I2C

Every device has a 7-bit address. A transaction:

1. START — SDA falls while SCL is high.
2. Address + R/W bit; the device ACKs (pulls SDA low).
3. Data bytes, each acknowledged.
4. STOP — SDA rises while SCL is high.

Wire.beginTransmission(0x68);   // MPU-6050
Wire.write(0x3B);               // register address
Wire.endTransmission(false);    // repeated start
Wire.requestFrom(0x68, 6);

Lines are open-drain with pull-ups (4.7 kΩ at 100 kHz is typical; lower values for faster speeds or long buses). Speeds: standard 100 kHz, fast 400 kHz, fast-plus 1 MHz. Run an "I2C scanner" sketch to find device addresses.