A communication link manager for problem determination and recovery of a failing resource on a communication link segment in a data communication network. When a problem occurs on a link segment, the using node passes link event data to the communication link manager for analysis. The communication link manager interacts with a configuration data base to determine the physical configuration of the failing link segment and the controlling link connection subsystem manager. The communication link manager directs the appropriate link connection subsystem manager to initiate tests of the various link connection components on the link segment under its control. When the failing resource is identified, the communication link manager initiates the appropriate non-disruptive recovery procedure through the link connection subsystem manager and prompts the data link control to restart the line.

A communications interface exchanges serial bit signals between sources differentially, over redundant, two conductor, primary and secondary channels. The primary channel provides the communications link during normal operation, and is monitored to ensure differential operation. In response to primary channel failure, the secondary channel becomes the communications link.

A modem, or data communications device, that is testable independently of a DTE device permits remote testing of it automatically by a network management computer or controller reducing required operator intervention. A data communications network that has possibly hundreds of these modems requires substantially less operator intervention.

A primary transceiver having a driver/receiver initially drives a bus while a secondary transceiver having a driver/receiver receives the signals present on the bus. The system compares the signals received by the secondary driver/receiver and the inputs to the primary driver/receiver. If a difference is detected, the system controller disables the primary driver/receiver and enables the secondary driver/receiver which drives the bus.

A serial data communication loop system includes single input-output controllers and/or dual message transmission with a novel adaptor unit for converting from a single to dual message frame and/or from a dual to single message frame. The message frames are multiple bit binary coded signals. The adaptor unit includes first and second converting sections, the first converting from a single input to dual output message frames and the second from a pair of identical messages to a single message output. The first section includes an amplifier connected to a polarity and level sensor and to a time delay which activates a disable timer to the amplifier and a strobe timer to operate a pair of transmitters to transmit the dual message frames. The second section includes a pair of receivers connected to a logic selector in combination with individual monitors which encode the status at the receivers to normally transmit from one receiver and automatically change to the second if the first fails. The adaptor may connect the input and output ports of a controller to a pair of parallel transmission trunks, automatically switch between main and back-up loop controllers connected in either a single or multiple trunk system, and provide an individual or group remote unit with message continuity if an individual or group loop is broken.