 They carried overflow traffic when the small Junctor Groups of an office with many LN could not cope.Digit transmitters, receivers, other complex service circuits, and some complex trunks including those using E&M signaling, were permanently mounted in relay racks similar to those of 5XB rather than plug-in frames.The computer received input from peripherals via magnetic scanners, composed of ferrod sensors, similar in principle to magnetic core memory except that the output was controlled by control windings analogous to the windings of a relay.