Problem with MC35i

Dear Friends.

I am facing a problem with MC35i GSM modem from Siemens.

I need to connect the MC35i GSM modem with my ARM7 based microcontroller which is LPC2148. I have one evaluation board of LPC2148 which is MCB2140 from KEIL.

I need to connect the MODEM to one COM port which is connected with two wires only Tx and Rx using MAX563 as charge pump.

When I am sending these AT commands connecting the modem to Computers COM port it's responding easily but while connecting this MC35i GSM modem to my evaluation boards com port there is no response to any AT command.

So, I have tried to send the data from my evaluation board to Computer and this time it's taking the data.

But sending the same to Modem is not giving responce. I am just sending a command ATD<number>; for a voice call to my cell phone but there is no ring so showing no responce from Modem.

Should I make some changes in hardware?? I want to do it with two wires only, even though I have tried to give +12V permenantly to the PIN7(RTS) of the modem. But not able to get anything.

Is this because MAX563??? should I replace it with MAX3232 or problem is somehwere else.

Please guide me as struggling from long time now.


My email address is bhimanipankaj(at)gmail(dot)com

Thanks and Regards,

Pankaj