I2C can be used to connect up to 127 nodes via a bus that only requires two data wires, known as SDA and SCL.

  • SDA=4 => D2.
  • SCL=5 => D1


SPI is much simpler than I2C. Master and slave are linked by three data wires, usually called MISO, (Master in, Slave out), MOSI (Master out, Slave in) and M-CLK.

  • M-CLK => D5
  • MISO => D6
  • MOSI => D7

(SPI Bus SS (CS)is D8.)