I'm using AT89C51 to control MC35i to send sms, my problem is the program keeps on looping to receive an 'OK' from the MC35i, which means I can't receive a reply from the modem after I've sent AT commands to it. I know that to interface between these 2 devices, I have to use a MAX232 to convert level between TTL and RS232, but I'm not sure about the connection between MC35i (which has a RS232 port) and MAX232, is it only the Tx, Rx and ground of RS232 are needed to be connected? How about other 6 pins such as RTS, CTS, DTR and DSR? Do they need to be connected?

Below is my program, is there any problem in my program? The program keeps on looping at the section in bracket, is it because of the MC35i does not receive any AT commands from the microcontroller?

ORG 0000H ;Power up
SJMP MAIN

ORG 50H
MAIN: MOV P2,#01H
ACALL DELAY
MOV PCON,#80H
MOV SCON,#52H ;8-bit, 1 stop bit, REN enabled
MOV TMOD,#20H ;Timer 1 in mode 2
MOV TH1,#-3 ;9600 baud rate
SETB TR1 ;Start timer 1
MOV DPTR,#ST ;Get address of Station
MOV R6,DPH ;Save higher data pointer
MOV R7,DPL ;Save lower data pointer
MOV P2,#81H
ACALL DELAY
NextMessage:
MOV DPTR,#AT ;Get address of AT
MOV P2,#11H
ACALL DELAY
ACALL SingleCommand ;Send AT+CMGF
MOV DPTR,#CMGF ;Get address of AT+CMGF
MOV P2,#22H
ACALL DELAY
ACALL SingleCommand ;Send AT+CMGF
MOV DPTR,#CSCA ;Get address of AT+CSCA
MOV P2,#33H
ACALL DELAY
ACALL SingleCommand ;Send AT+CSCA
MOV DPTR,#CMGS ;Get address of AT+CMGS
MOV P2,#44H
ACALL DELAY
ACALL ATcommand ;Send AT+CMGS

WaitReady:
ACALL SerialIn ;Get the status of the phone
CJNE A,#'>',WaitReady ;Repeat if symbol '>' not received

MOV DPH,R6 ;Retrieve higher address of station
MOV DPL,R7 ;Retrieve lower address of station
ACALL ATcommand ;Send station
MOV R6,DPH ;Save current higher address of station
MOV R7,DPL ;Save current lower address of station
CLR A ;Clear accumulator
MOVC A,@A+DPTR ;Get data to be saved into accumulator
JNZ NextMessage ;Jump to NextMessage if '0' is reached
MOV P2,#88H
ACALL DELAY
FINISH: JMP FINISH ;If yes, end of program

SerialOut:
CLR TI
MOV SBUF,A ;Load the data
WaitTI: JNB TI,WaitTI ;Stay until last bit sent
CLR TI ;Get ready for next character
MOV P2,#55H
ACALL DELAY
RET

SendIt: ACALL SerialOut ;Jump to send
ATcommand:
CLR A ;Clear accumulator
MOVC A,@A+DPTR ;Get data to be saved into accumulator
INC DPTR ;Point to next data
MOV P2,#99H
ACALL DELAY
JNZ SendIt ;Jump to send if it is not zero
RET

SingleCommand:
ACALL ATcommand ;Jump to ATcommand
Wait0DH:
ACALL SerialIn ;Jump to receive
CJNE A,#0DH,Wait0DH ;Jump if not equals to carriage return
MOV P2,#77H
ACALL DELAY
RET

(SerialIn:
MOV P2,#00H
ACALL DELAY
MOV P2,#0FFH
ACALL DELAY
JNB RI,SerialIn ;Wait for another character)

MOV A,SBUF ;Save it in accumulator
CLR RI ;Get ready for next character
MOV P2,#66H
ACALL DELAY
RET

DELAY:
ACALL DELAY1
ACALL DELAY1
DELAY1:
MOV R2,#0FFH ; 1 machine cycle,1.085us
LONG1:
MOV R3,#0FFH ; 1 machine cycle,1.085us
LONG2:
DJNZ R3,LONG2 ; 2 machine cycle .
DJNZ R2,LONG1 ; Total delay = 256X256X2X1.085u = 142ms
RET

AT: DB 'AT',0DH,0
CMGF: DB 'AT+CMGF=1',0DH,0
CSCA: DB 'AT+CSCA="+60120000015"',0DH,0
CMGS: DB 'AT+CMGS="+60122521131"',0DH,0
ST: DB '01',1AH,0,'02',1AH,0,'03',1AH,0,0

END

Thanks in advance.


Is this connection correct?