The SIU is optional equipment. An SIU is needed if, for example, door status cannot be provided using a serial or Ethernet interface on the VDR.

A standard non-modular compact SIU has 8 serial interfaces, 8 analog interfaces and 64 digital interfaces. A modular SIU also exists which can be configured to have other combinations of interfaces. The SIU must be installed indoors. It must be connected to the DAU with an Ethernet cable up to 100m long.