Connecting an RS485/Modbus Radar Level Meter to a PLC

A Modbus RTU radar can communicate reliably only when the electrical bus and the data interpretation are both correct. Successful polling proves little if the PLC reads the wrong register, swaps bytes or scales distance as level. Commission in two stages: first establish a clean RS485 exchange, then validate each engineering value against the instrument display.

Match every serial parameter

The PLC master and radar slave must use the same baud rate, data bits, parity and stop bits. Give every device a unique slave address. Record the settings before connecting a multi-device bus; duplicate addresses can create intermittent or corrupted replies that resemble wiring noise.

Start with one instrument on a short known-good cable. Confirm requests and valid replies using the PLC diagnostic or a trusted Modbus test tool. Add the field cable and remaining devices only after the basic exchange is proven.

Build a proper RS485 bus

Use a daisy-chain trunk rather than a star, keep stubs short and maintain the twisted pair polarity consistently. Terminal naming varies among vendors, so A/B labels alone are not conclusive; use the signal definition or test safely. Include a signal reference conductor where the interface design requires it.

Termination is normally placed at the two physical ends of a long bus and should match the cable impedance. Biasing should exist at one designed location. Too many terminators or bias networks load the driver, while none can allow reflections or an undefined idle state. Follow the network design rather than adding resistors by trial and error.

Resolve register addressing

A manual may show holding register 40001 while a PLC function block asks for offset 0 or 1. Confirm whether the documentation uses logical reference numbers or protocol offsets. Also verify the function code: reading input registers and holding registers are different operations even when displayed addresses look similar.

Request only supported contiguous ranges. An illegal-address exception is useful evidence, not a reason to increase timeout. Record the exact request and response during acceptance so future software changes can reproduce the working map.

Interpret data type and byte order

A level value may be a 16-bit integer, signed integer, 32-bit integer or IEEE 754 floating-point number. Multi-register values require the correct register order and byte order, often described as ABCD, CDAB, BADC or DCBA. A plausible but impossible value frequently indicates wrong interpretation rather than a sensor fault.

Apply the documented scale factor and units. Determine whether the register represents distance, level, percentage, signal quality or device status. Compare the decoded value with the local display at more than one level.

Design polling and fault handling

Set a polling interval that leaves bus capacity for every slave and allows response time under process conditions. Excessive requests create collisions or timeouts; a long timeout hides failed devices. Track CRC errors, exception codes, consecutive timeouts and last-good timestamps separately.

Define the PLC response to stale data and communication loss. Hold-last-value may be acceptable for display but unsafe for a control interlock. Use status registers where available and keep protective functions independent when required by the risk assessment.

Commissioning checklist

Match baud rate, data bits, parity, stop bits and unique slave address.
Prove one device on a short cable before rebuilding the full bus.
Use daisy-chain topology with designed termination and biasing.
Confirm function code and zero- or one-based register offset.
Validate data type, byte order, scaling and units.
Test stale-data, timeout and recovery behavior in the PLC.

Frequently asked questions

Why does the PLC receive a value that is far too large?

Check data type, signedness, register order, byte order and scale factor. Compare the raw register words with the documented example.

Will a slower baud rate fix every RS485 problem?

No. It can improve tolerance, but it does not correct star wiring, duplicate addresses, polarity errors, poor reference grounding or excessive termination.

Should the PLC use the last value after communication loss?

Only under a documented control philosophy. Mark it stale and define a safe action; a frozen value must not silently operate a critical interlock.

Need help checking a level-measurement loop? Send the instrument model, vessel drawing, wiring diagram, PLC input details and required range through our contact page. METRAVON can review the signal chain and acceptance scope before commissioning.