Curious if anyone in the group had any strong feelings about how to go
about RF-proofing RS-4XX interfaces? (RS-232, 422, 485)
I have a couple of microprocessor projects in the works that will need
to operate in close proximity to strong HF fields. In normal operation,
loads of Mix 31 ferrite on the twisted pair should keep the RF down to
levels that the RS485 differential can overcome, but in a few rare cases
the interfaces may just have to survive being saturated. The key word
here is survive. :-)
What I've done in the past is follow an old National (Semi, not Radio)
app note that discusses using a small amount of series current-limiting
resistance and back-to-back zeners between each side of the differential
pair and ground. A newer TI app note states basically the same thing,
except they discuss using specific TVS (Transient Voltage Suppressor)
devices instead of discrete zeners. I've also seen designs that use
MOV's, but the National note suggests they aren't as fast as zeners.
Any thoughts?
Thanks,
Jim, N7CXI
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