An early series of products that we did several decades ago used=20 single-ended current-mode signalling. This was used on simple=20 2-digit "Nursery Call" systems installed in churches and the=20 like. The transmit end used simple NPN saturated switches on the Clk=20 & Data lines; the receivers used PNP transistors with Emitter tied to=20 the incoming 12V line as simple current sensors. We could tolerate=20 voltage drop in the 0V (Ground) line up to the point where the=20 receivers wouldn't have sufficient supply voltage to operate. The entire system used Telephone Station "Z" wire (4 conductors #20=20 in a PVC jacket) because the wire was so darned inexpensive. This technology pre-dated the microcontroller era and worked=20 extremely well for distances greater than 1000 feet. That said: the=20 data rate we used was fairly slow (low KHz). You mention that you have a 12V rail available. Current-mode=20 signalling might just work out for you. dwayne At 01:07 PM 5/17/2017, Neil wrote: >I'm doing one-way digital communications between 2 PICs about 12 feet >apart. ~50 kHz max. Cable currently has 12V, ground, and one data >wire (5V direct from a PIC 18F output). Nothing is twisted yet -- just >straight wires. At this stage, it's functional and works fine as it is, >but I'd like to add EMI/noise immunity (against possible external noise >from other nearby signals). Shielding is noticeably increasing that >cabling cost, so I'm thinking of differential signaling instead. >Looking for a simple way to do this. > >First thought was to use RS-485 or similar differential transceivers, >but that adds a few dollars, which is higher than I'd like. CAN >transceivers are lower cost (~$0.40 each side). However, CAN is a >relatively low-level signal, so I'm thinking that I can just use 2 I/O >pins on the sender to create my own differential 5V-ish signal, and use >the comparator on the receiving PIC to process the differential signal. >I would need to add some resistors at the receiving end as the (PIC32) >comparator inputs can only handle up to 3.3V on that side. From >anyone's experience, would this even make a dent in the system's ability >to reduce noise effects? Additionally, I am hoping to have the sender >be open-drain, and pull-up at the receiver. Should this be hard-driven >to 5V instead for better noise immunity? > >Another thought is that I have 12V available at the sender, so I can use >that for the signaling level, but that adds a couple transistors. Or I >can stay open-drain at the sender and pull up to 12V at the receiver, >then voltage divide. Would it make a difference for differential signalin= g? --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA 780-489-3199 voice 780-487-6397 fax 888-489-3199 Toll Free www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .