Abstract: The present invention preferably places a hardware circuit between the SCU's ARCNET transceiver and the backplane bus rather than a parallel snooper circuit. This circuit builds the map based on the tokens it observes. When the circuit detects the token for the SCU, it blocks the token transmission to the SCU's ARCNET transceiver. It then sends a minimum length “ping” message to each unit that was present on the bus during the last token rotation, but is not present during the token rotation that just ended. Since units that have lost their token can still respond to a Free Buffer Enquiry message, the circuit can use it as a ping to verify whether the missing unit(s) are actually missing or have just lost their token due to noise. After verification, the token is forwarded to the SCU's ARCNET transceiver.

Abstract: A system for providing 1:n protection for common processing units in a star bus architecture multiplexer or switching system and providing one redundant protection common processing unit to protect up to n working common processing units has a pair of first and second common processing slots (C) for common processing units, a plurality of tributary interface slots (T) for tributary interface units, and at least one universal slot (U) for accommodating either an additional common processing unit or a tributary interface unit. If the working on-line common processing unit of the dedicated pair fails, the protection common processing unit takes over for the failed common processing unit. If one of the additional common processing units fails, the dedicated protection common processing unit takes over, thereby providing 1:n protection regardless of where each common processing unit is located.