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Resources · Interactive Guide
Daisy-chain, terminate, bias, ground — in that order. Why the differential pair shrugs off noise, where the 120 Ω resistors really go, and the six mistakes that cause almost every dead bus in the field.
RS-485 sends every bit twice, as mirror images — when A goes high, B goes low. The receiver reads only the difference between the wires. Noise from motors, drives and contactors hits both wires of the twisted pair equally, so the difference — your data — survives untouched. That is the whole trick, and it is why RS-485 runs 1200 m through electrically hostile plants.
The two signal dots travel as mirror images. Inside the noise zone both get pushed the same way at the same time — but their difference never changes, so the receiver output stays clean.
RS-485 is a transmission line: the cable must run from the master through each device to the last one — in and out of every terminal block. Branches, stars and long stubs reflect signal edges back into the bus and corrupt frames at random.
✓ Correct — daisy-chain
one continuous pair, terminated at the two far ends
✕ Wrong — star / stubs
every branch point reflects edges back into the bus
Down at the terminals it looks like this: the three conductors — A, B and signal common — arrive at each device, land on its terminals, and continue out the same terminals to the next device. Two conductors under one screw (or one ferrule) per terminal: the incoming wire and the outgoing wire.
A lands on every D+, B on every D−, common on every G — one continuous run, device to device. The only places a 120 Ω resistor sits across D+/D− are the two physical ends (many masters and converters can switch theirs in internally). Watch the mirrored pair travel the whole trunk through every device.
Doing it neatly in the panel: twist the incoming and outgoing conductor together into one bootlace ferrule per terminal — or use devices with separate IN/OUT terminals. Never run a long pigtail from the trunk to a device (that is a stub). And remember: unscrewing a device wired this way breaks the chain for everything after it — join the pair through a terminal block if a device must be removable.
A fast edge travelling down a cable behaves like a wave. If the far end is open, the wave bounces back and collides with the next bits. A 120 Ω resistor across A–B at each far end matches the cable impedance and absorbs the wave. Only the two physical ends get resistors — never every device. On short, slow buses (≤ 9600 bps, tens of metres) you can usually skip them; past that, terminate.
✕ unterminated — edge reflects
the bounce collides with the next frame — random CRC errors
✓ terminated — edge absorbed
the resistor soaks up the wave — clean bus at any length
Fail-safe biasing is the quieter cousin of termination: with no driver active, an unbiased pair floats and receivers read phantom characters. The master / converter end should pull A up and B down (typically 560 Ω–1 kΩ) so the idle bus rests in a defined state. Most Wiman gateways and converters bias the bus for you.
| From (master / gateway) | To (every slave) | Note |
|---|---|---|
| D+ / A / Data+ | D+ / A on every device | one wire of the twisted pair |
| D− / B / Data− | D− / B on every device | the other wire of the same pair |
| GND / COM | signal common (3rd wire) | recommended — holds common-mode in range |
| Shield | earth at ONE end only | panel / master end — never both ends |
A to A, B to B — always straight through, never crossed. Beware: some vendors label the pins backwards (TI convention vs. the standard). If a wired device stays silent, swapping A/B at that device is the first thing to try — it is harmless.
Use shielded twisted pair with ~120 Ω characteristic impedance (24 AWG industrial RS-485 cable — not flat telephone wire, not two cores picked from a power cable). The twist is not cosmetic: noise couples hardest into whichever wire is nearer the source. Twisting swaps the nearer wire every half-turn, so A and B collect the same noise — and the receiver’s A − B subtraction erases it.
✕ parallel wires — the near wire soaks it all up
✓ twisted pair — the twist shares the noise equally
Noise pulses rain down from the power cable above. On parallel wires every pulse lands on the near wire (A) — the pair picks up unequal noise. On the twisted pair the top position alternates, so pulses land A, B, A, B — equal on both, and the difference cancels it out.
Keep the pair away from mains runs; cross them at 90° if you must. One bus segment supports 32 unit loads — modern ⅛-load transceivers stretch that to 256 devices.
| Baud rate | Reliable length | Typical use |
|---|---|---|
| 9600 bps | ~1200 m | metering, slow sensors — the classic |
| 19200 bps | ~1200 m | still full length |
| 38400 bps | ~1000 m | fast polling on long trunks |
| 115200 bps | ~300 m | short runs, panel-level links |
Rule of thumb: the full 1200 m holds up to ~100 kbps; above that the usable length falls roughly in proportion to speed (the classic curve: 1 Mbps ≈ 120 m) — and field practice derates further, which is what the table reflects. Need more distance or more devices? Split the bus with a repeater, or move the trunk to Ethernet with a converter (below).
RS-485 is differential but not floating — receivers only tolerate roughly −7 to +12 V of common-mode. A signal-common wire between all nodes keeps every transceiver inside that window. Two-wire-only buses work until the day the potentials drift.
Land the shield on earth at one end (panel / master side). Grounding both ends invites a ground loop — circulating current that couples hum straight into the pair.
Between buildings, across big machines, or anywhere earth potentials differ, use galvanically isolated ports — the fault current flows through the isolation barrier’s nothing instead of your transceiver.
The #1 field fault. Nothing works, or you get pure garbage. Vendors label A/B inconsistently — if a device stays silent, swap the pair at that device first.
Branches and stars reflect every edge back into the bus. RS-485 is a line: in and out of each device, stubs as short as possible (< 30 cm).
Works on the bench, fails at site. Long, fast buses need 120 Ω at both far ends — and only there, never at every device.
An idle, unbiased bus floats — receivers read noise as random characters. The master / converter end should bias the pair (pull-up on A, pull-down on B).
Earth potential differs between panels — grounding the shield twice makes a ground loop that hums right through your data. One end only.
A on one twisted pair and B on another throws away the whole noise immunity. A and B must share one twisted pair — never split them.
Every Wiman gateway, converter and IO card speaks RS-485 natively — with biasing on board and screw terminals sized for field cable: