Modbus Poll Bytes Missing Error Fixed ((exclusive)) Direct
Always prefer converters utilizing genuine FTDI or Silicon Labs (CP210x) chipsets over cheap CH340 clones. 4. Poor Wiring and Electrical Noise (EMI)
Insufficient power to an RS485 converter or the slave device itself can lead to truncated or "missing" data packets. Ensure your transceiver has a stable common ground with the PC.
This error indicates that Modbus Poll sent a request and received a response, but the incoming data packet contained fewer bytes than the Modbus protocol requires for that specific command. Because the packet is incomplete, Modbus Poll cannot parse the data and drops the connection or flags an error. modbus poll bytes missing error fixed
Based on common troubleshooting for Modbus RTU communication, the "bytes missing" or "bytes not received" error in Modbus Poll
Even though RS-485 is differential, a floating ground can cause enough noise to drop bytes. Ensure a signal ground wire connects all devices. 2. Adjust Timing and Timeouts Always prefer converters utilizing genuine FTDI or Silicon
While a complete mismatch usually causes a "Timeout," a slight clock drift at high baud rates (like 115200) can cause missing bits/bytes. Try lowering the Baud Rate to 9600 for testing.
If the master and slave are not running at the exact same baud rate, parity, or stop bits, the data will be interpreted incorrectly, causing the serial port to cut off the message prematurely. Ensure your transceiver has a stable common ground
Some PLCs / meters send incomplete frames. Workaround:
If you are using a laptop with a USB-to-RS485 converter, the error might not be Modbus at all—it might be the USB driver. Windows USB drivers often introduce a high latency (delay) that breaks the strict timing required for Modbus RTU.
: Use the Display > Communication window in Modbus Poll to see the raw Hex strings. Look for the RX (receive) line to see if the slave is actually sending fewer bytes than requested.
Cheap USB-to-RS485 converters frequently drop bytes under heavy traffic due to poor internal buffer management. Update Drivers Open Windows Device Manager. Locate your COM port.
