Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 66)

Operating Your PLC-5 Backup System
Chapter 5
5-2
Figure 5.1
Automatic Transfer of Data Over Remote I/O
HSSL
Primary Secondary
PLC-5 PLC-5
1785-BCM 1785-BCM
Remote I/O
11053I
In addition to providing the secondary processor a copy of the remote discrete
input image table, the 1785-BCM modules in both systems work together to
provide the secondary processor a copy of block transfer read (BTR) data from
the remote I/O chassis.
The primary 1785-BCM module provides the secondary 1785-BCM module
with information that a BTW command has been accomplished. The processor
can store in memory up to 256 BTW done bits (16 racks x 16 slots=256,
maximum) per channel. The primary 1785-BCM module has sufficient memory
to store up to 92 BTR instructions of a possible 256 per channel. When the
primary processor executes a BTR from any module in a remote I/O chassis, a
copy of the data is stored. Should the secondary processor attempt to read that
module, it will receive the same data. If the secondary processor attempts to
read from a module that the primary processor has not read, the done bit (DN)
will not turn on.
Transfer of Data with Block Transfer Instructions
Although not required to implement backup, you can transfer data in the data
table by including block transfer instructions to the 1785-BCM module in your
application program. We describe transfer of data in the data table by
explaining the:
sequence of transfer
method of transfer
words in the BTR and BTW instructions used by the PLC-5 backup system
input image table auxiliary diagnostics/status words