[Coco] Weird CoCo3 problem...
gene.heskett at verizon.net
Tue Jan 5 00:47:30 EST 2010
On Monday 04 January 2010, Brian Blake wrote:
>Okay guys, got the o'scope, schematic and CoCo3 service manual, along with
> Frank's 'Tandy's Little Wonder' out for good measure and started reading
> and testing. The results have been posted to my blog at the following
> address so I could post pictures of the actual scope traces.
>I'm not sure if these will be much help or not, as the test results are...
> interesting. I've got the exact same signal from the GIME to places I
> wouldn't think they should be (i.e.: GIME output to TP2, 3, 4 & 5 appears
> to be identical, and NOT what the service manual says they should be). I
> freely admit there are folks here who have forgotten more than I know
> about the CoCo family, so I hope these images help.
First thing, either that scope is slow, or your ground leads from the probes
are way too long and connected too far away. Rise and fall times are slower,
by 5x, than anything I've seen on my Hitachi V1065, a dual trace 100mhz
scope. Using 200mhz rated 10x probes, properly compensated. You are using
1x probes maybe?
Second, the E and Q clocks will look identical, _unless_ the timebase is
always triggered from the same clock. Use the E clock for the triggering
signal, and if you have dual trace on that tek, which it should, look at both
clocks at the same time in chopped mode. The you should see the 90 degree
phase angle between them.
Third, one of the lower pix/traces seems to indicate a data clash, at the
left end of the trace is a fairly square pulse that just doesn't make it down
to a solid logic 0, but the whole pix looks like its a slow scope or 1x
probes so its not cut and dried. Is that the one created by running the
basic program I saw suggested in an earlier post in this thread? That might
indicate a stuck input when that chip is enabled, like one of the 74ls245's
in an mpi that is not properly reversing the data path when told to, and
while not in any of my coco stuff, I have seen that failure in a 74ls245
Last, when posting the pix, we need the gain settings of the vertical display
for both channels, and the sweep speed settings info too. That adds much to
the data we can interpret.
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