#include<reg52.h>
unsigned char flag,a;
void main()
{

        TMOD=0x20;//设置定时器1为工作方式2
        TH1=0xfd;
        TL1=0xfd;
        TR1=1;
        REN=1;
        SM0=0;
        SM1=1;
        EA=1;
        ES=1;
        while(1)
                {
                /*        if(RI==1)
                        {
                        RI=0;
                        P1=SBUF;
                        }*/
                        if(flag==1)
                                {
                                        ES=0;
                                        flag=0;
                                        SBUF='1';//28;        
                                        while(!TI);        
                                        TI=0;
                                        SBUF='2';//28;        
                                        while(!TI);        
                                        TI=0;
                                        SBUF='3';//28;        
                                        while(!TI);        
                                        TI=0;
                                        ES=1;
                                }
                }
}

void ser() interrupt 4
{
        RI=0;
//        P1=SBUF;
        a=SBUF;
        flag=1;
}
这是郭天祥版本的，看样子好像你没有开中断的使能，，，