EN Enable line is used to turn on/off the IC chip. It shuts internal circuit off so that it will not use power when it is not needed and to isolate the chip from other circuits in the network. It places the device into high impedance state. It is especially useful when it is in the large complex bus where there are many signals that share same signal lines. For most simple circuit, the Enable line is held always ON via either tied to ground or VCC depending on the IC chip.