Module 01 ~50 min Interactive

Introduction & Interface Comparison

Before you route a single trace, you need to know why RS485 can run a kilometre of cable at high speed while RS232 gives up after a few metres. The answer is the difference between single-ended and differential signaling. This module builds that intuition visually, then lays all three interfaces side by side and gives you a decision guide.

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All three are TIA/EIA serial standards that define the electrical layer — voltages, drivers, receivers and cabling — not the data format. You still send the same UART bytes; these standards just decide how those bits survive the trip down the wire.

RS232 — the desktop veteran

Single-ended, point-to-point, ±3 to ±15 V swings. Simple and everywhere (DE-9 port), but noise-prone and limited to short cables and modest baud rates. One driver, one receiver.

RS485 — the industrial bus

Differential, multidrop. Up to 32 (often 256 with modern parts) nodes on one twisted pair, ~1200 m reach, high common-mode immunity. Usually half-duplex. The backbone of Modbus, PROFIBUS and building automation.

RS422 — the long full-duplex link

Differential, point-to-point (one driver, up to 10 receivers), full-duplex with separate TX and RX pairs. Same distance/noise benefits as RS485 when you need simultaneous two-way traffic.

The common thread

RS485 and RS422 both send data as the difference between two wires. That single idea is what buys them distance and noise immunity — and it's what the next section makes visible.

A single-ended signal is measured against ground. Any noise picked up on the wire rides straight into the receiver. A differential pair sends the signal on two wires (A and B) that carry equal-and-opposite voltages. Noise couples into both wires almost equally, so when the receiver subtracts B from A the noise cancels — this is common-mode rejection.

Injected noise40%
Single-ended (RS232) — noise adds directly to the received waveform. Crank up the slider and the logic levels start to smear.
Differential (RS485/RS422) — A and B both pick up the same noise; the receiver's subtraction (A − B) recovers a clean signal.
Why this matters for your PCB: because rejection depends on both wires seeing identical noise, differential pairs must be routed close together, length-matched, and over a continuous ground plane. Break that symmetry and you throw away the very advantage you paid for. (Module 7 covers layout.)

The single most useful reference in this whole course. Keep it on your bench.

Parameter RS232 RS485 RS422
Signal type Single-ended Differential Differential
Logic / voltage levels ±3 V to ±15 V (driver) Diff. ±1.5 V to ±5 V Diff. ±2 V to ±5 V
Max cable length* ~15 m (50 ft) ~1200 m (4000 ft) ~1200 m (4000 ft)
Max data rate* ~20 kbps (up to ~1 Mbps short) ~10 Mbps (short) / ~100 kbps @ 1200 m ~10 Mbps (short) / ~100 kbps @ 1200 m
Topology Point-to-point Multidrop / multipoint bus Point-to-point (multi-receiver)
Max drivers / receivers 1 driver / 1 receiver 32 drivers / 32 receivers (unit loads) 1 driver / 10 receivers
Duplex Full duplex Half duplex (typ.) / full with 2 pairs Full duplex (separate TX/RX pairs)
Bus termination Not required 120 Ω at each end 120 Ω at receiver end

* Cable length and data rate are inversely related — the reference points above are common industry figures, not simultaneous maximums. The next section makes that trade-off interactive.

You can have distance or speed — rarely both. As cable gets longer, capacitance and reflections force you to slow down. This log-log Plotly chart plots the practical operating envelope for each interface. Hover any point for the exact values.

Reading the chart: RS485/RS422 (the differential pair) hold a high data rate far longer before the length wall forces them down, and they reach ~1200 m. RS232 collapses early — its single-ended swing simply can't fight cable capacitance and noise over distance.

A one-line decision aid for the schematic-capture stage:

Choose RS232 when…

Short cable (<15 m), a single device, full-duplex, and you want the simplest possible link — e.g. a bench instrument, a console/debug port, or a legacy PC peripheral.

Choose RS485 when…

Long cable and/or multiple devices on one bus (multidrop) — e.g. Modbus RTU, building automation, motor networks, sensor strings. Half-duplex is fine and you want maximum node count and noise immunity.

Choose RS422 when…

Point-to-point over distance and you need simultaneous two-way (full-duplex) traffic, or one transmitter broadcasting to several receivers — e.g. encoder feedback, long full-duplex links.

Rule of thumb: the moment your cable leaves the enclosure or exceeds a few metres, reach for a differential interface (RS485/RS422). Distance and electrical noise are exactly where single-ended RS232 falls apart.

Five questions. Click an answer to check it — feedback appears instantly.