Thank you very much. Very insightful
Yes I did pretty much get to grips with the SIM900 now. I'm getting round the power-up problem using a relay as mentionedd before. It is very cumbersome, and wastes a GPIO pin, but it will do for now.
I also managed to hit the USB power limit finally. Determined to find the limit, I dropped in a shift register with LEDs attached ... once I got to 8 LEDs on the SIM900 wouldn't power on any more
to power it up, just declare Pin 8 as output, False. then you can control the power button through pin 9. send High from netduino for 500ms.
regards