Plural null digital interconnections

摘要

Apparatus and method for interconnecting a controlling unit and a controlled unit include providing a plurality of operational nulls. One of the nulls is a logically disconnected null; another of the nulls is a logically connected null. All operations proceed in a predetermined sequence from either of the two nulls. The two units interchange signals indicating the interconnecting state; that is, both units supply the null signal simultaneously except when there is a desire by one of the units to change to another operational state. Unique status recovery procedures and apparatus are disclosed that enable recovery of status and emergency control of units during error prone or degraded operations. All of the functions are based upon an encoded set of tag signals being exchanged. For reliability purposes, complementary tag signal combinations provide diverse functions.