9. Connecting PLCs to .NET applications

using System.Net.Sockets;
using NModbus;

using var tcp = new TcpClient("192.168.0.10", 502);
var master = new ModbusFactory().CreateMaster(tcp);
ushort[] regs = master.ReadHoldingRegisters(slaveAddress: 1, startAddress: 0, numberOfPoints: 2);
float temperature = BitConverter.ToSingle(BitConverter.GetBytes((regs[0] << 16) | regs[1]), 0);  // word order per device manual
master.WriteSingleRegister(1, 10, 750);   // write setpoint 75.0 (scaled ×10)

var endpoint = CoreClientUtils.SelectEndpoint(config, "opc.tcp://plc1:4840", useSecurity: true);
using var session = await Session.Create(config, new ConfiguredEndpoint(null, endpoint), false, "MES", 60000, new UserIdentity(), null);

var subscription = new Subscription(session.DefaultSubscription) { PublishingInterval = 1000 };
var item = new MonitoredItem(subscription.DefaultItem) { StartNodeId = "ns=3;s=\"Line1\".\"Temperature\"", SamplingInterval = 500 };
item.Notification += (mi, e) =>
{
    foreach (var value in mi.DequeueValues())
        Console.WriteLine($"{mi.StartNodeId} = {value.Value} ({value.StatusCode}) @ {value.SourceTimestamp}");
};
subscription.AddItem(item);
session.AddSubscription(subscription);
subscription.Create();

PLCs (Modbus/S7/EtherNet/IP) ─► Gateway / OPC UA server ─► Edge service (.NET Worker, buffering)
     ─► MQTT broker or Azure IoT Hub ─► Stream processing ─► SQL Server / time-series DB ─► Dashboards, MES, ML