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Resources · Interactive Guide
Three serial ports, three different jobs. Why the differential pair changed everything, who is allowed to talk on each wire, how far each one really reaches — and which port to use where.
All three move serial bytes — the difference is the electrical wire underneath. RS-232 swings a single wire against ground: simple, but short and noise-prone. RS-422 went differential: one strong talker, kilometre reach, many listeners. RS-485 added the last piece — drivers that can let go of the wire — so many devices can share one pair, taking turns. That is why the field bus of industrial automation is RS-485.
RS-232 measures its signal against ground — whatever noise lands on the wire adds straight onto the data, and once it crosses the ±3 V receiver threshold the byte is garbage. RS-422 and RS-485 send the signal and its mirror image on a twisted pair: noise shoves both wires together, the receiver subtracts, and the data walks out clean.
RS-232 · single-ended
the hit knocks the signal off its reference — and it stays off
RS-422 / RS-485 · differential
both wires shoved together — the difference never changes
What the receiver actually measures
RS-232 — volts against ground
RS-422 / RS-485 — A minus B
The RS-232 receiver needs the wire to swing clear past ±3 V — everything between is undefined. A differential receiver decides the bit on just ±200 mV of difference, so even a signal attenuated by a kilometre of cable still reads clean. Detail worth knowing: RS-232 logic is inverted — a 1 (mark) is the negative level, a 0 (space) the positive one.
Electrically similar, operationally worlds apart. RS-232 is two devices in a private call — both may speak at once. An RS-422 driver is always on: one voice, many ears, nobody else may drive the pair. RS-485 drivers can tri-state — release the wire — so any node may transmit, one at a time. That single feature is what makes a multi-drop bus possible.
RS-232 — full-duplex, exactly two
two wires, both directions at the same time — and nobody else can join
RS-422 — one talker, many listeners
the always-on driver broadcasts; every tap hears it — none may answer on this pair
RS-485 — everyone may talk, in turn
the question goes out, S2 answers back on the same two wires — tri-state drivers taking turns
Single-ended signalling runs out of headroom fast: the RS-232 spec caps cable capacitance (≈2500 pF — about 15 m of typical cable). The differential receiver only needs 200 mV of difference to decide a bit, so RS-422 and RS-485 carry on to ~1200 m — the full length holds up to ~100 kbps, and above that the usable length falls roughly in proportion to speed (1 Mbps ≈ 120 m).
Same start, same cable — the violet RS-232 signal is out of spec past the 15 m mark, while the 200 mV differential receivers still read the attenuated pair at 1200 m.
| RS-232 | RS-422 | RS-485 | |
|---|---|---|---|
| Electrical | |||
| Signalling | Single-ended, measured against ground | Differential pair | Differential pair |
| Driver levels | ±5 … ±15 V swing | ≥ ±2 V differential (into 100 Ω) | ≥ ±1.5 V differential (into 54 Ω) |
| Receiver threshold | ±3 V — anything between is undefined | ±200 mV | ±200 mV |
| Common-mode window | n/a — both ends share one ground | ±7 V | −7 V … +12 V |
| Noise immunity | Low — noise adds straight onto the signal | High — common-mode rejected | High — common-mode rejected |
| Bus & topology | |||
| Devices | 1 driver : 1 receiver | 1 driver : up to 10 receivers | 32 unit loads — up to 32 drivers & receivers (256 with ⅛-UL parts) |
| Duplex | Full — separate TX and RX wires | Full — two one-way circuits (a pair each) | Half on 2-wire (4-wire variant exists) |
| Topology | Point-to-point only | Point-to-point / one-to-many broadcast | True multi-drop bus, daisy-chained |
| Driver off-bus | n/a | Driver always on — cannot share the pair | Tri-state drivers — talkers take turns |
| Termination | None | 100 Ω at the far (receiver) end | 120 Ω at both ends of the trunk |
| Practical | |||
| Distance | ~15 m (cable ≤ 2500 pF) | ~1200 m | ~1200 m |
| Speed | 20 kbps by the spec; real ports run 115.2 kbps on short leads | Up to 10 Mbps (speed–length trade-off) | Up to 10 Mbps (speed–length trade-off) |
| Signal wires | TX, RX + GND (plus handshakes) | 2 pairs (4 wires) + common | 1 pair (2 wires) + common |
| Typical use | Config / console ports, modems, lab gear | Encoders, long full-duplex links | Modbus RTU field buses — meters, drives, sensors |
Handy mental model: RS-485 is a superset of RS-422 — a 485 transceiver can listen on a 422 line, and 4-wire RS-485 recreates the 422 topology while keeping tri-state drivers. The reverse is not true: a 422 driver must never sit on a shared 485 bus, because it cannot release the wire.
Building an RS-485 bus? The RS-485 Wiring Guide covers termination, biasing, grounding and daisy-chain wiring hands-on.
The three are not electrically compatible — RS-232 swings up to ±15 V against ground and would stress a differential transceiver, so never join them with bare wire. Converters and gateways translate cleanly, and most Wiman hardware speaks more than one standard already: