Valid data appears briefly, followed immediately by the error message. Top 5 Causes and How to Fix Them 1. Incorrect Serial Communication Settings
Do not guess. Use this diagnostic checklist.
Ensure the Master and Slave are identical (e.g., 9600). modbus poll bytes missing error fixed
Ensure you aren't trying to read "holding registers" (4xxxx) using a "discrete input" (1xxxx) function code. 5. Inspect for Inter-Char Timeouts
Her breakthrough came when she added timestamps to the sniffer’s capture. The missing bytes always coincided with short spikes on the bus — micro-interruptions lasting fractions of a millisecond. Not long enough to break the physical signal visibly, but enough to clip a few bytes from the frame. Valid data appears briefly, followed immediately by the
Some microcontrollers and PLCs require time to process a request before they can turn the RS485 line around to transmit. Implement Turnaround Delay Check the firmware of your slave device.
Fixing "Bytes Missing" is rarely about the software being "broken"—it’s about harmonizing the speed of light with the limitations of copper and silicon. Once you find that sweet spot of timing and shielding, the error vanishes like smoke. Use this diagnostic checklist
Use Shielded Twisted Pair (STP) cable and ground the shield at one end only . Summary Checklist Potential Cause Reflections Add 120Ω termination resistors. Noise Use shielded cables; separate from power lines. Timing Increase Response Timeout in Modbus Poll. Framing Match Baud, Parity, and Stop Bits exactly. Hardware Replace cheap USB converters with FTDI-based ones.
: Add small delays in your slave's code or reduce the number of registers requested in a single poll to lighten the load. Local Echo Issues
In Modbus Poll, go to Connection -> Connect and increase the Response Timeout (e.g., from 1000ms to 3000ms or higher).