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Keep it simple

– at least to begin with.

I have received the stuff (from Lawicel in Sweden since they had everything in stock) so let's fire it up...
Having read about how many problems can arise with these modules, I have decided to try out a simple-simple test first.
That means connecting the GE865 to a 1000 mAh LiPo battery (without the low ESR cap which I haven't received yet), connecting a manual push button on the on-off pin and a LED to the status LED pin (and a SIM card, obviously).
And then use the Arduino's serial monitor to send and receive commands.
However, the Arduino runs on 5V so I will need a voltage divider to step down to 2.5V on the TX line (which is close enough to the nominal 2.8V required by the 2.8V CMOS logic) and a 74LS04 to step up to 5V on the RX line (since, according to the datasheet, 2.8V is just enough to be seen as high when the ATmega runs on 5V. And I can't be sure that the GE865 will set high as 2.8V – it could be as low as 2.2V which won't be recognized as high by the Arduino).
Note: R3 is a 100k pull-up resistor on the RX line. This is because the Arduino powers up first and starts listening for serial communication (The RX line is output from the GE865 and input on the Arduino). This means that without the pull-up, the RX would be in an unknown state and we might get garbage. With the pull-up, the line will be high (which, as we all know is UART idle) when the C104/RXD pin is in floating state.
I haven't added a pull-up on TX to protect the GE865 from garbage comms from the Arduino since the Arduino will always be running with UART initialized when the GE865 is on.
It looks like this:

(click to view full size)

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