Hello!

I'm trying to implement a very basic MIDI repeater using a circular buffer for the incoming bytes.
Somehow the buffer implementation or the way I handle the serial interrupts is wrong, because the program never transmitts anything.

I have spend few hours to debug the program, but still cannot figure out what is wrong. I will be very gratefull if you can spare few minutes and give a clue where is the problem:

#INCLUDE "8051EQU.INC" ;include predefined constants

BUFFER .equ 40H
BUFFER_SIZE .equ 12

;**************************************************************************
;
; RESET ;reset routine

.ORG 0H ;locate routine at 00H
AJMP START ;jump to START
;
;**************************************************************************
;
; INTERRUPTS ;place interrupt routines at appropriate
; ;memory locations
.ORG 03H ;external interrupt 0
RETI
;
.ORG 0BH ;timer 0 interrupt
RETI
;
.ORG 13H ;external interrupt 1
RETI
;
.ORG 1BH ;timer 1 interrupt
RETI
;
.ORG 23H ;serial port interrupt
JB RI,SERIN ;JUMP TO RECEIVE INRUP
JB TI,SEROUT

;
;******************************************************;
.ORG 030H ;locate beginning of rest of program
;
;******************************************************;
INITIALIZE: ;set up control registers

MOV PCON,#00H
MOV TMOD,#020H ;Set Timer 1 for Mode 2, Auto Reload
MOV TCON,#040H ;Turn timer 1 on
MOV PSW ,#00H ; Set Bank 0 ???
MOV SCON,#050H ;set serial port to 8 bit UART
MOV TH1 ,#0FEH ;Reload value for timer 1 for 31.25k baud (24MHz xtal)
MOV P3 ,#003H ;Enable Serial Port Transmit and Receive

SETB ES ; Enable Serial Interrupt
SETB PS ; Set Serial Interrupt Priority to High

RET

;******************************************************;
; SERIAL INPUT INTERRUPT ROUTINE
;
SERIN:

PUSH ACC
PUSH PSW

; receive

MOV A, SBUF ; store byte in A
CLR RI ; Clear receive flag

MOV @R0,A ; BUFFER[R0++] = A
INC R0
CJNE R0,#BUFFER+BUFFER_SIZE,ISR_EXIT ; if(RO == BUFFER+BUFFER_SIZE)
MOV R0,#BUFFER ; RO == BUFFER

ISR_EXIT:
POP PSW
POP ACC

RETI


;**************************************************************************
;
; SERIAL OUTPUT ROUTINE
;
SEROUT:
; CLR TI ;CLEAR TRANSMIT INRUP
RETI ;JUST EXIT ROUTINE


;**************************************************************************
;
; MAIN

START: ;main program (on power up, program starts at this point)
MOV SP,#02FH ;Set Stack Pointer = 2FH
ACALL INITIALIZE ;set up control registers

MOV R0,#BUFFER
MOV R1,#BUFFER

CLR RI ; Clear Receive Flag
CLR TI ; Clear Transmit Flag
SETB EA ; Global Interrupt Enable

MAIN_LOOP:

CLR C
MOV A, R0
SUBB A, R1

JNZ SEND_BYTE
; CJNE A, R1, SEND_BYTE
SJMP MAIN_LOOP

SEND_BYTE:

MOV A, @R1 ; A = BUFFER[R1++]
INC R1

CLR TI
MOV SBUF,A ; move outgoing value from A to SBUF
JNB TI, $ ; Wait till done transmitting

CJNE R1,#BUFFER+BUFFER_SIZE,MAIN_LOOP ; if(R1 == BUFFER+BUFFER_SIZE)
MOV R1,#BUFFER ; R1 == BUFFER

SJMP MAIN_LOOP
.END

The CPU is a 24MHz 8051 clone.
Any input will be apreciated.

Thanks!