Dial backup

A modem can also be used as an alternate or redundant communication path to a remote. If a remote’s primary line of communication degrades, the system can automatically fail over to a backup modem line.

This is true if the field device is capable of communicating over the primary and alternate connections simultaneously. Otherwise, the communications must be manually switched by the Controller from the primary connection to the alternate.

Note: To switch the remote to an alternate communication path, the Controller sends a request to the remote to switch communications to the alternate path. The Controller does not have to specifically select the modem as the alternate.

Communications occur on both the primary and alternate line if the alternate connection is established in automatic mode. The primary line continues polling for a response from the remote to test the primary connection, but the data that is received is not processed. To inform the Controller of the line conditions, communication status and percentages of successful communications are maintained for both the primary and alternate paths. The alternate line polls the remote and processes all received data. Control commands are passed to the remote through the alternate line.

When the alternate connection is no longer required, automatic failover can disconnect the modem, or the Controller can request the modem to disconnect, making it available for future use. Communications then resume on the primary line.