c#进行com口编程无法收到响应数据

现要对PMAC725多功能电表进行串口编程,获取响应数据并解析, 通过com调试工具发送010300000001840A十六进制数据可获取响应01 03 02 56 42 07 D5,但通过自己写的程序可以发送, 无法获取响应.
sing System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO.Ports;
using System.Threading;

namespace SerialPortDemo
{
    class Test2
    {
        private SerialPort comm = new SerialPort();
        private StringBuilder builder = new StringBuilder();//避免在事件处理方法中反复的创建，定义到外面。  
        private long received_count = 0;//接收计数  
        private long send_count = 0;//发送计数  

        public Test2()
        {

            comm.PortName = "com11";
            comm.BaudRate = 9600;
            comm.StopBits = StopBits.One;
            comm.DataBits = 8;
            comm.Parity = Parity.None;
            comm.NewLine = "\r\n";

            //添加事件注册  
            comm.DataReceived += new SerialDataReceivedEventHandler(comm_DataReceived);
            comm.Open();
        }

        void comm_DataReceived(object sender, SerialDataReceivedEventArgs e)
        {
            int n = comm.BytesToRead;//先记录下来，避免某种原因，人为的原因，操作几次之间时间长，缓存不一致  
            byte[] buf = new byte[n];//声明一个临时数组存储当前来的串口数据  
            received_count += n;//增加接收计数  
            comm.Read(buf, 0, n);//读取缓冲数据  
            builder.Clear();//清除字符串构造器的内容  
            //因为要访问ui资源，所以需要使用invoke方式同步ui。  

            //判断是否是显示为16禁止  

            //依次的拼接出16进制字符串  

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