sim900 sending response multiple times

[devil6600]

hi

i am using sim900 module connected to max232 to pc, when ever i am sending any command to it i get its response multiple times, about 50-60 times a second. i have triple checked power supply but no issue found. i am using lm317 to power it up. also i am using it without antenna for now just to check everything.
any help would be appreciated.

thanks

 

[horace1]

sounds more like a programming problem than hardware - could you be sending the command to the sim900 multiple times hence getting multiple responses? have a look the Rx and Tx lines with an oscilloscope

 

[devil6600]

hi horace1
thanks for your comment. now i have tried powering it without the tx line connected, still i am getting the default response many time, more than 50-60 times per second.

RDY

+CPIN: NOT INSERTED

+CFUN: 1

this is the message sent by the module on startup
do you mind taking a look at my schematic? i think i have done some connection wrong
thanks

 

[devil6600]

hi
i was checking continuity of the tracks and found antenna pin short with ground, so i removed the module and again checked its continuity (directly on the module pads itself) between antenna and ground and again i found it short, strange.. antenna is connected to ground internally... is it normal? I have few extra modules and with them on checking continuity i found them also short. can any one check this on their modules? I suspect all the modules i have are defective...
can anyone help... plz..
thanks

 

[Sparked]

Devil,

I had a similar problem. I was using the IComSat PCB with an Arduino Mega. I wrote a simple sketch that sends data received from the SIM900, by the arduino, to my terminal program through my usb port. It also sends data received from the terminal program, by the arduino, to the SIM900. When I first tested this I had the same problem as you. It turns out that I was trying to use the same rx/tx pins for both. Once I realized this I switch to using two sets of rx/tx pins and all was well. Hope this helps.