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