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Hello sir/madam, My name is srinivas.for my microcontroller interfaing to GSM modem application, first i tested GSM modem with PC that time , i am getting a new problem while interfaing PC with GSM modem.when i am interfacing PC to GSM modem with total 9 pin DB9 connector RS232 cable, it working fine. But when i am interfaing GSM modem to PC with(Rx,Tx and GND) 3 pins , its not working. how i overcome this problem . please help me, bcoz for application i will provide those three pins only.i.e,when i will GSM interface to microcontroller , i will provide those three pins only. please help me and if you give through suggestions this mail also oltc@inteluxindia.com. Thank you very much.
"But when i am interfaing GSM modem to PC with(Rx,Tx and GND) 3 pins , its not working."
Actually, it almost certainly is working!
All modems (not just GSM) will use the full set of RS232 control lines - if you don't assert the required signals, the modem will correctly interpret this as a "command" to not communicate!
Your modem may have an AT Command (or commands) to allow it to work with just the 3 wires - check the Manual - but many require certain lines to be active.
If not, you will need to loop-back CTS to RTS at the modem:
Modem 3-Wire cable to PC TXD in <-------( 3 =-----------------< TXD out RXD out >-------( 2 =-----------------> RXD in CTS out >-------( 8 =--+ | RTS in <-------( 7 =--+ GND ------------( 5 =------------------ GND
You might also want/need to loop-back DTR to DSR:
DSR out >-------( 6 =--+ | DTR in <-------( 4 =--+
Again, check the modem's Manual
Remember that you will also need a suitable RS232 Transceiver (MAX232 or suchlike) between your microcontroller and the modem...