Hardware Wiring & PLC Settings

RS-485 Connection

• Connect the PLC’s RS-485 terminals: + to converter +, – to converter –.
• On some Panasonic models, jumper terminal E to –.

DIP-Switch Baud Rate

Switch 1-4	Baud	Data bits	Stop bits	Parity
1=ON	9600	8	1	None
2=ON	19200	8	1	Even
3=ON	38400	8	1	Odd
4=ON	115200	8	1	Mark

PLC Data Regisetrs for Modbus RTU

D1000 = 1      ; Mode = Modbus RTU
D1001 = 1      ; Slave address
D1002 = 9600   ; Baud rate
D1003 = 0      ; 8 data bits
D1004 = 0      ; 1 stop bit
D1005 = 0      ; No parity

C# Implementation with NModbus4

Core Communication Class

using System;
using System.IO.Ports;
using Modbus.Device;

public sealed class PanasonicRtuClient : IDisposable
{
    private readonly SerialPort _port;
    private IModbusMaster _master;

    public PanasonicRtuClient(string com, int baud = 9600,
                              Parity parity = Parity.None,
                              int dataBits = 8,
                              StopBits stopBits = StopBits.One)
    {
        _port = new SerialPort(com, baud, parity, dataBits, stopBits);
    }

    public void Open()
    {
        _port.Open();
        _master = ModbusSerialMaster.CreateRtu(_port);
        _master.Transport.ReadTimeout  = 1000;
        _master.Transport.WriteTimeout = 1000;
    }

    public ushort[] ReadWords(byte slave, ushort start, ushort length)
        => _master.ReadHoldingRegisters(slave, start, length);

    public void WriteCoil(byte slave, ushort coil, bool value)
        => _master.WriteSingleCoil(slave, coil, value);

    public void WriteWords(byte slave, ushort start, ushort[] data)
        => _master.WriteMultipleRegisters(slave, start, data);

    public void Dispose()
    {
        _master?.Dispose();
        _port?.Close();
    }
}

Console Sample

class Demo
{
    static void Main()
    {
        using var plc = new PanasonicRtuClient("COM3");
        plc.Open();

        // Read 10 holding registers starting at 40001 (index 0)
        var regs = plc.ReadWords(1, 0, 10);
        Console.WriteLine($"Values: {string.Join(", ", regs)}");

        // Turn on coil 00001
        plc.WriteCoil(1, 0, true);
    }
}

| Modbus Address | Zero-based Index | Type | PLC Register | |---|---|---|---| | 40001 | 0 | UInt16 | D1000 | | 40002 | 1 | UInt16 | D1001 | | 30001 | 0 | UInt16 | A1000 | | 10001 | 0 | Bool | C1000 |

Diagnostics & Troubleshoooting

• InvalidOperationException: parameter mismatch.
• TimeoutException: check wiring and baud rate.
• IOException: COM port already in use.

Tools: Modbus Poll for register testing, Wireshark with serial tap, Panasonic FPWIN to live monitoring.

Advanced Patterns

Async Batch Operations

public async Task<ushort[]> ReadWordsAsync(byte slave, ushort start, ushort len)
{
    return await Task.Run(() => _master.ReadHoldingRegisters(slave, start, len));
}

Coil Array Write

bool[] coils = Enumerable.Repeat(true, 16).ToArray();
plc.WriteMultipleCoils(1, 0, coils);

Performance Tips

• Reuse the SerialPort instance; avoid repeated open/close.
• Cache frequently read registers in memory.
• Aggregate multiple writes into a single request.
• Use hardware CRC acceleration when the PLC supports it.

Project Layout Example

PanasonicModbus/
├── src/
│   ├── PanasonicModbus.Core/     # RTU logic
│   ├── PanasonicModbus.UI/       # WPF dashboard
│   └── PanasonicModbus.Tests/
├── config/
│   └── PLC_Settings.xlsx
└── docs/
    └── AddressMap.pdf