Standards for Connectors

I've developed different standards for cables colours, I2C and serial connectors to speed-up development and ease control.

Colours for Cables

I use the following colour standard:
  •  red  for VCC=+5V 
  •  orange  for VCC=+3,3V
  •  black  for ground
  •  blue  for SCL = I²C clock
  •  green  for SDA = I²C data
  •  yellow  for RX=Serial UART receive
  •  white  for TX=Serial UART transmit
Left, a full I2C implementation. Right, I use the colour standard even for boards.

Connections and Connectors

I use the standard defined by fischertechnik for I2C connections on the Ext 2 plug of the TX controller.
Left, the Ext 2 plug for I2C on the TX controller (safety rightmost). Right, the pins of the plug, with the safety hole on the left (as below).
Left, the I2C connector with the safety hole on the rear face. Right, the connector plugged-in. 

Connections and Connectors

I use a Berg connector for serial UART connections.
Left, the connector with the cables attached (+5V rightmost). Right, the connector plugged-in (+5V leftmost). Note the safety system of the plug.