A-BPLC-5 Backup Communication Module

and is used to pass I/O, status, and data table information.

Primary System is the PLC-5 processor that controls the I/O and the other

equipment associated with that processor.

Secondary System is the PLC-5 processor that is ready to assume control of

the I/O and the equipment associated with that processor.

Backup System is the primary and secondary systems.

Switchover is the transfer of I/O control from the primary processor to the

secondary processor.

Bumpless switchover is the transfer of I/O control from the primary

processor to the secondary processor where the operation of the process

being controlled is not affected.

Asynchronous processing is processing where the ladder program scan and

the block-transfer scan operate independently of each other within the same

scan time.

BTW is block-transfer write; transfer of up to 64 words from the processor to

the block-transfer module.

BTR is block transfer read; transfer of up to 64 words from the block transfer

module to the processor.

Smart Switch Interface is a remote I/O interface in the 1785-BCM module

that responds to secondary scanner poll commands with actual input values

from the remote I/O link, making the scanner think it is talking with 

remote I/O.

For more information about components used with the 1785-BCM module in a

PLC-5 backup system, refer to the following publications:

Publication Title Publication Number

Enhanced and Ethernet PLC-5 Programmable Controllers 

User Manual

1785-6.5.12

Classic 1785 PLC-5 Family Programmable Controllers Hardware

Installation Manual

1785-6.6.1

PLC-5 Programming Software Documentation Set 6200-N8.001

PLC-5 Programming Software: Installing and

Configuring the Software

 6200-6.4.6

PLC-5 Programming Software: Programming 6200-6.4.7

PLC-5 Programming Software: Instruction Set Reference

PLC-5 Programming Software: I/O Configuration Software

 6200-6.4.11

 6200-6.4.12

Remote I/O Adapter Module (cat. no. 1771-ASB) Installation Data 1771-2.162

This chapter describes concepts for using a backup system with your

programmable controller. In addition, this chapter provides an overview of

a PLC-5 processor backup system that uses 1785-BCM backup

communication modules and a system that also uses 1785-BEM backup

expansion modules.

The objective of any redundant system (backup system) is to improve the

amount of up-time of a machine or process by ensuring consistent

availability of that machine, and by reducing costs associated with

equipment failure. By using this backup system, you can guard your

application against shutdowns caused by the programmable controller.

ATTENTION: Backup does not protect you from faults caused

by programming errors or system timeouts because such an

error or timeout will also occur in the secondary processor.

The backup option is used where you must transfer the control of the

process to a secondary system, without thereby interrupting the

machine/process operation.

To guard against system shutdown, a backup system must provide:

 equipment with exceptional reliability

 automatic fault isolation

 minimal disturbance of the process when switching from the primary to

the secondary system

C

A PLC-5 system configured with 1785-BCM modules provides high speed

backup communication and switchover of the Data Highway Plus and

remote I/O links. In this section we:

 show a typical PLC-5 backup configuration

 explain how the backup system works

 describe the role of the 1785-BCM module (including 1785-BEM

module)

A Typical PLC-5 Backup Configuration

You must use BCM series B, revision C or later if you are using PLC-5/20,

-5/30, -5/40, or -5/60 processors.

A PLC-5 backup system contains two of each of the following hardware

components:

 PLC-5 processor module

Processor:* Catalog Number:

PLC-5/11 series A, revision B or later 1785-L11B

PLC-5/15 series B, any revision 1785-LT series B

PLC-5/20 series A, revision B or later 1785-L20B

PLC-5/25 any revision 1785-LT2

PLC-5/30 series A, revision C or later 1785-L30B

PLC-5/40 series A, revision F or later

or series B, revision C or later

1785-L40B

PLC-5/60 series A, revision F or later 

or series B, revision C or later

1785-L60B

PLC-5/80 series C, revision A or later 1785-L80B

*

 Contact your Allen-Bradley Sales Office or your Allen-Bradley distributor

for additional PLC-5 processors that can be configured with the

1785-BCM modules.

 1785-BCM module

 1785-BEM module (when applicable)

 power supply

 local chassis

Figure 1.1 shows a typical PLC-5 backup configuration using PLC-5/15

processors and 1785-BCM modules. Figure 1.2 shows a typical PLC-5

backup configuration using PLC-5/40 or PLC-5/60 processors, 1785-BCM

modules, and 1785-BEM modules.

A

A PLC-5 system configured with 1785-BCM modules provides high speed

backup communication and switchover of the Data Highway Plus and

remote I/O links. In this section we:

 show a typical PLC-5 backup configuration

 explain how the backup system works

 describe the role of the 1785-BCM module (including 1785-BEM

module)

A Typical PLC-5 Backup Configuration

You must use BCM series B, revision C or later if you are using PLC-5/20,

-5/30, -5/40, or -5/60 processors.