LOW PORTA cause serin2 not working.

[mikewen]

Really need some help here.
I spent the whole weekends still not able to figure out the problem:
LOW PORTA.4
will cause serin2 stop working

If I comment out the ' LOW PORTA.4 ' in my code.
It works fine.

Can someone shed any light onto the problem ?
BTW, I use PBP250A. the chip is PIC12F635.

Include "modedefs.bas"

OSCCON = $60
TRISA = %11111110
PORTA = GPIO
BaudRate    CON 188

STR1 VAR BYTE[12]  'Use as Lat and other
STR2 VAR BYTE[12]   'Use as Log 
T Var BYTE

RS232_out var PORTA.1
GPS var PORTA.2  
RS232_in var PORTA.5

GPIO.0 = 0
GPIO.1 = 0
GPIO.2 = 1
GPIO.3 = 1
GPIO.4 = 0
GPIO.5 = 1

Init:
    SERout2 RS232_out,BaudRate,["at+clip=1",10,13]
    SERIN2 RS232_in,BaudRate,2000,Init,[WAIT("OK"),T]
    
    
read 1,STR1[0]
if STR1[0] = "1" THEN
    LOW PORTA.4   'Here is the problem
    SERout2 RS232_out,BaudRate,["On",10,13] 
else
    HIGH GPIO.4
    SERout2 RS232_out,BaudRate,["Off",10,13] 
endif

PAUSE 1000

SERIN2 RS232_in,BaudRate,[WAIT("+CLIP: ",34,"+8600"), STR STR2\12]
'Not enough RAM, use Lat to get caller id

    select case STR2[0]
        Case "1"
            write 1,STR2[0]
        Case "2"
            write 1,STR2[0]
        case "3"
            'Get Tracking
            SERout2 RS232_out,BaudRate,["ATA",10,13]
            PAUSE 1000
            SERIN2 GPS,BaudRate,2000,Reboot,[WAIT("$GPGGA,"),Skip 11,STR STR1\12,STR STR2\12] ' Fill the arrays with received data.
            SERout2 RS232_out,BaudRate,["AT+vts=*;+vts=",STR1[0],";+vts=",STR1[1],";+vts=",STR1[2],";+vts=",STR1[3],";+vts=",STR1[5],";+vts=",STR1[6],";+vts=",STR1[7],";+vts=",STR1[8],10,13]
            PAUSE 5000
            SERout2 RS232_out,BaudRate,["AT+vts=#;+vts=",STR2[0],";+vts=",STR2[1],";+vts=",STR2[2],";+vts=",STR2[3],";+vts=",STR2[4],";+vts=",STR2[6],";+vts=",STR2[7],";+vts=",STR2[8],10,13]
            PAUSE 5000
    end select
    SERout2 RS232_out,BaudRate,["ATH",10,13]

Reboot:
@ goto $