End.
Industrial environments are noisy. If your Modbus cable is running parallel to high-voltage power lines or VFD (Variable Frequency Drive) cables, the "missing bytes" are likely being "killed" by interference.
The most frequent cause is trying to use to talk to a device that is actually sending Modbus RTU data over an Ethernet connection (often via a serial-to-Ethernet gateway). modbus poll bytes missing error fixed
If you've worked through all the steps, use these final checks:
In serial communications, some devices need a small delay after the request is sent before they can switch from "sending" to "receiving" mode. A Pre-Delay of 50ms often stabilizes the connection. 2. Hardware and Wiring Fixes The most frequent cause is trying to use
The error would appear when the received frame length didn’t match the expected byte count based on the function code + register count. In my case, the inverter sometimes responded with a 3-byte delay between the slave address and CRC. Modbus Poll’s older parser was too strict—it timed out between bytes, assuming the frame was truncated. The result: dropped data, retries, and logging gaps.
Between frames, there must be a of at least 3.5 character times. This silence tells the slave device: “The previous message is complete; the next byte starts a new command.” If you can tell me (PLC
A: Yes. Some clones or poorly implemented slaves send incomplete frames. Test with a known-good slave to isolate the issue.
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.
Before diving deep, check:
Mara arrived to a sea of sticky notes on the operator console: “Check wiring,” “Rebooted RTU 3,” “Tried new master timeout.” She pulled up the log: a repeating pattern—reads to Slave 12 occasionally returned fewer bytes than requested. Not every poll, not in any neat cadence. Random, subtle, corrosive.