RS-485 termination and bias checker

Check where terminating and bias resistors belong on a Modbus RS-485 network, for two-wire and four-wire wiring.

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Wiring
Wiring
Signal common conductor
Resistors fitted

Count every one switched on, including inside devices

Four-wire RS-485

2 things to fix

  • Check: TerminationsTerminate both ends of each pair and nowhere else.2 of 4
  • Fail: Bias (polarisation)Bias the bus at one point only. Switch off the extra bias resistors.2 locations
  • Pass: Signal commonA common conductor runs with the pair

These checks follow the Modbus serial line guide. The device manuals decide whether termination and bias are built in or switchable.

How it’s calculated

2-wire trunk two end terminations4-wire trunk two terminations per balanced pairPolarization one location when required

Fix before commissioning: 2 terminations where 4 belong, one at each end of each pair; bias at 2 points instead of one.

Tip: Many meters ship with a termination switch turned on. On a bus of ten meters, turn off all but the one at the far end.

How RS-485 termination and bias work

A Modbus RS-485 bus needs a terminating resistor at each end of the trunk and nowhere else, bias at no more than one point, and a common conductor run with the pair. Break one of these rules and the usual symptom is CRC errors and timeouts that come and go with traffic, cable length or the number of devices polled. The commonest cause on site is a row of meters that each arrived with its termination switched on.

The checker tests a bus against the Modbus over Serial Line Specification and Implementation Guide V1.02 (Modbus Organization, 2006), called the guide below.

Termination

Two-wire: 2 terminations; four-wire: 2 per pair, 4 in total

A termination matches the end of the cable to its characteristic impedance, so the signal is absorbed there and does not reflect back along the line. The guide puts one near each end of the trunk, allows no more than two on one pair, and never puts one on a branch (derivation) cable. Use a 150 Ω, 0.5 W resistor, or 120 Ω (0.25 W) in series with 1 nF (10 V minimum) where the pair is also biased.

Bias (polarisation)

At most 1 location; pull-up to 5 V on D1, pull-down to common on D0, 450 Ω to 650 Ω each

When no driver is active, the pair floats and noise can look like a start bit. If any device needs the idle line held in a known state, one pair of resistors does it for the whole bus, usually at the master. Every other device must have its bias switched off. The guide allows 32 devices on a bus without a repeater, and 4 fewer when the bus is biased.

Signal common

D0, D1 and Common

The guide defines three circuits for two-wire Modbus: D0, D1 and Common. The common holds every transceiver inside the EIA-485 common-mode range of -7 V to +12 V. Without it, the earth potential difference between two panels can push a receiver outside that range. Connect the common to protective earth at one point only, generally at the master. If you bond it at every panel, it carries ground-loop current.

The guide limits the trunk to 1,000 m at 9,600 baud on AWG26 or thicker cable, and halves that when a four-wire cable is used as a two-wire bus. Category 5 cable is good for 600 m. Each branch must be shorter than 20 m, and a multi-port tap with n branches limits each one to 40 m ÷ n. Longer runs, higher baud rates and more devices are possible with the right cable and transceivers, but they need a network design, not a checklist.

RS-485 wiring examples

Ten meters, every termination switched on

Ten energy meters share one pair, and each arrived with its 120 Ω termination switched on. Ten 120 Ω resistors in parallel put 12 Ω across the line. EIA-485 drivers are specified into 54 Ω, so the differential voltage collapses and replies fail at random. Bias and common are correct, so there is 1 thing to fix. Switch off the termination in nine meters and keep it on in the meter at the far end of the trunk. Terminate the near end at the master.

Two-wire RS-485 1 thing to fix Open in the calculator

A four-wire bus biased twice

A four-wire bus has two pairs, one from the master to the devices and one back, and each pair needs a termination at both ends: four in all. This bus has one on each pair at the master end, and bias is switched on in both the master and a meter. That makes 2 things to fix: the two far-end terminations are missing, and there is one bias point too many. Terminate both pairs at the far end and switch off the bias in the meter.

Four-wire RS-485 2 things to fix Open in the calculator

A correctly wired two-wire bus

One termination at each end of the pair, bias at the master only and a common conductor in the cable: all three wiring checks pass. If the bus still logs errors, check the polarity at each device next, then the cable type and the branch lengths.

Two-wire RS-485 Wiring checks pass Open in the calculator

How many RS-485 terminations a bus needs

What the checker reports for each number of terminations switched on, on a two-wire and a four-wire bus.

Terminations fittedTwo-wireFour-wire
02 missing4 missing
11 missing3 missing
2Correct2 missing
31 too many1 missing
42 too manyCorrect
64 too many2 too many

Download this table (CSV)

Questions about RS-485 termination and bias

Does a short RS-485 bus need termination?

A short bus often works without it, because reflections die away before the receiver samples the bit. The guide still requires a termination at both ends, and it costs little. Two 150 Ω terminations on a bus biased with 650 Ω resistors draw about 3.6 mA from the 5 V bias supply and leave about 0.27 V across the idle pair. That is above the 200 mV EIA-485 receiver threshold. A 120 Ω plus 1 nF termination draws no DC current at all.

Where should the termination go?

At the two physical ends of the trunk cable. These are not always the master and the highest address. A master in the middle of a run needs no termination, and the devices at the two ends of the cable do. Trace the cable before you set any switches, and never terminate a branch.

Why does too much bias cause problems?

The guide counts one bias point as the load of 4 devices. Three meters with their bias switched on take up roughly 12 of the 32 device loads on the bus. The drivers then deliver less differential voltage, and the devices furthest along the cable usually stop replying first.

Is 120 Ω or 150 Ω correct?

Both are allowed. The guide gives 150 Ω, 0.5 W as a plain resistor, and 120 Ω in series with 1 nF when the pair is also biased. The capacitor stops the bias current flowing through the termination, so the idle voltage stays well above the receiver threshold. Twisted-pair data cable is usually about 120 Ω, and most meters with a built-in termination use 120 Ω. Use what the device provides, as its manual says.

Why does one device not reply when the others do?

Reversed polarity is the usual cause. The guide names the lines D0 and D1: D0 is the EIA-485 A line, D1 is B, and D1 is the more positive line when the bus is idle. Many device and transceiver makers print A on the positive line instead, so a device wired A to A can be reversed. A reversed device does not reply, and the other devices keep working. Swap its two wires, and label the pair D0 and D1 on the drawings.

Limits of this result

  • This does not size resistors or replace the device manufacturers’ instructions.
  • Branch lengths, repeaters, isolation, earthing and surge protection need their own design.
  • Passing these checks does not prove the bus is error-free; check the error counters at commissioning.

Read Modbus devices on the Gateway

A ZMB reads Modbus RTU equipment on its own short RS-485 line and sends the registers over the wireless mesh, so no long trunk has to run back to the panel. The Industrial version of the ZGW-20 Gateway takes an I/O board with an RS-485 port for Modbus devices wired to it directly.

Related guides

Sources

  1. Modbus Serial Line Protocol and Implementation Guide (opens in a new tab) (PDF) Modbus Organization, V1.02, 2006-12-20