ReadInputDiscretesResponse error (Jamod,Android)

Question

I'm using the library Jamod and I have trouble reading the record, what I want is to read only the record number 300 PLC I'm connected, but I get read error (enters the catch). Thanks for your help

package com.JR.scada;

import java.net.InetAddress;

import net.wimpi.modbus.Modbus;
import net.wimpi.modbus.io.ModbusTCPTransaction;
import net.wimpi.modbus.msg.ReadInputDiscretesRequest;
import net.wimpi.modbus.msg.ReadInputDiscretesResponse;
import net.wimpi.modbus.msg.ReadMultipleRegistersResponse;
import net.wimpi.modbus.net.TCPMasterConnection;
import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;




public class Main extends Activity{
    TextView text, depurar;
    EditText IP;
    Button boton;
    int i=0;


    TCPMasterConnection con = null;     //the TCP connection
    ModbusTCPTransaction trans = null;  //the Modbus transaction

    InetAddress addr = null;        //direccion del esclavo
    int port = Modbus.DEFAULT_PORT;//puerto por defecto 502 


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        text = (TextView) findViewById(R.id.lblRegistro);
        IP = (EditText) findViewById(R.id.txtIp);
        depurar = (TextView) findViewById(R.id.txtdepurar); 
        boton = (Button)findViewById(R.id.btnVerRegistro);


    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }   
    @Override
    protected void onStop() {
        super.onStop();
        //Close the TCP connection
        con.close();
    }


    public class conectar extends AsyncTask<String,String,Integer>{
        ReadInputDiscretesRequest req = null; //the request
        ReadInputDiscretesResponse res = null; //the response

        int startReg;
        protected void onPreExecute() {
            try {
                //IP address;
                addr = InetAddress.getByName("212.170.50.238");
            } catch (Exception e) {
                Log.d("MODBUS","IP error", e);

            }
         }
        protected Integer doInBackground(String... urls) {
            try {

                // Open the connection
                con = new TCPMasterConnection(addr);
                con.setPort(port);
                con.connect ();

                try {

                    startReg = 300;
                    // Prepare the request
                    req = new ReadInputDiscretesRequest (startReg, 1);

                    // Prepare the transaction
                    trans = new ModbusTCPTransaction(con);
                    trans.setRequest(req);
                    // execute the transaction
                    trans.execute();
                    // get the response
                    res = (ReadInputDiscretesResponse) trans.getResponse ();


                } catch (Exception e) {
                    Log.d("MODBUS", "Error in reading/writing");
                    return 1;
                }   


            } catch (Exception e) {
                Log.d("MODBUS","connection error", e);

                return 1;
            }
            return 0;
        }
         protected void onPostExecute(Integer bytes) {
             if(con.isConnected()){

                 depurar.setText("conecta");
            }
             text.setText("Digital Inputs Status=" + res.getDiscretes ().toString () );

         }
    }

    public void onClick(View v)  {

       // int startReg;
        conectar conectamos = new conectar();
        conectamos.execute("hola");
}

error:

08-21 10:01:57.554: D/MODBUS(3322): Error in reading/writing

Answer 1

Without knowing more about the modbus configuration on your slave PLC, my first suggestion is to try a different value for startReg, and see if the error persists. This will rule out problems with your Java.

Some test numbers that should work: 0,1,7,8

They may not all work, but at least one of them should return successfully.

If none of them return successfully, there may be a problem on the PLC configuration, or a problem with your request code.

If one of the test numbers is successful, you should post your results. If so, this means that you are attempting to access a memory location that does not exist on the PLC. If you access undefined PLC memory locations, often the PLC will abort the request (it does not just send back '0').

Followup remarks:

If you find that your code works when using different values for startReg, but not for 300, the reason has to do with memory mapping on the PLC, and this can be different for each brand/model of PLC (post your PLC brand/model if available).

You say in your question 'record 300'. When working with PLC, you usually don't refer to memory as a record. Are you trying to access Bit 300, Byte 300, Word 300, DoubleWord 300?

The real question you need to be asking, is 300 the actual modbus address you want to read, or is 300 how the PLC address is mapped (such as the 300th I/O slot, not necessarily the 300th WORD). It may be required to convert between octal, decimal, and hexidecimal addresses. Or, you may need to re-index an address (some PLCs like to start counting at 1, but generally modbus starts counting at 0).

Perhaps you meant to read BIT 300 (not record 300), which would then be 12th BIT in the 18th WORD, so it would look like this:

// 
startReg = 17;

// Prepare the request
// Your 300th bit should be the last value returned.
req = new ReadInputDiscretesRequest (startReg, 12);

The different values I suggested for startReg are meant to help you discover how your PLC Inputs are mapped into modbus addresses by your Java library. This might help with number conversions.

Answer 2

If you keep getting exceptions in your catch block, you might want to find out more about the error.

Try changing this line from your original code:

            try {
                //...
            } catch (Exception e) {
                Log.d("MODBUS", "Error in reading/writing");
                return 1;
            }   

into this:

            try {
                //...
            } catch (Exception e) {
                Log.d("MODBUS", e.getMessage() );
                return 1;
            }

Hopefully the exception will tell you more about exactly why its failing. Post those results.

If you are getting a NULL message, you might try using a debugger to manually inspect your connection instance.