HEUREKA !!!!!!!! it works fine
I can communicate without any problem with an ASCON J1 type indicator
of temperature
  
www.ascon.it .
This library is a spectacle.
This is my simple code and for the moment THANKS TO ALL .
// MainWindow.cs created with MonoDevelop
// User: franaria at 11:21 15/05/2008
//
// To change standard headers go to Edit->Preferences->Coding-
>Standard Headers
//
using System;
using System.Diagnostics;
using System.IO.Ports;
using System.Net;
using System.Net.Sockets;
using System.Threading;
using Gtk;
using Modbus.Device;
public partial class MainWindow: Gtk.Window
{
	public MainWindow (): base (Gtk.WindowType.Toplevel)
	{
		Build ();
	}
	protected void OnDeleteEvent (object sender, DeleteEventArgs a)
	{
		Application.Quit ();
		a.RetVal = true;
	}
	protected virtual void Butt1Start (object sender, System.EventArgs e)
	{
		try
		{
			using (SerialPort port = new SerialPort("/dev/ttyS0"))
			{
				Console.WriteLine("I am in the procedure");
				//configure serial port
				port.BaudRate = 9600;
				port.DataBits = 8;
				port.Parity = Parity.None;
				port.StopBits = StopBits.One;
				Console.WriteLine("Port configured");
				port.Open();
				Console.WriteLine("Port opened");
				// create modbus master
				IModbusSerialMaster master = ModbusSerialMaster.CreateRtu(port);
				byte slaveID = 1;
				ushort startAddress = 1;
				ushort numRegisters = 5;
				// read registers
				ushort[] registers =
master.ReadHoldingRegisters(slaveID,startAddress,numRegisters);
	
Console.WriteLine("==================================================");
				Console.WriteLine("Serial test");
Console.WriteLine("==================================================");
				for (int i = 0;i < numRegisters;i++)
					Console.WriteLine("Register {0}={1}",startAddress +
i,registers[i]);
	
Console.WriteLine("==================================================");
			}
		}
		catch (Exception ec)
		{
			Console.WriteLine(ec.Message);
			Console.WriteLine("Test by ARWEN");
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