A central processing unit polls a plurality of room control units (in e.g. each room of a hotel) by periodically transmitting, over the neutral and ground lines of the A.C. power distribution network, an interrogation signal having an address portion and a data portion. The room control unit addressed sends data to the central processing unit. A sequence of successive, identical addresses is used to allow the room control unit to accept data from the central processing unit, which also performs check comparisons on data it receives. IMAGE
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