The main board in a computer Accessory boards are plugged into "slots" on the mother board, and may have connections on the "back plane" of the computer for connecting external devices The mother board contains the microprocessor, the memory, and a variety of other components Newer computers have more of their components on the mother board, meaning that fewer accessory boards are necessary Thus, newer computers can get by with fewer "slots" than older ones
The heart of the alarm interface/isolation bay design consisting of diode sets, relays, terminal strips and lamps which are housed in a framework intended to be located in the power board line