请教一个51串口通信的问题

本意是设计一个交互的串口通信程序，以前没有写过单片机程序，遇到一些问题来请教大家。
单独的收和发都是好的，但同时启动收发时，主程序得不到执行，推断应该是一直在中断中，但不知道是什么原因引起的，如何清掉，还请指教，谢谢！
#include <reg51.h>
#include <stdio.h>
#include <string.h>
#define FOSC  40000000
#define BUF_LEN 10
static unsigned char prompt[4] = {0x0d, 0x0a, '$', ' '};
static unsigned char inbuf[BUF_LEN]; // last 2 char: 0x0d, 0x0a
static unsigned char incount = 0, inlen = 0;
static bit read_flag = 0;
void delay_1ms(void)                 //1ms延时函数，不一定准哈，只是delay一下
{
        unsigned int i;
        for (i=0; i<40000/12; i++);
}
  
void delay_nms(unsigned int n)       //N ms延时函数
{
        unsigned int i=0;
        for (i=0; i<n; i++)
                delay_1ms();
}
static void init_serialcomm(void)
{
        PCON |= 0x80;  // SMOD=1;
        SCON = 0x50 ;  // SCON: serail mode 1, 8-bit UART, enable ucvr
        TMOD &= 0x0f;  // TMOD: timer 1, mode 2, 8-bit reload
        TMOD |= 0x20;
        TH1 = 240 ;    // TH1: Baud:9600 fosc=40MHz. 0xea=234 can not work...
        TL1 = TH1;     // TL1: init Timer1
        TR1 = 1 ;      // TCON: timer 1 run
        TI = 1;        // SCON: Indicate TX0 ready
        IE |= 0x90;    // Enable Serial Interrupt
        ES = 1;        // IE: Enable serial recv
}
//向串口发送一个字符
static void send_char_com(unsigned char ch)
{
        SBUF = ch;
        while (TI == 0);
        TI = 0 ;
}
//向串口发送一个字符串，strlen为该字符串长度
static void send_string_com(unsigned char *str, unsigned int strlen)
{
        unsigned int k = 0;
        do {
                send_char_com(*(str + k));
                k++;
        } while (k < strlen);
}
//串口接收中断函数
static void serial() interrupt 4 using 3
{
        unsigned char ch;
        ES = 0; // 关串口中断
        /* send_char_com('?'); */
        if (RI) {
                IE = 0;
                RI = 0; // 软件清除接收中断
                ch = SBUF;
                send_char_com(ch); // echo
                inbuf[incount++] = ch;
                if (incount > BUF_LEN-2) { // leave last char to append 0x0a
                        incount = BUF_LEN-2;
                }
                if (ch == 0x0d) {
                        send_char_com(0x0a);
                        inbuf[incount++] = 0x0a;
                        read_flag = 1;
                        inlen = incount;
                        incount = 0;
                        // test
                        send_string_com(inbuf, inlen); // for test
                }
        }
        TI = 0; // ???
        
        ES = 1; // 开串口中断
}
void main (void)
{
        init_serialcomm();
        EA = 1; // enable global interrupts
        send_string_com(prompt, 4);
        while (1) {
                /* send_char_com('+'); */
                /* if (read_flag) { //如果取数标志已置位，就将读到的数从串口发出 */
                /*      send_string_com(inbuf, inlen); */
                /*      read_flag = 0 ; //取数标志清0 */
                /*      send_string_com(prompt, 4); */
                /* } */
                delay_nms(50);
                send_char_com('.');
        }
}