The controller represents the CPU of a controller and usually has only one communication channel. Signals reach the controller via this channel for processing in the CPU. In many systems, this channel corresponds to the backplane bus. The controller can be connected to couplers. Consequently, when a processor made by a controller manufacturer also has connections for external communication (e.g. for programming devices or decentralized peripherals), these can be - logically - connected to the CPU via various couplers.