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I am trying to connect a sim-card (GSM) module to my RPi. I've connected its serial pins, to the serial pins on the Pi. I then edited the /boot/config.txt file by adding enable_uart=1 to the end. After this, I installed the ppp screen application (see ppp.samba.org) to communicate with the serial device. I then used the command: sudo screen /dev/serial0 115200 to open the communication interface (115200 is the baud rate for the GSM module). However, upon entering the command, I am faced with a blank screen which does not accept any inputs. I am unable to enter any commands. I would appreciate if you could give any ideas.

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Based on the scant information you've provided, I'm not sure if this is "in the ballpark" or not, but here goes anyway:

Have you looked at Wammu & Gammu?. I was looking for a way to send SMS messages from my RPi, and convinced myself the package was worth a closer look. From outward appearances, it looks like a currently maintained project, and he has a load of documentation - which is just to say the module libraries appear credible.

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At first, what is your GSM module? Is it on a third party board for easy connection or just a module like SIM900 or SIM800. If it is on a third party board I suggest you check its tutorials or other existing projects on the internet to how to configure it. If it is not on a board make sure you give enough voltage or current. Or connect pins to connect inputs.

Also If you are sure about the module is on and working, make sure that you implemented PPP correctly by trying pon or chat codes on raspberry.

By the way, on most of the modems, if you establish a PPP connection, it means it is in a data mode. In data mode, you can not control or read by serial. It means the serial bus is using for packet transfer from RPI to the modem. To connect or read serial make sure your modem is in command mode. You can check how to open data mode by checking the datasheet of your modem.

Also for the serial read on raspberry pi, use minicom. It is an old program but it is reliable. After establishing serial try writing "AT". Most of the modems use AT commands to work. If it returns you "OK", that means it is working.

If you have any other problems ask at comments.

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