I'm doing one-way digital communications between 2 PICs about 12 feet=20
apart.  ~50 kHz max.  Cable currently has 12V, ground, and one data=20
wire  (5V direct from a PIC 18F output).  Nothing is twisted yet -- just=20
straight wires.  At this stage, it's functional and works fine as it is,=20
but I'd like to add EMI/noise immunity (against possible external noise=20
from other nearby signals).  Shielding is noticeably increasing that=20
cabling cost, so I'm thinking of differential signaling instead. =20
Looking for a simple way to do this.

First thought was to use RS-485 or similar differential transceivers,=20
but that adds a few dollars, which is higher than I'd like.  CAN=20
transceivers are lower cost (~$0.40 each side).  However, CAN is a=20
relatively low-level signal, so I'm thinking that I can just use 2 I/O=20
pins on the sender to create my own differential 5V-ish signal, and use=20
the comparator on the receiving PIC to process the differential signal. =20
I would need to add some resistors at the receiving end as the (PIC32)=20
comparator inputs can only handle up to 3.3V on that side.  From=20
anyone's experience, would this even make a dent in the system's ability=20
to reduce noise effects?  Additionally, I am hoping to have the sender=20
be open-drain, and pull-up at the receiver.  Should this be hard-driven=20
to 5V instead for better noise immunity?

Another thought is that I have 12V available at the sender, so I can use=20
that for the signaling level, but that adds a couple transistors.  Or I=20
can stay open-drain at the sender and pull up to 12V at the receiver,=20
then voltage divide.  Would it make a difference for differential signaling=
?

Thanks,
-Neil.

--=20
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