问net.wimpi.modbus.ModbusSlaveException:错误代码= 2，当尝试使用Modbus协议连接单元时

EN

*******************Sample Code***********************


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

public class modbus_conn {

    public static void main(String args[]){
                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
                        /* Variables for storing the parameters */
                        InetAddress addr = null;  // the slave's address
                        int port = 502;  // the default port
                        //int coil = 1;  // one of the coils (D0 1 for this address) to switch ON/OFF

                        //Setup the parameters
                        addr = InetAddress.getByName("127.192.6.31");  // ** The address assigned to the module **

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

                        //Prepare the READ request
                        int k = 30001;  // register address starting from 30001
                        rreq = new ReadInputRegistersRequest(k, 2);  // Reading 8 bytes

                        //Prepare the READ transaction
                        trans = new ModbusTCPTransaction(con);
                        trans.setRequest(rreq);

                        //Execute the READ transaction
                        trans.execute();

                        rres = (ReadInputRegistersResponse) trans.getResponse();
                        System.out.println("Hex Value of register " + "= " + rres.getHexMessage());

                        //Close the connection
                        con.close();

                }
                catch (Exception ex) {
                        System.out.println("Error");
                        ex.printStackTrace();
                }           
    }
}

Error
net.wimpi.modbus.ModbusSlaveException: Error Code = 2
    at net.wimpi.modbus.io.ModbusTCPTransaction.execute(ModbusTCPTransaction.java:207)
    at modbusConn.Control_ADAM.main(modbus_conn.java:48)