Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 65)

Chapter
5
5-1
Operating Your PLC-5 Backup System
In this chapter we describe how the primary system transfers data to the
secondary system. We do this by describing the operation of the:
1785-BCM module
PLC-5 backup system
In addition, we provide procedures for:
starting your system
powering up your system
disconnecting a faulted system
restarting a repaired system
changing the processor’s operating mode
editing a program on-line
In a PLC-5 backup system, the 1785-BCM module performs three distinct
functions:
automatic transfer of remote input data over the High-Speed
Serial Link (HSSL)
transfer of data in the data table by block-transfer instructions
secondary processor remote programming capability
Automatic Transfer of Remote Input Data Over the HSSL
With automatic transfer, the primary 1785-BCM module is continually updating
a copy of the remote input image table in an interface located in the secondary
1785-BCM module. This remote input data includes discrete data as well as
block-transferred data. Automatic transfer is carried out over the High Speed
Serial Link (HSSL) and is independent of the application program.
The secondary 1785-BCM module responds with current data to the remote I/O
scan requests of the secondary processor, making the secondary processor
“think” that it is communicating with remote I/O chassis. This response
prevents remote I/O faults in the secondary processor, since the secondary
processor is not physically connected to the remote I/O link (Figure 5.1).
Chapter Objectives
How the 1785-BCM Module
Operates