interfacing gsm modem to microcontroller
Hi,
I suggest you to remove the present connections to pins 4 and 7 of the 9 pin connector. Then:
1. You short pins 4 and 7 of the connector and connect to pin 7 of Max 232 IC .
2. Connect pin 10 of Max 232 to ground.
3. You also connect pin 8 of Max 232 to Vcc through 1K resistor.
Points 1 and 2 ensures that DTR and RTS will be enabled high (10V) if your modem looks for that for communication. This will ensure that the status of PC's connector pins match with your controller connector. Point No.3 is just to ensure that the rx gate is not left open.
In one of the recent discussions in the Forum, it was pointed out that MAX 232 should use all 1uF capacitors only in the place of 10uF caps used to be seen with some of the earlier designs. I am not sure how far the value of these caps will affect, however, it is true that all recent data sheets of Maxim/TI etc. show only 1uF capacitors.
Also, with these changes, you can connect your microcontroller card using a cross connected cable to PC and check if it works with hyper terminal under identical condition of Baud rate etc., which you used while connecting PC to the modem. It will ensure that your micro is working as expected.
Please give us your test results.
Regards,
Laktronics