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Modbus Poll Bytes Missing Error Fixed [upd] -

The Ultimate Guide: How to Fix the "Modbus Poll Bytes Missing" Error (Step-by-Step)

Introduction

If you work in industrial automation, SCADA systems, or PLC programming, you have likely encountered the dreaded "Modbus Poll Bytes Missing" error. This message, appearing in debugging tools like Modbus Poll, ModScan, or custom Python scripts, can bring data acquisition to a grinding halt.

  • In Modbus Poll: Setup → Serial Communication → Response Timeout Inter-Character Timeout
  • Increase inter-character timeout to 50–100 ms (or 3.5 char times).

: Match the serial parameters (e.g., 9600 baud, 8N1) exactly between Modbus Poll and your slave device. Buffer Overflows or Hardware Limitations modbus poll bytes missing error fixed

The Fix: Increase the Response Timeout in your Connection Setup. Giving the device an extra 500ms can often bridge the gap. The Ultimate Guide: How to Fix the "Modbus

6. Address or Function Code Mismatch

Sometimes the error is simply that you are asking for data the device doesn't have, causing it to send a truncated exception response. In Modbus Poll: Setup → Serial Communication →

  • Go to Device Manager (Windows) → Ports (COM & LPT) → Right-click your USB Serial Port → Properties → Port Settings → Advanced.
  • Increase Receive Buffer to maximum (if available).
  • Alternatively, replace with an industrial-grade converter (e.g., Moxa UPort, Advantech).