Modbus Poll Bytes Missing Error !!install!! Guide
If your slave is configured for Modbus ASCII (which uses start/end colons and LRC checksum) and your master is in RTU mode (using silent intervals and CRC), the ASCII response will look like a truncated RTU frame. Modbus Poll will receive a few bytes, see no valid CRC, and throw the missing bytes error.
What is Modbus and How does it work? | Schneider Electric India modbus poll bytes missing error
Misconfigured baud rate, parity, data bits, or stop bits. If your slave is configured for Modbus ASCII
| Cause Category | Specific Issue | Probability | | :--- | :--- | :--- | | | Baud rate mismatch between master and slave | Very High | | | Incorrect character format (stop bits, parity) | High | | | Slave response timeout too short in Modbus Poll | Medium | | Noise / Signal | Poor cable shielding or ground loops | High | | | RS485 termination resistor missing/misplaced | Medium | | Slave Firmware | Slave device power cycles during response | Low | | | Slave CPU overload (delayed response) | Medium | | Frame Collision | Half-duplex contention (two masters) | Low | | | Echo on RS232-to-RS485 converter | Medium | | Schneider Electric India Misconfigured baud rate, parity,
If serial problems persist and your budget allows, convert the network to using serial-to-Ethernet gateways (e.g., Moxa NPort series). TCP/IP handles packet integrity, retries, and error checking, eliminating "bytes missing" errors caused by serial timing.
A clean, continuous stream of bytes rules out the physical layer, pointing to firmware or configuration.
Essentially, the software knows how many bytes to expect based on the function code and register count; if it gets fewer, it triggers this specific error instead of a full "Timeout". 🛠️ Primary Causes & Fixes 1. Incorrect Register Quantity