A client had a Schneider Electric Altivar 32 VFD connected via RS485 to a PC running Modbus Poll. Every 22 seconds, "Bytes Missing" appeared.
By systematically working through this list, starting with the and USB Latency , you will likely resolve the "Bytes Missing" error and establish a stable Modbus connection.
Physical layer problems are extremely common in RS-485 networks. This includes crossed A/B wires, the absence of a common ground reference between devices, a missing or improperly sized termination resistor (especially on long cable runs), or a faulty USB-to-RS485 converter (adapters with CH340 chips are known to occasionally cause framing issues). Any of these can lead to incomplete packet reception. modbus poll bytes missing error fixed
Fixing this issue requires checking serial timings, wiring integrity, and device configurations. 1. Adjust Timing Settings (Most Common Fix)
Requesting too many registers in a single Modbus function code call can exceed the slave device’s buffer capacity. A client had a Schneider Electric Altivar 32
Some slow slaves need more than 3.5 characters of silence. If your master sends the next poll too quickly, the slave might still be processing, leading to a truncated response.
What is sitting between your PC and the device? Physical layer problems are extremely common in RS-485
If all parameters match and the physical wiring is sound, the issue may be specific to the software environment.
A: Yes. Some clones or poorly implemented slaves send incomplete frames. Test with a known-good slave to isolate the issue.
Add a across the Day+ and Day- lines at both physical ends of the RS485 network. This stops signal reflections that mimic missing bytes. Address Electrical Noise Use Shielded Twisted Pair (STP) cables for all serial runs.
Most "Bytes Missing" errors are born in low-quality USB-to-RS485 converters. These devices often struggle with "direction control"—the split-second switch from sending to receiving.