hardware//electronics//differential signaling

Differential signaling is a way of sending a digital or analog signal as the difference between two wires driven in opposite directions, rather than as the voltage of one wire against ground, and it is used to carry data reliably over long cables through electrically noisy places such as factories, vehicles and drone frames. The receiver ignores the absolute voltage of each wire and reads only which one is higher, and by how much.

Differential signaling is a way of sending a digital or analog signal as the difference between two wires driven in opposite directions, rather than as the voltage of one wire against ground, and it is used to carry data reliably over long cables through electrically noisy places such as factories, vehicles and drone frames. The receiver ignores the absolute voltage of each wire and reads only which one is higher, and by how much.

The reason is noise. A motor cable, a variable frequency drive switching at kilohertz or a lightning surge nearby induces a voltage in any conductor it passes; in a twisted pair both wires sit side by side, so they pick up almost the same disturbance. Added to both, it vanishes from the difference. A single wire referenced to ground has no such defence, and a few volts of induced noise on a 5 V line flips bits.

Noise that hits both wires equally cancels in their difference.

That property, called common-mode rejection, is what lets RS-485 run about a kilometre at low speed and lets a car's CAN bus survive the ignition system next to it.

On RS-485 the two lines are called A and B, and a receiver needs only about 200 mV of difference to decide, while each wire may wander over several volts of common-mode range with respect to the receiver's ground. That tolerance matters in plants whose ground potentials differ from one cabinet to the next.

On CAN the lines are CAN-H and CAN-L. In the recessive state both rest near 2.5 V; in the dominant state CAN-H rises to about 3.5 V and CAN-L falls to about 1.5 V. Because a dominant driver overrides a recessive one on the pair, the same electrical fact gives CAN its arbitration.

The pair must be twisted and kept together all the way; splitting A and B into different cables, or pairing A with a wire of another circuit, throws away the cancellation and gives intermittent errors that look like software faults.

A long differential line behaves as a transmission line with a characteristic impedance of about 120 ohm for typical twisted pair, and it needs bus termination at both ends or its edges echo.

The same idea runs well beyond field buses: USB, Ethernet, LVDS camera links and the high-speed lanes inside a data center are all differential, for the same reason.