C#多线程串口收发报错由于线程退出或应用程序请求

使用串口进行数据收发，多线程启动时会报错 “由于线程退出或应用程序请求，已中止 I/O 操作。”

尝试加了LOCK锁也一样，当发送数据不新开线程时可以正常使用，但多线程就会报错

求各位解答

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using Modbus.Device;
using System.Threading;

namespace modbus
{

    public partial class Form1 : Form
    {
        public Form1()
        {
            Control.CheckForIllegalCrossThreadCalls = false;
            InitializeComponent();
            btnStop.Enabled = false;
        }

        private void btnStart_Click(object sender, EventArgs e)
        {
            serialPort1.Open();
            btnStart.Enabled = false;
            btnStop.Enabled = true;
            timer1.Enabled = true;
        }
        public static object locker = new object();
        Thread th1;
        Thread th2;
        private void timer1_Tick(object sender, EventArgs e)
        {

            th1 = new Thread(Recive);
            th1.Start();

        }

        private void Recive()
        {
                ModbusSerialMaster master1 = ModbusSerialMaster.CreateRtu(serialPort1);
                ushort[] t = master1.ReadHoldingRegisters(1, 0, 10);
                string s = null;
                for (int i = 0; i < t.Length; i++)
                {
                    s += t[i].ToString();
                }
                textBox1.AppendText(s + "\r\n");

        }

        private void btnStop_Click(object sender, EventArgs e)
        {
            serialPort1.Close();
            btnStart.Enabled = true;
            btnStop.Enabled = false;
            timer1.Enabled = false;
        }

        private void textBox2_TextChanged(object sender, EventArgs e)
        {
            btnSend.Enabled = true;
        }

        private void btnSend_Click(object sender, EventArgs e)
        {
            th2 = new Thread(Send);
            th2.Start();
            //Send(); 发送时不新开线程就不报错
        }

        private void Send()
        {
            lock (locker)
            {
                string s = textBox2.Text;
                for (int i = 0; i < s.Length; i++)
                {
                    ushort c = Convert.ToUInt16(i);
                    ushort b = Convert.ToUInt16(s[i].ToString());
                    ModbusSerialMaster master = ModbusSerialMaster.CreateRtu(serialPort1);
                    master.WriteSingleRegister(1, c, b);//由于线程退出或应用程序请求，已中止 I/O 操作。
                }
            }
        }
    }
}

odbusSerialMaster master = ModbusSerialMaster.CreateRtu(serialPort1);拿到外面去

master这东西全局唯一