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Fault 16
Pro

Fault 16 — RS-485 Terminator Disconnected

What you'll learn

**Reported symptom:** Modbus works at low baud rate but becomes intermittent at the normal rate, especially on long cable runs.

Lab time: ~12 minutes.

Lesson briefing

RS-485 Terminator Disconnected

Reported symptom: Modbus works at low baud rate but becomes intermittent at the normal rate, especially on long cable runs.

The circuit was commissioned from a known-good drawing. Treat every terminal as suspect, but test from source to load instead of guessing. Use voltage mode on an energised control circuit and continuity only after the simulated circuit is isolated.

Your task

Power down the network and verify end-to-end termination resistance and both terminator conductors.

Choose the fault category only when the measurements support it.

Hints

Hint 1

A correctly terminated de-energised bus normally measures about 60 Ω across A/B when two 120 Ω terminators are present.

Hint 2

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Hint 3

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This lesson uses 10 placed components on the lab canvas. Components are vendor-neutral (no proprietary trademarks); the wiring rules apply to any equivalent industrial part.

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Competency and practice field guide

Open RS-485 termination fault lesson: practice plan

Direct answer

The learner can map the bus, identify intended end points, separate bias and termination and correlate physical-layer evidence with frame errors and timeouts.

Written for controls and instrumentation learners diagnosing a serial network that works at low speed or short distance but fails intermittently after a wiring change.

an industrial communications bench used to compare serial and Ethernet evidence, requests, responses, addressing and device state while studying RS-485 termination loss, reflections and intermittent Modbus evidence
System map / 02

NODE 01observable

Scope

Two-wire or four-wire topology, trunk, stubs, physical end points, characteristic impedance, termination, bias, common reference, shield, baud rate, cable length and error evidence.

NODE 02observable

Signal path

Transmitter edge through differential cable, reflections and receiver threshold to serial frame, CRC, Modbus response, timeout and application state.

NODE 03observable

Baseline practice

Documented requests succeed repeatedly at the intended rate and topology with stable error counters and device values.

NODE 04observable

Edge cases

One termination open, excess termination, long stub, reversed pair, missing common, duplicate bias, noise, baud change, device restart and reconnect.

NODE 05observable

Fault practice

A topology, cable, termination, bias, reference, signal, serial, framing, identity or application mismatch.

NODE 06observable

Transfer to the job

The installed network inspected and captured against current device and physical-layer documentation.

Answer surface / 07

Where should RS-485 termination be installed?

Typically at the two physical ends of the main bus, using values appropriate to the cable and devices; do not treat every device as an end point.

Why can an unterminated RS-485 network sometimes work?

Short, slow or lightly loaded networks may have enough margin, but reflections can create intermittent failures as length, rate, topology or environment changes.

Motor, wiring and VFD path

Connect the control command to physical motor behavior

Move from safe control wiring and contactors into VFD parameters, measurements, faults and PLC command paths.