Modbus Poll Bytes Missing Error Fixed [2021] -

If using , you must use 2 Stop Bits to maintain the 11-bit frame character structure. 3. Inspect Physical Wiring and Hardware

A common mistake is setting a single, aggressive scan rate for all registers on all devices. If you are polling a slow device faster than it can respond, you will constantly slam into timeouts and see incomplete responses. On a busy 9600 baud network, if one transaction takes 50ms, a 500ms scan rate across many devices creates a queue, leading to data collisions and errors.

Modbus RTU strictly requires 8 data bits. Modbus ASCII uses 7 data bits.

The "bytes missing" message indicates that the master (your computer) sent a request, but the response received from the slave device did not contain the expected number of bytes defined by the Modbus protocol, or it was cut off prematurely. modbus poll bytes missing error fixed

Last updated: October 2025 – tested against Modbus Poll 9.6.0 and Windows 11 24H2.

: If using a USB-RS485 adapter, ensure you are using the latest FTDI or Prolific drivers .

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. If using , you must use 2 Stop

If the Slave device is a bit slow (perhaps it’s an old PLC or a complex sensor), it might start sending data, pause to process, and then finish. Modbus Poll might see that pause as the end of the message.

This public link is valid for 7 days and shares a thread, including any personal information you added. This link or copies made by others cannot be deleted. If you share with third parties, their policies apply. Can’t copy the link right now. Try again later.

Increase this from the default (usually 1000ms) to 3000ms to see if the error clears. If you are polling a slow device faster

: A mismatch in baud rate, parity, or stop bits can cause framing errors where bytes are misinterpreted or dropped.

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.

This error means the slave device started to send a response, but the data packet was cut short, corrupted, or took too long to arrive. The master (Modbus Poll) expected a specific number of bytes based on the Modbus protocol rules but simply didn't get them.

The "bytes missing" error often indicates that the master device (Modbus Poll) stopped listening before the slave device finished sending its data. Increasing the response timeout is the most frequent fix. How to deal with Modbus communication problem? - XWiki