get_modbus_input (Manual Mode)

This section explains how to use get_modbus_input during Manual (Teach) operations to read real-time values from external PLC or inverter Modbus registers for diagnostic or verification purposes.

Typical usage

• Monitor status bits (e.g., READY, RUN, FAULT) from coils or discrete inputs.

• Read analog feedback (e.g., current speed, torque, or position) from Input or Holding Registers.

• Check whether external Modbus communication is functioning correctly during manual setup.

Note

• Applicable for DISCRETE_INPUT and INPUT_REGISTER types.

• For writable points, use set_modbus_output.

• Ensure the device uses the correct address base (0/1-based) and byte/word order.

Example: Read PLC READY and inverter speed feedback

// Preconditions:
// - Connection established (open_connection)
// - Manual (Teach) mode active
// - Modbus symbols registered beforehand

// 1) Register input tags
drfl.add_modbus_signal("PLC_READY", "192.168.0.10", 502,
                       MODBUS_REGISTER_TYPE_DISCRETE_INPUT, 0, 0);
drfl.add_modbus_signal("INV_SPEED_FB", "192.168.0.10", 502,
                       MODBUS_REGISTER_TYPE_INPUT_REGISTER, 120, 0, 1);

// 2) Poll device until READY or timeout
bool ready = false;
for (int i = 0; i < 30; ++i) {
    unsigned short val = drfl.get_modbus_input("PLC_READY");
    if (val == 1) { ready = true; break; }
    std::this_thread::sleep_for(std::chrono::milliseconds(100));
}
printf("[PLC_READY] = %s\n", ready ? "ON" : "OFF");

// 3) Read inverter feedback (e.g., speed in 0.01 Hz units)
unsigned short fb = drfl.get_modbus_input("INV_SPEED_FB");
printf("[INV_SPEED_FB] = %u (%.2f Hz)\n", fb, fb / 100.0);

// 4) Cleanup
drfl.del_modbus_signal("PLC_READY");
drfl.del_modbus_signal("INV_SPEED_FB");

Tips

• If read value stays 0, confirm device mapping and IP/port configuration.

• Periodically polling too fast may cause timeout or CRC errors; use 50–200 ms intervals.

• For multi-word data (e.g., 32-bit), combine successive registers per device documentation.