Since CAN Bus is a differential signalling system, what is the significance of GND/COM signal? Can I get by without it?
I've got a very simple 2-node system that has some weird happenings going on. (see here for that issue, if you're interested) The two nodes are my device and a USB-CAN monitor; both have 120ohm termination resistors, and the wire run between them is <1m.
I just remembered that in my earlier tests I only had the CAN_H and CAN_L signals connected between the nodes; I did not have anything connecting the CAN_COM of my device to the ground pin on the USB-CAN monitor.
A (the?) point of differential signaling is to be much less (not?) effected by common mode noise. So by not having the ground connected, I would guess we lose the absolute reference for CAN_H and CAN_L… but does that make a difference?
Best Answer
A perfect, theoretical, differential system can run with no ground, because the input can always do (A-B) without worrying about the absolute values of A or B.
Unfortunately, CAN is not such a system, and the receiver will have limits to its 'common mode' capability, beyond which it be damaged. So you have to have a ground to keep the common mode voltages within limits. 'Common mode' just refers to stuff (voltage, in this case) which is common to both wires.
There are other ways of building receivers which are far freer of this sort of constraint - for example an opto isolator can do the (A-B) detection with maybe 1000s of volts of common mode offset. Transformers are often used too (e.g. in twisted-pair Ethernet, which has no ground). But for normal CANbus, you need a ground.