#define  COMM_TIMEOUT 0xff

unsigned char g_ch=0;
unsigned char g_flag=0;
void InitComm()
{
            

            T2CON=0x30;            
                            RCAP2H=255;
                            RCAP2L=184;
                       
            
            SCON=0x50;
            ES=1;
            EA=1;
            TR2=1;
}

////////////////////////////////////////////
//function:Receive 1 byte
//return:0--ok  1---failure
unsigned char ReadComm(unsigned char *ch)
{

        unsigned int timeout = COMM_TIMEOUT;
       

        timeout = COMM_TIMEOUT;
        while(timeout--)//µ÷ÊÔÍê³Éºó£¬ÇëʹÓó¬¹ýʱ
        {

        
             if(RI==1)
             {
                 RI=0;
                 for(i=0;i  <5;i++)
                 {

        
                 }
                 *ch = SBUF;
                 RI=0;
                 return ID_OK;
             }
        }
        return   ID_ERR;
}
////////////////////////////////////////////
//function:Send 1 byte
//return:0--ok  1---failure
unsigned char WriteComm(unsigned char ch)
{
        unsigned int xdata timeout=COMM_TIMEOUT;

        SBUF=ch;
        while(timeout--)
        {
             if(TI==1)
             {
                 TI=0;
                 return ID_OK;
             }
        
        }
        return ID_ERR;
}
//
void  Comm_ISR() interrupt 4
{
          

     ES=0;
     if(RI==1)
     {
                ReadComm(&g_ch);
                g_flag=1;
                
    }
    ES=1;

}

void main(void)
{
        unsigned char xdata testbuf[100]="12345678901234567890";
        unsigned char xdata command=0;

       g_flag=0;

        InitComm();//Baud9600);
        SendResponse(0x01,testbuf,3);
        
        EA=1;
        ES=1;
        
        while(1)
        {
         if(g_flag)
         {
          g_flag=0;
          WriteCom(g_ch);
         }
        }
}      

/////////////////////////////////////////////
对照一下程序，如果有问题，可能就是你的RXD线没有接通，
或者，你的+5V电源没有接对。