sim900 serial communication doesn't work

[Alessandro Capezzera]

Posted on April 16, 2017 at 13:03

Hi, I'm trying to configure sim900 serial communication with stm32f401re but it seems that it doesn't receive any serial command, source code i'm using: 

https://pastebin.com/bvrYxEn9

 (tried with differents port combinations and with

http://m2msupport.net/m2msupport/module-tester/

but it doesn't work and probably I'm forgetting something). I attach a photo of my stm32 connected with sim900

Thanks for the support

PS: the status of the sim900 blink with 1sec interval and I know that it should be 1 blink each 3 seconds, what am I missing?#stm32f4 #sim900

[Tesla DeLorean]

Posted on April 17, 2017 at 13:56

Do you have a schematic for the modem board? What are the power requirements? Have you mixed rx and tx pins? What pins and USART are you using?

[Alessandro Capezzera]

Posted on April 18, 2017 at 21:22

No, I don't have any schematic for the modem board. I bought it here:

https://www.amazon.it/Quad-Band-Modules-Development-Arduino-adattatore/dp/B01DVYFVXC/ref=sr_1_1?ie=UTF8&qid=1492542089&sr=8-1&keywords=sim900

and in the packet there was only the modem board and sim adapters. Power requirements are 2Amp, 3.2V to 4.8V,

https://developer.mbed.org/media/uploads/bcostm/nucleo64_revc_f401re_mbed_pinout_v3_arduino.png

I'm using D2 and D8 pins in stm32 and pins 7 and 8 in the modem board as indicated here:

http://linksprite.com/wiki/index.php5?title=SIM900_GPRS/GSM_ShieldPins_usage_on_Arduino

, you can see pins connection above, attached to the topic. I noticed that there's no 2G network for my operator so I understood the wrong led blink but why AT serial commands doesn't work? (waiting for new sim operator)

[Tesla DeLorean]

Posted on April 18, 2017 at 22:21

It shouldn't need a SIM or service for the AT commands to work. Things like ATE1 and ATI4 should work

If you have TX/RX wired correctly, then make sure the modem is actually powered up, use the power button or D9

Check the VDD_EXT

https://s3.amazonaws.com/linksprite/Shields/SIM900_GPRS/GPRSshield_Schematic.pdf

 

[Alessandro Capezzera]

Posted on April 19, 2017 at 21:09

The modem is powered up(because netlight blinks). What you mean with check the vdd ext? How can i check if I

 have TX/RX wired correctly?(you can see them in the image attached to the topic) I'm trying also with this simple code but I get no serial answer: 

https://pastebin.com/najSsUYZ