If you can tell me (PLC, Sensor, VFD) you are trying to communicate with and whether you are using RS485 or TCP , I can provide a more specific troubleshooting strategy.
cabling specifically rated for RS485.
In RS485, if two devices talk simultaneously, you get partial frames → bytes missing.
Modbus RTU and TCP protocols require specific frame lengths. A standard Modbus RTU response frame contains a slave ID, function code, data bytes, and a 2-byte CRC check. If Modbus Poll expects an 8-byte response but only receives 6 bytes, it triggers the error. This differs from a "Timeout" error, where no data arrives at all. 1. Eliminate Serial Timing and Timeout Issues modbus poll bytes missing error fixed
In industrial automation and control systems, Modbus is a widely used communication protocol for exchanging data between devices. However, during a recent deployment, our team encountered a critical issue with Modbus polling, where bytes were reported missing. This write-up details the problem, the investigation, and the solution implemented to resolve the issue.
Sometimes, the "Bytes missing" error is actually a disguised exception response. If the slave device encounters an error (e.g., trying to read an illegal data address), it should reply with an Exception Code. If this exception frame is malformed or interrupted, it can manifest as a missing bytes error in your testing software. 5. Character Framing (Stop Bits and Parity)
Look at the exact hex bytes being sent and received. If you see the master send a request for 10 registers, and the slave replies with an Rx frame that is noticeably shorter than it should be, you've confirmed the missing bytes. Step 4: Check for Ground Loops and Noise If you can tell me (PLC, Sensor, VFD)
Fixing this issue requires checking serial timings, wiring integrity, and device configurations. 1. Adjust Timing Settings (Most Common Fix)
is usually resolved by addressing timing, hardware, or configuration issues rather than a software bug. Here is a review of how this issue is typically fixed: Common Fixes for "Bytes Missing" Error Adjust Serial Port Settings:
Add a across the Day+ and Day- lines at both physical ends of the RS485 network. This stops signal reflections that mimic missing bytes. Address Electrical Noise Use Shielded Twisted Pair (STP) cables for all serial runs. Modbus RTU and TCP protocols require specific frame lengths
What is the you are trying to use (01, 02, 03, 04, etc.)?
The Modbus standard dictates that if you use No Parity , you must use 2 Stop Bits . If you use Even or Odd Parity , use 1 Stop Bit . Many users fail this "8-N-2" vs "8-E-1" rule. 4. Address Buffer Overruns and USB Converters
To truly master Modbus diagnostics, you should be able to calculate and read a Modbus frame manually. Let's look at what a standard read request and response look like.
Use a tool like Modbus Tools' Serial Monitor to capture the raw data and see exactly what the slave is sending back. Reduce Poll Rate: Lower the frequency of requests. Summary Table Potential Cause Baud Rate Mismatch Reconfigure Master and Slave to match. No Termination Resistors Add 120Ω resistors at ends of RS485 bus. Low Response Timeout Increase timeout in Modbus Poll Settings. EMI/Noise Check shielding, ground properly. Too many registers requested Reduce register count in Read Definition.
Add a (typically 5ms to 10ms) in the slave code before it transmits the response.