Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 92)

Switchover Considerations
Chapter 6
6-7
The secondary processor (remote I/O scanner or adapter) is not part of the
remote I/O link, but is communicating with the secondary 1785-BCM’s
smart switch interface. When the secondary processor is configured for
scanner mode, this interface responds to a poll from the secondary
processor’s scanner with actual data it receives from the primary processor
over the HSSL. The scanner “thinks” that it is communicating with the real
remote I/O adapters. The function of the interface is to respond to the
secondary processor with current data from the primary processor,
enabling the input image table of the secondary processor to be a copy of
that of the primary processor.
Similarly, when the secondary processor is configured for adapter mode,
the 1785-BCM module polls the secondary processor’s adapter, updating
the input image table of the secondary processor to be a copy of that of the
primary processor. Also, a BTR can be performed in the secondary
processor to receive data from the supervisory processor.
Important: The smart-switch interface communicates with the secondary
processor only when the 1785-BCM module belongs to the secondary
system. When the module is primary, the smart switch only monitors the
remote I/O link to obtain input/output data.
The smart switch interface also includes a carrier-detect circuit to monitor
the activity of the remote I/O link. The purpose of this circuit is to detect
an idle remote I/O link before the secondary relays are closed. This
guarantees that the former primary system was really disconnected from
the link and the remote adapters have completed their response to the last
poll from the primary scanner.
In addition, in the event of an HSSL break, the carrier-detect circuit
prevents the secondary system from taking control of the link along with
the primary system. Figure 6.1 shows the block diagram of the remote I/O
switch.