User manual

Understanding the 1785-BCM
Hardware Components
Chapter 2
2-4
T able 2.A
Communication Ports
Communication Port: This link is used to:
High Speed Serial Link (HSSL) permit two-way alternating communication (half-duplex)
between the two 1785-BCM modules of the backup system
at a distance of up to 15 feet.
Channel 1A connect the primary PLC-5 processor to the Data Highway
Plus network or to the remote I/O link; the secondary
processor is isolated from this link.
Channel 1B connect the primary PLC-5 processor to the Data Highway
Plus network or to the remote I/O link; the secondary
processor is isolated from this link.
Channels 1A and 1B have a default communication mode which can be
changed by resetting switches, if necessary. Table 2.B describes the default
communication mode for each of the two channels.
T able 2.B
Default Communication Modes
Channel Default Communication Mode
1A Data Highway Plus
1B Remote I/O – Scanner mode at 57.6 kbaud
With the exception of the HSSL, all of the connections in the 1785-BCM
module have an internal relay whose contacts are closed when the
controller is primary and open when the processor is secondary. Figure 2.3
shows these relays for processor A and processor B of a PLC-5 backup
system.