What could cause such micro-interruptions? Mara’s eyes drifted to the power supply racks. They shared a route with the RS-485 bus, and last month a new VFD had been installed near Slave 12. VFDs were notorious for generating EMI if not filtered or grounded correctly. She walked the route at different times, watching the VFDs shift motor speeds as production loads changed. At moments when the VFD switched frequency, tiny spikes appeared on her oscilloscope across the bus lines.
In Modbus Poll, increase the if using a hardware flow control line. 5. Isolate Software and Driver Conflicts modbus poll bytes missing error fixed
If the error follows the slave device, the problem is likely in its firmware, configuration, or processing speed. Advanced Troubleshooting: Analyzing Modbus Frames What could cause such micro-interruptions
Many cheap converters (e.g., Prolific PL2303) have a default latency timer of 16 ms. Modbus Poll expects a clean stream. If the converter buffers 16 ms of silence, it may chop the final 2 bytes of the CRC. VFDs were notorious for generating EMI if not
Modbus Poll defaults to 1000 ms (1 second) timeout. But some slaves (especially older PLCs or wireless modems) need 1500–2000 ms. The old "fix" (increase to 3000 ms) worked sometimes but introduced latency.
Timing errors mimic byte-missing conditions, especially if the slave is slow to compute or the network path includes wireless links.
This is perhaps the most common reason for this specific error when using Ethernet gateways.