Noise that moves together

Common mode

Noise from a motor reaches both wires of a pair. If it lifts A and B by the same amount, A minus B has not changed, and the receiver sees the same bit as before. A voltage that both wires share is called a common-mode voltage, and the receiver rejects it by subtracting.

A single wire has no partner to cancel against.

The window

The rejection has limits. A standard receiver works while both inputs, measured against its own ground, stay between about minus 7 V and plus 12 V. Inside that range the difference is trusted. Just outside it the receiver is no longer guaranteed, even if the difference is large. Further out, past the absolute maximum (minus 8 V to plus 12.5 V on a MAX485), the chip can be damaged.

A large enough common voltage can ruin a signal whose difference is perfectly good.

Twisting

A twisted pair makes the two wires trade places along the cable, so each sits nearly the same distance from a noise source and picks up nearly the same amount. Nearly: some noise still lands on one wire more than the other. That leftover is a real difference, and the receiver reads it as signal.

Twisting makes the noise more equal. It does not make it equal.

Ground offsets

The two ends of a long run rarely share the same ground voltage. That gap is added to whatever noise is present, and the sum has to fit in the window.

The Modbus serial specification asks for a third conductor beside the pair, the common, joined to every device and connected to protective earth at one point only, normally at the device that starts each exchange. That limits how far the grounds can drift. The RS-485 book shows where it lands on our boards.

Common mistakes

• Believing RS-485 ignores all noise. It ignores noise that lands equally on both wires and stays inside the window. A straight pair or a big ground offset leaves some noise that does not cancel.
• Skipping the common wire on a long run. A two-wire pair on its own can let the grounds drift out of the window, and then nothing is reliable.
• Treating twisting as a guarantee. It lowers the leftover. It does not remove it, and a bus can still fail beside a loud motor.