Modbus RTU on RS-485: CRC Errors and Intermittent Communication

The symptom

A PLC polls eight devices on an RS-485 Modbus RTU network. Most replies arrive, but the error counter keeps climbing: CRC errors, timeouts, and now and then a device that drops out for a few seconds. It gets worse when the drives are running.

Settings must match exactly

Every device on the bus needs the same baud rate, data bits, parity and stop bits (for example 19200, 8, Even, 1). A device set to 8-N-1 on an 8-E-1 bus misreads every frame: depending on the device it either stays silent, so the master logs timeouts, or answers with frames the master rejects as CRC or framing errors. Check every device, and especially any replacement that arrived with factory defaults.

Termination: exactly two resistors

RS-485 needs a 120 Ω terminating resistor at each physical end of the bus, which makes two in total. The common mistakes are:

With the bus powered off, measure between A and B. About 60 Ω is correct: that is two 120 Ω resistors in parallel.

Biasing

When no device is transmitting, the bus floats unless something biases it, and noise on a floating line looks like random start bits. Bias resistors should come from the master or from one dedicated device. Check the master's documentation for a bias setting.

Polarity

Manufacturers label the lines inconsistently: A/B, D+/D−, TxD+/TxD−. A device with swapped polarity never answers. If one device is silent while the others work, swap its two data wires.

Topology and cable

RS-485 is a daisy chain, not a star. Long stubs to each device cause reflections, so keep them short. Use shielded twisted-pair cable made for RS-485 (around 120 Ω characteristic impedance), and route it away from motor cables.

Ground and common

RS-485 is differential, but it still needs a common reference. Connect the signal common (0 V / GND) between devices, or use isolated transceivers. A large ground potential difference between cabinets will destroy transceivers.

Protocol timing

At low baud rates, slow devices may need a longer timeout and an inter-frame delay. If you poll too fast, without the 3.5-character silent interval between frames, devices miss requests. Lengthen the poll interval and see whether the error rate falls.

Measure, then decide

Put an oscilloscope across A and B. A healthy bus shows clean square edges; a badly terminated one shows ringing or rounded edges. The frame itself is covered in the RS-232/RS-485 article.