor-detection circuits
as well as one or more PICs.

To provide diagnostic information, _one pin_ of the PIC is dedicated to 
performing a dump of
PIC register memory, independent of its normal operation. The data is 
pumped out at a rate
of one bit every 4mS, slow but very reliable; to pump out 512 registers 
it takes just over 2
seconds, using a Manchester scheme, with a 100mS "dead band wait" for 
resyncing. I use
just 35 words of PIC code and just ONE register for this valuable 
feature, which is
interrupt-driven.

The way I use the data is that I have the operator install a known 
sensor into the PCB then
read the diagnostic information to see the internal results. If the 
results are within nominal
range, I pronounce that board section good, then go to the next. It 
saves the technicians
an immense amount of time, yet doesn't interfere with normal product 
function. It also allows
the technician to monitor the PIC itself for internal problems (I have 
found a few PIC16F877A
PICs with stuck port bits).
 
In years past I first used DOS PCs to read the diagnostic information 
into the tester software,
written almost always in Borland Pascal 5.5 for DOS. The data is slow 
enough that DOS can
extract it without error. I then went to Delphi 5. Of late, Delphi is 
not well supported, so I
find myself going back to old DOS machines. The data is read through the 
PC's Parallel Port,
and it works on even the OLDEST parallel port (uses the ACK pin).

I am astonished at how reliable old DOS machines are (and were)! Why did 
we EVER get
caught up with windows? I needed 3 old DOS systems, and my old resources 
of the past
(pawn shops) didn't have any, so to get them, I had to ask friends and 
relatives to search
their storage lockers and closets for them. Out of 5 received, I cobbled 
together the 3
I needed by shifting hard disks around, parallel port cards, etc, 
tossing the broken stuff.

FWIW, FreeDOS is the operating system of choice.

--Bob A
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