Modbus RTU Configuration on IDEC FC6A Using WindLDR

Quick Answer

The IDEC FC6A supports Modbus RTU as both master and slave on its built-in RS-232/RS-485 serial port. Configure serial parameters in WindLDR, use the MSTR instruction for master (polling) operations, and configure slave address for remote access from SCADA systems.

Modbus RTU and RS-485 Basics

Modbus RTU is a serial communication protocol using RS-232 (point-to-point) or RS-485 (multi-drop bus) physical layers. It is the most common protocol for connecting field instruments, drives, and meters to PLCs. For SCADA-side Modbus configuration, see the Geo SCADA Modbus setup guide.

Serial Port Configuration

In WindLDR, navigate to Communication Settings → Serial Port. Configure baud rate (9600, 19200, 38400, etc.), data bits (8), parity (None or Even), stop bits (1 or 2), and RS-485 mode enable. These settings must match all devices on the bus.

FC6A as Modbus RTU Master

Use the MSTR instruction in ladder logic to read or write registers on slave devices. Configure the function code (FC03 read holding registers, FC06 write single register, FC16 write multiple registers), slave address, starting register, quantity, and destination R registers in the FC6A.

FC6A as Modbus RTU Slave

Configure the FC6A slave address (1-247) in WindLDR Communication Settings. The FC6A maps its R registers to Modbus holding registers, allowing external devices to read and write FC6A memory via Modbus.

Register Mapping

FC6A R registers map directly to Modbus holding registers. R0 = Modbus register 40001, R1 = 40002, etc. For Modbus coils, M relays map to coil addresses. This direct mapping simplifies SCADA integration.

Example: Reading a Flow Meter

MSTR instruction reads 2 holding registers (FC03) from slave address 1, starting at register 40001 (flow rate as 32-bit float), and stores the result in R100-R101 in the FC6A.

Troubleshooting

• No response — check wiring, baud rate, parity match, and slave address
• CRC errors — verify RS-485 termination resistors and cable length
• Timeout — increase MSTR timeout parameter for slow devices