A-BPLC-5 Backup Communication Module

Important User Information

Because of the variety of uses for this product and because of the differences

between solid state products and electromechanical products, those responsible

for applying and using this product must satisfy themselves as to the

acceptability of each application and use of this product. For more information,

refer to publication SGI-1.1 (Safety Guidelines For The Application,

Installation and Maintenance of Solid State Control).

The illustrations, charts, and layout examples shown in this manual are intended

solely to illustrate the text of this manual. Because of the many variables and

requirements associated with any particular installation, Allen-Bradley

Company cannot assume responsibility or liability for actual use based upon the

illustrative uses and applications.

No patent liability is assumed by Allen-Bradley Company with respect to use of

information, circuits, equipment or software described in this text.

Reproduction of the contents of this manual, in whole or in part, without written

permission of the Allen-Bradley Company is prohibited.

Throughout this manual we make notes to alert you to possible injury to people

or damage to equipment under specific circumstances.

ATTENTION: Identifies information about practices or

circumstances that can lead to personal injury or death, property

damage or economic loss.

Attention helps you:

- Identify a hazard

- Avoid the hazard

- recognize the consequences

Important: Identifies information that is critical for successful application and

understanding of the product.

I

Summary of Changes

This release of the publication contains new updated information.

To help you find updated information in this release of the manual, we

have included change bars as shown to the left of this paragraph.

Using This Manual

This manual shows you how to use 1785-BCM series B backup communication

modules with a PLC-5 programmable controller (PLC-5/15 series B, -5/20,

-5/25, -5/30, -5/40, and -5/60 processors). These modules enable high-speed

communication transfer between two PLC-5 processors and provide system

backup should the processor or other equipment in the system fail.

In this manual we describe:

 backup system concepts

 procedures for installing and operating your modules

 various programming techniques

Before you read this manual or attempt to use 1785-BCM modules, you should

be familiar with PLC-5 programmable controllers. In addition, you need to be

familiar with:

 6200 Series Software

 remote I/O system

 Data Highway Plus network

 block-transfer instructions

The following table lists each chapter of this manual and describes the contents

of each.

If you want to read about: Refer to chapter:

an overview of backup concepts; description of backup

system that uses 1785-BCM module.

1 – Backup Concepts for the PLC-5 System

a description of 1785-BCM module hardware. 2 – Understanding the 1785-BCM Module Hardware Components

procedures for installing the1785-BCM module; procedures

for connecting the backup system.

3 – Installing Your 1785-BCM Series B Backup System

description of 1785-BEM backup expansion module and

procedures for installing

4 – Installing Your 1785-BEM Module

an overview of how the 1785-BCM module and the backup

system operate; procedures for starting, powering-up,

disconnecting, and restarting a repaired system.

5 – Operating Your PLC-5 Backup System

considerations for timing, divergence, I/O forces, Data

Highway Plus switching, Remote I/O switching, special

sections of the data table, and data integrity.

6 – Switchover Considerations

If you want to read about: Refer to chapter:

two methods you can use to program your backup system;

considerations for using instructions that can cause

problems in your backup system.

7 – Programming Techniques

1785-BCM module faults (as indicated by the module’s

status indicators and bits of the system status word) and

procedures for correcting faults.

8 – Diagnosing Faults

specifications for the 1785-BCM module. 9 – Specifications

sample Programs for method 1 (transferring one block of

data at a time) and method 2 ( transferring multiple blocks

of data at a time); data table transfer times for both

programming methods.

Appendix A – Sample Programs

answers to common PLC-5 backup system questions;

delay times between the primary and the secondary

processor for both programming methods and formulas for

determining delay times.

Appendix B – Reference Information

describes differences between a 1785-BCM series A

backup module and a 1785-BCM series B backup module

Appendix C – Using a Series A 1785-BCM Module

PLC-5 Processor Reference

In this manual, the term PLC-5 processor refers to the following processors:

 PLC-5/11

 PLC-5/15

 PLC-5/20

 PLC-5/25

 PLC-5/30

 PLC-5/40

 PLC-5/60

 PLC-5/80

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.

Some terms used in this manual may be unfamiliar to you. We list these terms

with a brief definition of each.

HSSL is the 1785-BCM module’s High-Speed Serial Link. It is a dedicated

communication link between the primary and secondary 1785-BCM modules

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.

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 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.

Figure 1.2

PLC-5 Backup System Configuration Using 1785-BCM 

and 1785-BEM Modules

How the PLC-5 Backup System Works

In the PLC-5 backup configuration, one system (consisting of one PLC-5

processor, 1785-BCM module, power supply, and chassis) controls the

operation of the remote I/O. This system is referred to as the primary

system. The other system is ready to take control of the remote I/O in the

event of a fault in the primary system. This is referred to as the

secondary system. The PLC-5 backup system does not back up local I/O;

therefore, do not install I/O in the local chassis.

Data Transfer

During normal operation, the primary system sends remote input and data

table data to the secondary system so that in the event of a switchover, the

secondary system (which becomes the new primary system) has the same

data.

Remote I/O data is automatically transferred over the High-Speed Serial

Link (see Figure 1.1). This transfer is independent of the application

program.

Data table values are transferred from the primary to the secondary system

with block transfer instructions that you include in your ladder program.

You do not have to transfer data table values if not necessary for your

application. Figure 1.3 shows how data table data is transferred from the

primary to the secondary system.

Figure 1.3

Transfer of Data Table Data From the Primary to Secondary System

Switchover

Should a fault occur in the primary processor, control switches to the

secondary system in less than 50 ms (maximum). When a switchover

occurs, the outputs in the remote I/O maintain their last state until they

come under the control of the secondary processor.

However, keep in mind that the program scans of the two processors are

not synchronized. This means that the secondary processor may be

scanning all, none, or only part of the program (at your discretion). This

manual explains the switchover process, and provides guidelines for

developing programs for your PLC-5 backup system. (For more

information about switchover, refer to Chapter 6, “Switchover

Considerations.”)

Role of the 1785-BCM Series B Module

As an integral part of the backup system, the 1785-BCM modules enable

high speed communication between the two PLC-5 processors, and permit

the secondary processor to assume control of the process. In addition, the

1785-BCM module provides:

 high speed transfer of the data table values from the primary to the

secondary system, to ensure that the secondary system’s data table is a

copy of the primary system’s

 a buffer of 4K words for data table values

 exchange of information on the status of the primary and secondary

systems

 automatic transfer to the secondary system of the remote input and

block transfer read values (analog values, etc.)

 transfer of control from the primary processor to the secondary

processor when one of the following conditions occur:

- power failure

- processor fault

- 1785-BCM module fault

- change in the primary processor’s mode from:

 RUN to PROGRAM (manual switchover)

 REM RUN to REM PROG

 REM RUN to REM TEST