BytesToWrite=0

我手头有一些通过485通讯的设备，接线完毕后用设备自带的Demo发送、接受数据均正常。前段时间我写的代码往设备上发送信息也都是正常的，但是今天不行了，发现BytesToWrite=0。代码如下：

//打开端口

 private void btn_open_Click(object sender, EventArgs e)
        {
            if (com.IsOpen == true)
            {
                label_state.Text = "状态：端口已打开";
            }
            else
            {
                com.PortName = "COM1";
                com.BaudRate = 57600;
                com.Parity = Parity.None;
                com.StopBits = StopBits.One;
                com.DataBits = 8;
                com.Open();
                label_state.Text = "状态：端口已打开";
            }
        }

//往设备上发送数据

 private void btn_sent_Click(object sender, EventArgs e)
        {
            if (com.IsOpen == true)
            {
                try
                {
                    com.DiscardOutBuffer();
                    byte[] bytesend = new byte[13];
                    bytesend[0] = 255;
                    bytesend[1] = 1;
                    bytesend[2] = 1;
                    bytesend[3] = 1;
                    bytesend[4] = 1;
                    bytesend[5] = 1;
                    bytesend[6] = 1;
                    bytesend[7] = 1;
                    bytesend[8] = 1;
                    bytesend[9] = 1;
                    bytesend[10] = 1;
                    bytesend[11] = 1;
                    bytesend[12] = 254;
                    com.Write(bytesend, 0, bytesend.Length);
                    if (com.BytesToWrite > 0)
                    {
                        MessageBox.Show("ok");
                    }
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.ToString());
                }
            }
        }

 

搞了好大时间不知道问题出在哪里，请各位高手指点，谢谢。