CODEWITHSUNDEEP
c#modbus-tcp
pooja
pooja

Reputation: 23

Modbus TCP-Slave Null Exception,Object reference not set to an instance of an object

I am developing a windows form to function as a modbus tcp slave Simuator. I am trying to recieve write multiple coil request from modbus master. I am able to recieve the request correctly, but when I try to do the write operation it's throwing "object reference is not set to an instance of an object" exception. It's saying slave is null, but I have created an instance of slave and initialized it. I am using NModbus Library. Here is my code 

        private void Button1_Click(object sender, EventArgs e)
        {

              try
              {

                byte slaveAddress = Convert.ToByte(textBox3.Text);

                IPAddress address = IPAddress.Parse(textBox1.Text);
                int port = Convert.ToInt32(textBox2.Text);

                // create and start the TCP slave
                TcpListener slaveTcpListener = new TcpListener(address, port);
                slaveTcpListener.Start();


                ModbusSlave slave = ModbusTcpSlave.CreateTcp(slaveAddress, slaveTcpListener);


                slave.Listen();
                slave.DataStore = DataStoreFactory.CreateDefaultDataStore();
                DataStore dataStore = slave.DataStore;


                slave.ModbusSlaveRequestReceived += new EventHandler<ModbusSlaveRequestEventArgs>(Modbus_Request_Event);

                slave.DataStore.DataStoreWrittenTo += new EventHandler<DataStoreEventArgs>(DataStoreWrittenTo);



                // prevent the main thread from exiting
                listBox1.Items.Clear();

                Thread.Sleep(Timeout.Infinite);
                //slaveTcpListener.Stop();


              }
              catch
              {
                MessageBox.Show("Error in connection");
              }
        }


 private void DataStoreWrittenTo(object sender, DataStoreEventArgs e)
        {

            switch (e.ModbusDataType)
            {

                case ModbusDataType.HoldingRegister:


                    for (int i = 1; i < e.Data.B.Count; i++)
                    {

               //Line A         slave.DataStore.HoldingRegisters[e.StartAddress + i + 1] = e.Data.B[i];
                    }

                    break;
                case ModbusDataType.Coil:

                    for (int i = 1; i < e.Data.A.Count; i++)
                    {
 //Line B                       slave.DataStore.CoilDiscretes[e.StartAddress + i + 1] = e.Data.A[i];
                    }
                    break;
                    //case Modbus.


            }


        }

I am getting the exception at line A if I try to write Multiple Holding Registers and at line B if I try to write Multiple Coils. Can any one please help me to solve this issue?

Upvotes: 0

Views: 1028

Answers (1)

jegtugado
jegtugado

Reputation: 5141

It looks like you're not instantiating your slave correctly. You are creating a local variable slave with the scope of Button1_Click.

ModbusSlave slave = ModbusTcpSlave.CreateTcp(slaveAddress, slaveTcpListener);

and you are accessing slave in DataStoreWrittenTo which should have a class scope.

You should change the above instantiation to: 

slave = ModbusTcpSlave.CreateTcp(slaveAddress, slaveTcpListener);

Upvotes: 1

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