Physical layer problems are extremely common in RS-485 networks. This includes crossed A/B wires, the absence of a common ground reference between devices, a missing or improperly sized termination resistor (especially on long cable runs), or a faulty USB-to-RS485 converter (adapters with CH340 chips are known to occasionally cause framing issues). Any of these can lead to incomplete packet reception.
If your timeout is set too aggressively (e.g., 50ms), the slave device might still be processing the request or preparing the response. Modbus Poll will time out, assume the bytes are missing, and move on. modbus poll bytes missing error fixed
Fixing the "Modbus poll bytes missing" error is a linear process: start with the physical layer (signal ground, termination, shielding), then verify communication parameters (especially parity), and finally use the Communication Traffic window to check frame integrity and CRC order. Physical layer problems are extremely common in RS-485