Control Board and Tester Work!

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Sam Wetterlin
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Joined: Sat Apr 28, 2018 6:48 pm

Control Board and Tester Work!

Post by Sam Wetterlin » Wed Nov 12, 2008 8:21 pm

I finally managed to get the control board attached to my computer.

Using my 'scope and the toggling feature of the tester software, I was

able to check for square waves at each latch output. Each output

looked great, so my control board appears operational.



I was hoping to be able to check rise and fall times, but I had to set

the sweep rate too slow to be able to do that. A sweep rate of

50us/div produced a good display, but there were fleeting images in

the background indicating that the wave timing was not perfectly

regular, so some sweeps got off sync. That jumpiness made it hard to

sync up at higher sweep rates. The wave transitions were as crisp as

they could be, but at that sweep rate even a 1 us rise/fall time might

look crisp. At low sweep rates, the first few waves looked stable but

the later ones jumped back and forth.



Scotty mentioned that he had a couple of bad bits with 15 ns fall

times. I could not check for such timing with this method. (Maybe I

could display a little of the pre-trigger wave form so I could look at

the full transition that caused the trigger, but I haven't done that

in so long I forgot how to do it.)



So I think toggling the latches is a quick way to check for

operational bits, but current draw probably has to be counted on to

uncover not-so-visible defects. Scotty's bad latch drew 15 ma. The

schematic shows normal current draw of 35 ma for the entire control

board, about 19 ma of which is for the LED. If a latch which should

draw 1 ma is drawing 15 ma, it will be obvious from the overall board

draw.



Bottom line, the Control Board Tester software works as it should and

is a handy way to get some confidence in your control board.



A word of warning: I can see how Scotty's ground clip fell off onto

the latch pins. So I clipped a ground wire to my 10:1 scope probe and

made sure I was hanging on to both the clip and the probe at the same

time.



Sam W.

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