I use PLC as Server and write the codes to hold values of Holding Register (DB10.DBW0) like the Picture attach below. And I use jamodModbus as client to write codes from PC to connect to PLC and read the values from of Holding Register (DB10.DBW0).

The result is: I can connect to PLC sucessful, send the request to PLC but the values out put respond I get from Holding Register (DB10.DBW0) of PLC always zero (00). How can I fix this problem and please tell which code I use to get only values from Holding Register (DB10.DBW0) and change them to Decimal?

<b>Moderator's Note:</b> You cannot attach illustrations to Control.com posts. I think he means the code below.

This is my codes from PC:<pre>
package testmodbus;

import java.lang.*;
import java.net.InetAddress;
import java.net.SocketTimeoutException;
import net.wimpi.modbus.Modbus;
import net.wimpi.modbus.ModbusSlaveException;
import net.wimpi.modbus.io.ModbusTCPTransaction;
import net.wimpi.modbus.msg.WriteCoilRequest;
import net.wimpi.modbus.msg.ReadInputRegistersRequest;
import net.wimpi.modbus.msg.ReadInputRegistersResponse;
import net.wimpi.modbus.net.TCPMasterConnection;

public class TestModbus {
public static void main(String[] args) throws ModbusSlaveException, SocketTimeoutException{
// TODO code application logic here

try {
/* The important instances of the class */
TCPMasterConnection con = null; // the connection
ModbusTCPTransaction trans = null; // the transaction
ReadInputRegistersRequest rreq = null; // the read request
ReadInputRegistersResponse rres = null; // the read response
WriteCoilRequest req = null; // the write request
int ref = 40000; //the reference; offset where to start reading from

int count =0;
int repeat = 2;

/* Variables for storing the parameters */
InetAddress addr = null; // the slave's address
int port = Modbus.DEFAULT_PORT; // DEFAULT_PORT = 502.

// 1. Setup ?":>the parameters
addr = InetAddress.getByName("192.168.0.1"); // ** The address of PLC

// 2. Open the connection
con = new TCPMasterConnection(addr);
con.setPort(port);
con.connect();
System.out.println("--- Message: Line:36 success --- ");
System.out.println("Connect to PLC sucessfull!!");
// ~~~~~~~~~~~~~~~~~~~~ The faulty Read Request ~~~~~~~~~~~~~~~~~~~~
// // 3r. Prepare the READ request
rreq = new ReadInputRegistersRequest(ref, count);
System.out.println("The reqquest values =" + rreq.getHexMessage());
System.out.println("--- Message: Line:42 success --- ");
//// // 4r. Prepare the READ transaction
trans = new ModbusTCPTransaction(con);
trans.setRequest(rreq);
/ System.out.println("--- Message: Line:46 success --- ");
//// // 5r. Execute the READ transaction
int k = 0;
do {
trans.execute();

rres = (ReadInputRegistersResponse) trans.getResponse();
System.out.println("The reqquest values =" + rreq.getHexMessage());
System.out.println("getHexMessage of resspond = "+ rres.getHexMessage());

k++;
}while (k<repeat);

// 6. Close the connection
con.close();

} catch (Exception e) {
e.printStackTrace();
}

}

} </pre>
And this is the out put I get from Console:
run:
--- Message: Line:36 success ---
Connect to PLC sucessfull!!
The reqquest values =00 00 00 00 00 06 00 04 9c 40 00 00
--- Message: Line:42 success ---
--- Message: Line:46 success ---
The reqquest values =00 01 00 00 00 06 00 04 9c 40 00 00
getHexMessage of resspond = 00 01 00 00 00 03 00 04 00
The reqquest values =00 02 00 00 00 06 00 04 9c 40 00 00
getHexMessage of resspond = 00 02 00 00 00 03 00 04 00 // always get 00 values form PLC with whatever address I try to read from PLC.
BUILD SUCCESSFUL (total time: 0 seconds)

 

If the client/master is requesting data from Holding registers, then Function Code 03 appears in both the request message and the response message.

If your examples are hex queries or responses

The reqquest values =00 00 00 00 00 06 00 04 9c 40 00 00
--- Message: Line:42 success ---
--- Message: Line:46 success ---
The reqquest values =00 01 00 00 00 06 00 04 9c 40 00 00
getHexMessage of resspond = 00 01 00 00 00 03 00 04 00
The reqquest values =00 02 00 00 00 06 00 04 9c 40 00 00

then I don't see a 03 Function code in any of the messages.

Are you sure the data in Holding Registers?
Are you polling (FC03) Holding Registers?