Loop communications system with method and apparatus for switch to secondary loop

摘要

This invention relates to a system for communicating between a central station, such as a host processor, and one or more remote stations, such as terminal devices, on a pair of communication loops. A remote station operates normally on a selected one of the loops (the primary loop for the station) and it has access to but does not normally use the other loop (the secondary loop for the station). An improved apparatus and method is provided for switching a remote station from its primary loop to its secondary loop when its primary loop is disabled. Means is provided for a remote station to listen to messages on its secondary loop. For switching the remote stations from a disabled primary loop, a unique command is transmitted on their secondary loop. Remote devices operating with a primary connection to the loop that carries this command recognize the command and respond in a non-interferring way. Remote stations having a secondary connection to the loop respond differently to this command and switch from one loop to the other. In a system called Synchronous Data Link Control (SDLC) the command is thirty-two consecutive 1 bits. The circuitry for detecting this command on a secondary loop is particularly simplified.