Re: Any OR gates with good noise immunity?

Your problem sounds an awfull lot like a variation of ground bounce
issue that I experienced in a design. In our of our products, we were
using a board that suffered from intermittant flase triggers when there
was a disturbance in the mains power, such as when electromechanical
devices would switch and would occur about 1 out of 10 times.

The problem in my case was due to a lack of a proper ground plane and
adequate bypass capacitors. The board designer believed that putting
one 10uF capacitor with a .1uF in parallel with it about 1 inch from a
logic device with four power pins was adequate bypass. Similarly, he
did not feel that it was necessary to put a bypass capacitor on any of
the logic ICs. The problem was resolved with a redesign of the board
that utilized a solid ground plane, on a two layer board, added proper
bypass caps, and switched the design from through hole to surface mount
which has a much lower lead inductance on the packages.

This brings me to the questions regarding your design. Are you using a
ground plane and what do you have for bypass capacitors on the driver
and receiver devices?

Another thing to consider is how long are the copper runs. You said
that you are running from one board to another through edge connectors,
which suggests that they may be of fairly decent length. It is likely
that the design will require proper termination of the transmissions
lines. Your runs will act as a transmission line, as opposed to a
lumped circuit, if the run length is more than about 1/6th of the
signal's propegation delay, which is a function of the PCB geometry and
material. It is a common misconception that a "high speed" circuit
means fast clock rate, when in fact it is a function of the signal rise
times. This brings me to my final questions, what do you have for
termination on the lines, how long are the copper runs, and what is the
driver rise time (either from a data*** or from measurement, though
you will need to measure with something other than a capacitive scope
probe). If you don't know these paramaters, I would guess that based
on typical PCB construction if your lines are more than about 4 inches
they will require termination.

I appologize if you are already familiar with the topics I descibed
above, but I figured it would be less offensive to tell you something
you already know that to leave something out that you needed to know.

.

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