Modbus RTU Device Cascading: Reading EMHeater VFD Data with Coolmay EX3S

A production line with five VFDs spread across 50 meters typically needs motor current, frequency, and fault status visible from a single HMI screen. Running individual 4–20 mA analog wires to each drive is expensive, noisy, and wastes analog input channels that could be used for process transmitters. A Modbus RTU daisy chain over RS-485 solves the problem with two wires and near-zero hardware cost—every EMHeater drive ships with RS-485 as standard.

Setting up the network is straightforward. The Coolmay EX3S acts as the Modbus master. Each EM10 or EM15 drive gets a unique slave address (set via parameter P0.03), and they're connected in a daisy chain using shielded twisted-pair cable. A 120 Ω termination resistor goes across the data lines at both ends of the bus. At 9600 baud, the bus supports cable runs up to 1200 meters.

Once the physical layer is working, reading drive data is a matter of polling standard Modbus registers. The running frequency lives at 0x1000 (scaled to 0.01 Hz), output current at 0x1001 (scaled to 0.01 A), and the fault code at 0x3000. The EX3S reads these registers using function code 03 and can display the values directly on its built-in HMI without any additional gateway hardware.

The Coolmay EX3S HMI/PLC communicates natively with EMHeater EM10 and EM15 series VFDs through its built-in RS-485 port. No protocol converter, no expensive comm module—just two wires and the correct parameter settings.

Download the EX3S Modbus library with pre-written ladder logic and HMI screen templates. Contact our integration team for assistance with multi-drive Modbus network commissioning.