Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 47)

Installing Your 1785-BEM Module
Chapter 4
4-4
As shown in Figure 4.2, there are two user-configurable ports (Channels
2A and 2B) that support remote I/O or Data Highway Plus modes.
Table 4.A lists the communication ports and describes how the system uses
each port.
Table 4.A
Communication Ports
Communication Port This link is used to:
Channel 2A connect the primary PLC-5/40, -5/60, or PLC-5/80 processor to the Data
Highway Plus network or to the Remote I/O link; the secondary PLC-5/40
or PLC-5/60 processor is isolated from this link.
Channel 2B connect the primary PLC-5/40, -5/60, or PLC-5/80 processor to the Data
Highway Plus network or to the Remote I/O link; the secondary PLC-5/40
or PLC-5/60 processor is isolated from this link.
The 1785-BEM module is shipped with Channel 2A configured for Data
Highway Plus mode and Channel 2B configured for Remote I/O scanner
mode at 57.6 kbaud (default configuration).
Important: Note that I/O, status, and data table communication between
the primary and secondary backup systems is accomplished over the
HSSL, which is connected between the primary and secondary 1785-BCM
modules only. Each 1785-BEM module communicates with the 1785-BCM
series B module, in its same rack, via a secondary backplane bus. Remote
I/O or Data Highway Plus is connected between the primary and secondary
1785-BEM modules through Channels 2A and 2B.
Channels 2A or 2B have an internal relay whose contacts are closed when
the controller is primary and are open when the processor is secondary.
Figure 4.3 shows these relays for processor A and processor B of a PLC-5
backup system.