I2c and UNO external power problem

Hi!

I'm trying to connect one master (M) and to slaves (S1 & S2) with the I2C protocol. I all ready do some tests and all works fine whit the three boards. In the tests I was using the three USB cords for each Arduino. The problem began when I was trying to connect an external power to the S1 & S2 boards, the M is connected to the computer with the USB (a processing sketch is sending serial data to the M). I'm using one external adapter (7.5V@1A) for the two slaves connected to Vin and Gnd. But... the arduinos (S1 & S2) don't work, they only work when I connect the USB cord to each of them (and disconnect the external power).

Trying to solve the problem I made a simple sketch that turns on and off some leds. First I power the arduinos with the USB cord, all were fine, and then with the external power (Vin = 7.5V) and also work!. I use a multimeter to try to figure out what was happening with the pins and I discovered that the pin 0 (Rx) and pin 1 (Tx) were at near 5V (In my sketch i wasn't using the Serial). With this in mind i get back to the original sketch, when i connect the external power i saw that pin 0 and pin 1 were at less than 100mV.

Whit all of these I have two conclusions (but no answers) 1.- My original sketch is wrong in the sense that doesn't sends any voltage to pins 0 and 1 2.- I need a voltage over 4V in pins 0 and 1 to get the arduino with the external power working

Anyone have a clue in how to solve this?

Regards

Are you connecting the grounds together or just SCL and SDA? If not, you must connect the grounds.

Also you may (probably will) require pull-up resistors. See this page: http://www.gammon.com.au/i2c

I still had my I2C Master-Slave on my table, http://arduino.cc/forum/index.php/topic,156639.msg1173834.html#msg1173834

This is what I did: SDA and SCL connected. Ground (GND) of both connected. Since it was a small test, I didn't use and external pull-up resistor yet. Master powered by USB, Slave powered with adapter. And it worked without problem.

You should not measure the RX and TX, but the 5V. If the 5V is okay, you probably don't have the Ground connected.