I got a RS232 Serial routine from this site. But when I tried on my actual hardware it did not work. I work OK using Dscope simulator. Could somebody be kindly to take a quick look at the code to see if why/how it dit not work on the actual hardware. My hardware uses a 14.318MHX oscillator, and I want to config the protocol for 9600 baudrate. Here is the code: //*--------------------------------------------------- SIO.C: Serial Communication Routines. #include <reg52a.h> #include <string.h> #include "sio.h" //------------------------------------- -----------------*/ //----- void main (void) { com_initialize (); /* initialize interrupt driven serial I/O */ com_baudrate (9600); /* setup for 9600 baud */ printf ("Interrupt-driver Serial I/O Example\r\n\r\n"); while (1) { unsigned char c; printf ("Press a key from the keyboard .\r\n"); c = getchar (); //store pressed character printf ("\r\nYou pressed '%c'.\r\n\r\n", c); //print out Character } } //end Main //-------------------------------------------------*/ char _getkey (void) { int k; do { k = com_getchar (); } while (k == -1); return ((unsigned char) k); } //*--- char putchar (char c) { volatile unsigned int i; while (com_putchar (c) != 0) { for (i=0; i<1000; i++) { /*** DO NOTHING ***/ } } return (c); } /////////// end main() ////// #define TBUF_SIZE 256 #define RBUF_SIZE 256 #define XTAL 14318000 //XTAL frequency static xdata unsigned char tbuf [TBUF_SIZE]; static xdata unsigned char rbuf [RBUF_SIZE]; static xdata unsigned char t_in = 0; static xdata unsigned char t_out = 0; static xdata unsigned char t_disabled = 0; static xdata unsigned char r_in = 0; static xdata unsigned char r_out = 0; //-----------------------------------------*/ static void com_isr (void) interrupt 4 using 2 { /*------------------------------------------------ Received data interrupt. ------------------------------------------------*/ if (RI != 0) { RI = 0; if ((r_in + 1) != r_out) rbuf [r_in++] = SBUF; } /*------------------------------------------------ Transmitted data interrupt. ------------------------------------------------*/ if (TI != 0) { TI = 0; if (t_in != t_out) SBUF = tbuf [t_out++]; else t_disabled = 1; } } //--------------------------------------------*/ void com_initialize (void) { /*------------------------------------------------ Setup TIMER1 to generate the proper baud rate. ------------------------------------------------*/ com_baudrate (9600); /*------------------------------------------------ Clear com buffer indexes. ------------------------------------------------*/ EA = 0; /* Disable Interrupts */ t_in = 0; t_out = 0; t_disabled = 1; r_in = 0; r_out = 0; /*------------------------------------------------ Setup serial port registers. ------------------------------------------------*/ SM0 = 0; SM1 = 1; /* serial port MODE 1 */ SM2 = 0; REN = 1; /* enable serial receiver */ TI = 0; /* clear transmit interrupt */ RI = 0; /* clear receiver interrupt */ ES = 1; /* enable serial interrupts */ PS = 0; /* set serial interrupts to low priority */ EA = 1; /* Enable Interrupts */ } /*------------------------------------------------------------------------------ ------------------------------------------------------------------------------*/ void com_baudrate ( unsigned baudrate) { EA = 0; /* Disable Interrupts */ /*------------------------------------------------ Clear transmit interrupt and buffer. ------------------------------------------------*/ TI = 0; /* clear transmit interrupt */ t_in = 0; /* empty transmit buffer */ t_out = 0; t_disabled = 1; /* disable transmitter */ /*------------------------------------------------ Set timer 1 up as a baud rate generator. ------------------------------------------------*/ TR1 = 0; /* stop timer 1 */ ET1 = 0; /* disable timer 1 interrupt */ PCON |= 0x80; /* 0x80=SMOD: set serial baudrate doubler */ TMOD &= ~0xF0; /* clear timer 1 mode bits */ TMOD |= 0x20; /* put timer 1 into MODE 2 */ TH1 = (unsigned char) (256 - (XTAL / (16L * 12L * baudrate))); TR1 = 1; /* start timer 1 */ EA = 1; /* Enable Interrupts */ } //---------------------------------------*/ char com_putchar ( unsigned char c) { /*------------------------------------------------ If the buffer is full, return an error value. ------------------------------------------------*/ if ((TBUF_SIZE - com_tbuflen ()) <= 2) return (-1); /*------------------------------------------------ Add the data to the transmit buffer. If the transmit interrupt is disabled, then enable it. ------------------------------------------------*/ EA = 0; /* Disable Interrupts */ tbuf [t_in++] = c; if (t_disabled) /* if transmitter is disabled */ { t_disabled = 0; TI = 1; /* enable it */ } EA = 1; /* Enable Interrupts */ return (0); } //------------------------------ int com_getchar (void) { int c; if (com_rbuflen () == 0) return (-1); EA = 0; /* Disable Interrupts */ c = rbuf [r_out++]; EA = 1; /* Enable Interrupts */ return (c); } //------------------------------------*/ unsigned char com_rbuflen (void) { return (r_in - r_out); } //---------------------------------------*/ unsigned char com_tbuflen (void) { return (t_in - t_out); } //---------------------------------------- ///////////////////////////////////////