A question about connecting 7610 through cable

[hswlfg]

I develop a programe use java for sending message in pdu mode,and now it can work.But the problem is ,the second time i can't send AT command to the cellphone when i run the programe twice and the interval overrun five minutes .How can i resolve this...

following is about my programme

/*****************

1)init the port
2)send "AT" to 7610(return ok)
3)set mode to PDU and send message (send ok but the
return string is the pdu code)
4)disconnect the port
*****************/