sahin5002, I am posting a schematic where I have modified the pin out to fit your 16F877. Since you are using a 3 volts modem you must be very carefull to connect it to the pic. The modem Tx can be connected to the Rx (input) directly without any problem, while the modem Rx must be connected to the pic via a resistors partition so that the TTL (5 Volts) will be reduced of 1/3 prior to input it to the modem. The rest of the pin out has been made compatibile with my project SMS IO Controller, but you can change them as you like to suit your need.
If you want to give a look to my project you can find it @ http://techni.caliti.es/blog/2008/12...ontroller.html.
Al.
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