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[SOLVED] Interfacing GSM module with the 8051 based controller

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Newbie level 6
May 13, 2012
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Hi Guys

I am interfacing a GSM module of some chinese company named ATWIN to a 8051 based controller. The GSM module is this AT139.

The Controller has two UART. But I am using the UART0 for the communication purpose.

The name of controller is MG84FG516 of Megawin.

I checked the communication of the AT139 development board with the PC and it is communicating fine with the PC and my serial port communication codes are also working fine i.e. they are sending and receiving the characters and strings both from the computer and to the computer.

But when I connect the GSM module with the microcontroller like this:

AT139 GSM Module Microcontroller ( MG84FG516 )

RX --------------------------------- TX
TX -------------------------------- RX
GND ------------------------------ GND

I am getting the pulses or data its at the RX pin of the AT139 GSM module but in return of that there is no response from it.

Also the micrcontroller is operating at the VDD of 3.17 V and the voltage pulses at the I/O data pins of the AT139 are around 2.9 V.

Actually the AT139 is interfaced with the serial connector for the PC with a voltage level shifter SP3238 and with all the pins connected means DTR,CTS,RTS etc.

But in my actual communication with the microcontroller I am using just three pins RX,TX and GND.

So can you guys tell me where is the problem lying.

I am not able to trace it out.

Thanks and regards

The handshake signals RTS and DTR must be jumpered to active state (low).


Thanks for your reply.

But I want to know one thing that the RTS is ready to send, and my controller is sending the data at regular interval of time.

So why should I ground the RTS at controller end.

Also I want to know fvm that what makes you think that the grounding the RTS and DTR will make the communication with my AT139 module.

Please explain me a little bit also that why my AT139 GSM module is not working or communicating normerly as the other microcontrollers or devices do.

Thanks and regards

Serial handshake signals are active by default. That means, the modem doesn't answer before the signals are asserted. Some GSM modules might have built-in pull-down resistors that activate RTS and DTR when unconnected.


thanks for your support , you helped me a lot and I appreciate this very much.

Also I want to thanks jS for providing me these two great links and sharing his sources.

Atlast I want to thanks all the edaboard team for theie cooperation.

The problem is solved.

Thank you all.

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