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I was hoping you guys could help me on connecting SIM900A GSM Modem with this TM4C123 micro controller. enter image description here I connected the RS232 ports of TX, RX, GND and power ports to GND and +12V. should I connect the RS232(TTL Levels) too? (the yellow pin connectors). As you can see in the picture, the red LED turns on for about 64ms and off for 3s, which based on this website: https://elementztechblog.wordpress.com/2014/10/10/interfacing-sim900a-gsm-modem-with-arduino/ means its registered to network or SIM card is active. but Green LED should be on to indicate that it is working properly and ready to send and receive messages, right? Could someone please explain the pin connection for this modem. thanks in advance

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  • \$\begingroup\$ You'd normally just connect one or the other. Are you sure you're using the right one? \$\endgroup\$ – Finbarr Apr 25 '17 at 20:15
  • \$\begingroup\$ It would be much easier if you drew a schematic, it's often impossible to tell how things are connected from a photo. \$\endgroup\$ – Finbarr Apr 25 '17 at 20:19
  • \$\begingroup\$ I followed this website for pin connection: instructables.com/id/… \$\endgroup\$ – Aliyar Attaran Apr 25 '17 at 20:40
  • \$\begingroup\$ That diagram would suggest you should be using the TTL port to connect directly to the MCU pins. The RS-232 voltage levels will probably damage the MCU. Without studying the datasheets for the GSM module and the Launchpad module it's hard to know. \$\endgroup\$ – Finbarr Apr 25 '17 at 20:57
  • \$\begingroup\$ Ok, could you please tell me these pins go where, SIMT, SIMR, T and R. \$\endgroup\$ – Aliyar Attaran Apr 25 '17 at 21:19
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The correct connections are: 3VR on GSM modem to TX port on launchpad, 3VT on GSM modem to RX port on launchpad and GND to GND. External Power supply too. you can test it with connecting the GSM modem to TTL to USB adapter and open putty and write "AT", if it replies "OK", its working.

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