Module 02 ~45 min Interactive

Signal Levels, Voltage & Duplex Modes

A "1" and a "0" are just voltages — but each interface defines them very differently. This module makes those voltage bands visible, shows why a differential pair has far more noise margin than it looks, explains the receiver common-mode limit that quietly corrupts data, and contrasts full- and half-duplex wiring.

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Serial standards inherit the historic terms mark (logic 1, the idle state) and space (logic 0). What differs is the voltage that represents each — and, crucially, whether it's measured against ground (single-ended) or between two wires (differential).

RS232 — single-ended, inverted logic

Driver output swings +5 V to +15 V for a space (0) and −5 V to −15 V for a mark (1). Receivers guarantee the correct state for inputs beyond ±3 V. The −3 V to +3 V band is an undefined "dead zone." Note the inverted convention: a negative voltage is a logic 1.

RS485 / RS422 — differential

The signal is the difference VA − VB. A driver produces at least ±1.5 V differential (typically 2–5 V); a receiver only needs ±200 mV to decide the bit. That huge gap between what's sent and what's needed is the noise margin that buys distance.

Why the ±200 mV receiver threshold matters: a signal can lose volts of amplitude to cable attenuation and still be read correctly, as long as the difference stays above 200 mV. RS232, needing a full ±3 V at the receiver, has nowhere near that headroom.

This chart plots the guaranteed logic regions for each interface on one voltage axis. Notice RS232's wide swing but large dead zone versus the tight, low-voltage differential decision bands.

Takeaway: RS232 needs big voltages because it fights ground noise directly. Differential interfaces work with small voltages because the receiver rejects whatever is common to both wires — the actual decision only depends on the tiny difference between A and B.

A differential receiver subtracts A from B — but it can only do that correctly if both voltages stay inside its input window. RS485 receivers are specified for a common-mode range of −7 V to +12 V. If ground potential differences or noise push both wires outside that window, the receiver saturates and the data is corrupted even though the differential voltage is perfect.

Ground offset between nodes0 V
Both wires sit inside the −7 V to +12 V window — receiver reads the bit correctly.
Design fix: when nodes can't share a solid ground (long runs, separate buildings, motor grounds), use an isolated transceiver so the bus reference floats with the signal. Covered in Module 8.

Duplex is about how many directions of traffic the wiring supports at once. This determines wire count, transceiver type, and whether you need direction control in firmware.

Full-duplex (RS232, RS422) — separate TX and RX paths. Both directions transmit simultaneously; no turnaround.
Half-duplex (typical RS485) — one shared pair. Only one node drives at a time; DE/RE flips direction between transmit and receive.
Interface Typical duplex Wires (data) Direction control
RS232 Full TX + RX + GND None
RS485 Half (2-wire) / Full (4-wire) 1 pair / 2 pairs DE/RE (half-duplex)
RS422 Full 2 pairs (TX, RX) None

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