问C#信号量超时期在写入虚拟串口时已过期。

EN

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO.Ports;

namespace ConsoleApplication1
{
    class Program
    {
        private static readonly SerialPort _serial = new SerialPort();
        static void Main(string[] args)
        {
            var portList = SerialPort.GetPortNames().ToList();
            _serial.PortName = portList[0]; 
            _serial.BaudRate = 9600;
            _serial.Handshake = Handshake.None;
            _serial.Parity = Parity.None;
            _serial.DataBits = 8;
            _serial.StopBits = StopBits.Two;
            _serial.ReadTimeout = 3000;
            _serial.WriteTimeout = 3000;
            _serial.DataReceived += ReceiveData;
            _serial.Open();

            while (true)
            {
                string str = Console.ReadLine();

                if (str.Equals("exit"))
                    return;
                SerialCmdSend(str);
            }

        }

        public static void SerialCmdSend(string data)
        {
            if (_serial.IsOpen)
            {
                try
                {
                    byte[] hexstring = Encoding.ASCII.GetBytes(data);
                    foreach (byte hexval in hexstring)
                    {
                        byte[] _hexval = { hexval }; 
    //The exception is thrown 3 sec. after the first call of the Write function
                        _serial.Write(_hexval, 0, 1); 
                    }
                }
                catch (Exception ex)
                {
                    Console.WriteLine(ex.Message);
                    throw;
                }
            }
        }


        private static void ReceiveData(object sender, SerialDataReceivedEventArgs e)
        {
            if (!_serial.IsOpen)
            {
                return;
            }
            try
            {
                while (_serial.BytesToRead > 0)
                {
                    var b = _serial.ReadChar();
                    //Handle read data
                }

            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
                throw;
            }
        }
    }
}