Modbus — Poll Bytes Missing Error Fixed [2021]
Common Causes & Solutions
This error occurs when the Modbus Poll master expects a specific number of bytes based on the requested function code (e.g., Read Holding Registers) but receives a truncated or incomplete message from the slave device. Response Timeout (Too Short) :
- Error rate went from ~5% of polls → zero in my 72h test.
- Logged data now aligns perfectly with oscilloscope captures on the RS485 line.
- No more “mystery” gaps – the debug mode shows partial frames and why they were retried.
"Bytes Missing" error in Modbus Poll typically indicates that the master received a response, but the data packet was smaller than what the Modbus protocol or the specific function code requires. This differs from a "Timeout" error, where no response is received at all. Modbus Poll: "Bytes Missing" Error Analysis Report 1. Core Cause: Partial Data Reception modbus poll bytes missing error fixed
3. Log Communication for Trend Analysis
Common Symptom Patterns
- The Fix: Check if your hardware has a "T1.5 / T3.5" inter-character timeout setting. If the slave device is sending bytes with tiny pauses between them, you may need to adjust the master to be more tolerant of these gaps.