Re: ISA Bus termination question

On Apr 24, 8:30 am, Matt <metal...@xxxxxxxxx> wrote:

Thank you all for the responses.

Dave, what you say is true. It's very possible that termination may
have only delayed the failure so there is quite a bit more testing to
be done. The reference I used for the measurments was the actual
ground on the motherboard's bus driver ICs and I am using a low
impedance/capacitance probe. I will let you all know if I find
anything else that stands out.

Thank you again,
Matt

One thing that a lot of people forget is that the clk signal on ISA is
not really a timing signal for the bus. It is typically used by chips
that interface to the bus, but not as a synchronous clock. Instead,
the read and write strobes are the timing signals that should be
treated as clocks. Depending on how a given chip detects the edges of
these "clocks" an over-shoot or under-shoot with ringing can result in
double clocking. So rather than looking at the bus clock, have you
looked carefully at the read and write strobes? Otherwise, I would
suspect the timing rather than the signal integrity on the bus.
Certainly ringing on the data and address lines is not significant
unless it is so severe that it causes latchup in the chips or delays
the settling time enough to cause a timing error.

The terminator likely created some timing differences. If you want to
explore the matter you can try removing terminators until you find a
particular one that can cause or fix the problem. With the problem
taking hours to occur, this may be a difficult problem to isolate.
But once you have, then you can explore the timing changes and see
exactly what is happening.

Oh, like David said, your ground for a ringing measurement is *very*
important. You need to pick a ground **very** close to the pin on the
receiver chip and use a ***very*** short ground lead. If you remove
the clip on the end of the scope probe and wrap a paper clip around
the ground sleeve, you can get the probe and the clip to contact the
board at the same time with a steady hand. This will give you an
accurate picture of the ringing on each receiver.

Rick
.

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