This is a draft of the the stand-alone GSM Module designed around the SIM800C module. I still want to see if I can get this into the Ethernet module, but made the diagram above to illustrate the content.
The primary interface to the MCU is UART + some GPIO. This uses the AT command set to control the GSM.
An USB is available from the GSM Module for debug and firmware download. The intention is to connect this to the USB on the MCU as well as making it available through an extern port. I have a loose end on how to do this that I need to dig into. I might also put this port a bit hidden as it is only optionally needed.
GSM antenna need to be added.
SIM Card holder need to be added.
Sound In/Out is usually to a mic/speaker. As this is not a phone, but a PLC we would like that digitalised so we can play sound files, do funny things like speech recognition etc. The later will require a Raspberry PI (Zero ) Add-on to do the processing. I am thinking of running an open source solution where the GSM act as a line card to FreeSWITCH or Asterisk + PocketSphinx to support a single line.
All in all I must admit that this module will require some space, so it might be a bit crowded to get in on the Ethernet Module – lets see. It’s a lot of loose ends here yet.