Rockwell Automation 1785 PLC-5 User Manual download pdf (Page 66)

Operating Your PLC-5 Backup System

Chapter 5

5-2

Figure 5.1

Automatic

ransfer 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

1 2 ... 61 62 63 64 65 66 67 68 69 70 71 ... 160 161

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Rockwell Automation 1785 PLC-5 User Manual Page 66