Modbus RTU is extremely sensitive to serial configuration mismatches. If the baud rate matches but the parity or stop bits do not, the master will receive "garbage" bytes, calculate that the frame is invalid, and report missing bytes.
How to fix it:
The developers behind Modbus Poll have listened. Starting from version 9.5.2 (released late 2023), a new option "Robust mode for broken slaves" was introduced.
Scenario: A water treatment plant engineer uses Modbus Poll to read flow meter data over a 500-meter RS485 cable. The error appears randomly every 30 seconds.
Symptoms:
Diagnosis:
Root Cause: EMI from VFDs and missing termination resistor.
Fix Applied:
Result: Zero "Bytes Missing" errors in 48 hours of continuous polling.
Sometimes bytes arrive late (due to slave processing delay or network latency). Modbus Poll may time out and report missing bytes.
In Modbus Poll software:
If the problem persists, post the Modbus Poll communication log (Display → Communication → Save) and the exact expected vs. received byte count — that usually reveals the exact cause.
The "Bytes Missing" or "Insufficient Bytes Received" error in Modbus Poll occurs when the software receives a response that is shorter than what the protocol requires for the requested command. This typically stems from a protocol mismatch, timing issues, or hardware interference. 1. Protocol Mismatch (Most Common)
The most frequent cause is trying to use Modbus TCP to talk to a device that is actually sending Modbus RTU data over an Ethernet connection (often via a serial-to-Ethernet gateway).
The Cause: Modbus TCP expects a 7-byte "MBAP" header before the data. Modbus RTU does not use this header. If your software is in TCP mode but the device sends RTU data, Modbus Poll sees the "missing" header bytes and triggers the error. The Fix: Go to Connection → Connection Setup.
Change the "Type" from Modbus TCP/IP to Modbus RTU over TCP/IP (sometimes called "Encapsulated RTU"). modbus poll bytes missing error fixed
Ensure your port number matches the gateway (common ports are 502 or 9999). 2. Timing and Timeout Issues
If the communication is slow (e.g., over a cellular/GPRS modem or long cable runs), the response may be split into multiple packets, and Modbus Poll might time out before the full message arrives. The Fix:
Increase the Response Timeout in the Connection Setup (try 1000ms or higher).
Add a Pre-delay or Inter-character delay (50ms–100ms) to allow the device more time to process the request. 3. Physical Layer and Hardware Issues
Hardware faults can truncate data packets or introduce noise that causes the software to reject "broken" bytes. Modbus RTU is extremely sensitive to serial configuration
It is structured to identify the problem, explain the root causes, and provide a step-by-step solution.
Poll a different register address or a different slave ID. If bytes are missing only for a specific register, the issue may be a slave firmware bug.