http://www.freewebs.com/maheshwankhede/rs232.html

In the circuit diagram of GPS GSM connection, we are not able to understand how switching is done by NAND gate logic.our problem is that GPS data is continuously received by controller.but when user send message to controller then only controller send message of GPS data via GSM.so switching of NAND gate logic is not in our hand!!
we want,
1) user send message (request) to controller via GSM to send GPS data.
2) controller detect that message and according to this it changes logic of nand gate (2:1 MUX).
3)so it stops to receive GPS data and start receiving GSM message which is sent by user to request data.
4)then only it send GPS data by GSM.
5)after that GPS receiver continue receiving data.
so,when user want data then only it get data.
please reply me and give me some details how to implement this logic.