Methods and apparatus to provide redundancy in a process control system

A network may consist of multiple terminals 402,404,406 which may each have two network interface cards (NIC) 302,304 connected to two different networks. A primary network 408 and a secondary network 410. The hosts transmit period network integrity messages (often known as heartbeat or keep-alive messages), possibly on both primary and secondary networks, whose receipt at other hosts indicates good operation of the network. Routing decisions are made accordingly. In the event of a failure 602, integrity messages will not be received within a timeout limit, a fault will be inferred, the network link declared bad and an alarm may be issued. Failover operations will switch traffic from the failed primary network to the secondary network for affected links. If faults are removed, restarting receipt of integrity messages re-establishes the good status of network links. Tables of link statuses may be maintained (see Figs. 7,8).