Scenarios

Poll a Modbus RTU device reliably

Build a repeatable request, verify CRC and separate timeout from framing failures.

  1. SPU poll command
  2. RS-485 adapter
  3. Modbus device
Reviewed: Applies to: Serial Port Utility 6.6.x
Serial, Network and Cloud Console Scenarios
Scenarios

Scenarios

Scenario 5: poll a Modbus RTU device reliably

Use this when you need a repeatable read request and must distinguish a silent device from an invalid frame.

  1. Match the RS-485 wiring, baud rate, parity and stop bits from the device manual; keep only one bias and termination arrangement on the bus.
  2. Build one known-good Modbus request in Hex, including slave address, function, register address and count.
  3. Let the CRC tool calculate the request CRC, then store the frame in Quick Send.
  4. Send manually first. A valid response must contain the same slave address and function, the expected byte count and a valid CRC.
  5. Only after the manual exchange is reliable, enable periodic send with an interval longer than the device response time.

A timeout means no complete response arrived; a CRC or length failure means bytes arrived but the frame is invalid. Check them separately. If several slaves share the bus, poll one at a time and leave an idle gap between frames.

Was this document helpful?

Expose a serial device over TCP

Let a remote application access an RS-232 or RS-485 device through a TCP server.

Read guide

Connect an isolated site outward

Use an outbound TCP client when the device site cannot accept inbound connections.

Read guide

Monitor a serial conversation inline

Capture both directions between an existing host and device without changing the protocol.

Read guide