A-BPLC-5 Backup Communication Module

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