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?