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I would suggest you look at either the FTDI port or the I²C port. MartinR used the I²C bus to communicate between 3 stacked emonTx's here: http://openenergymonitor.org/emon/sites/default/files/emonTx_CT123_Volta...

Is there a way to get only instantaneous current and voltage by using the normal RF communication? I want to eventually capture the data and find characteristics in the current and voltage curves for different home appliances.

regards 

jacodt357:

What do you mean by "instantaneous"?  The emonTx samples the voltage and current waves at about 2500 samples per second. Unfortunately, that is too fast for every reading to be sent by radio. The emonLib library has methods that read those values for a number of cycles that you can define - normally 10 - and gives you the rms current and voltage, average power and apparent power for that period.

If you want to record the inrush current over more than a cycle, even that might not be possible because of the limited memory in the Atmel ATmega328P. But if you only want the peak current during the first 10 or 20 half-cycles after switch-on (say, which should give you a reasonable profile) then you should be able to write some code that records that and later sends the values by radio. But you would need to ensure that the radio was not transmitting when the appliance switched on, or you would miss it.

I don't see how the "voltage curves" will help you - the voltage will dip, certainly, but how much depends as much on the supply system as it does on the appliance.