Communication network interface with screeners for incoming messages

Abstract

Each message carried by a communication network has a packet that contains data, a tag, and a generation count wherein duplicate packets sent in different messages have identical generation counts. A station includes a modem that connects to the communication network in order to receive messages. A fixed screener circuit has a first memory that stores a set of tag values and a fixed screener determines when a packet tag matches a tag value in the first memory. The station also has an interface through which tag values are received from an external device, such as a host computer. A general purpose screener includes a second memory with a plurality of storage locations in which to store tag values from the interface and a generation count associated with each tag value. A first comparator in the general purpose screener determines when a tag contained in a packet matches a tag value in the second memory, and a second comparator determines when a generation count in the packet matches the generation count in the second memory that is associated with the matching tag value. A control circuit produces a signal when the first comparator indicates a tag match and the second comparator indicates that the generation counts do not match. The station ignores the packet in the absence of the signal.