Function Description

The HSBY_SWAP function block is used to initiate the swapping between primary and standby CPU.

EN and ENO can be configured as additional parameters.

This function block is convenient to be used to activate a swap by program logic. This swap between primary and standby CPU can be performed in the Hot Standby mode only.

Please refer to:

This means that when the HSBY is running, the standby PLC becomes the primary PLC, and the old primary PLC becomes the standby PLC activated by the program logic.

NOTE: It is not mandatory to use this function for performing a hot standby swap. Indeed, you may rely on register %SW60, as described in the Conditions for Switchover for Premium or in the Conditions for Switchover for Quantum.
NOTE: The DFB should not be used in the first section of the application.

Advantages of the Swapping Function

The advantages of swapping could be the following:

  • The health of the standby PLC is monitored. It is checked that the standby PLC can take over.

  • The switchover could be tested at regular intervals.

Example of a Hot Standby Application

The illustration below shows an example of a hot standby application:

Steps for Changing Status

A Hot Standby swapping function is processed as follows:

Step

Action

1

Status: PLC-A is the primary controller, PLC-B is the standby controller.

PLC-A sets itself to offline.

Result:

PLC-B becomes the primary controller.

2

Status: PLC-A is offline, PLC-B is the primary controller.

PLC-B sets PLC-A to run-mode

Result:

PLC-A is the standby controller.

3

Status: PLC-A is the standby controller, PLC-B is the primary controller.

EFB outputs will be set.

Result:

Hot Standby swapping function is completed.

Representation in FBD

Representation

Representation in LD

Representation

Parameter Description

Description of the input parameter

Parameter

Data Type

Meaning

START

BOOL

START = 1 starts the HSBY_SWAP operation.

The value of 1 must be applied until the operation has finished or until an error has occurred.

Description of output parameters

Parameter

Data Type

Meaning

ACTIVE

BOOL

ACTIVE = 1 indicates that an HSBY_SWAP operation is in progress.

DONE

BOOL

DONE = 1 indicates that the HSBY_SWAP operation has been completed successfully.

ERROR

BOOL

ERROR = 1 indicates that an error has occurred, or that the current HSBY_SWAP operation has been aborted.

STATUS

INT

An error code (STATUS) is generated by the HSBY_SWAP block. A complete list is shown in the error code table.

Error Status Table

The following table explains the error codes:

Error Codes

Fault Description

0

OK

1

The function HSBY_SWAP has been aborted.

2

Hot Standby not activated (%SW61.15=0).

3

Standby does not exist.

5

The swap was unsuccessful.
NOTE: The system words %SW60 and %SW61 reflect the status of the primary and the secondary PLC.

Switchover Using Command Register System Bit %SW60.1 or %SW60.2

Another way of forcing a switchover is setting the bits in the Command Register. To achieve this, do the following:

Step

Action

1

Open file 1.

2

Connect to the primary,

3

Ensure the controller order of the primary is A or B.

4

Access

  • Command Register system bit %SW60.1

    If the connected controller order is A.

  • Command Register system bit %SW60.2

    If the connected controller order is B.

5

Set bit to 0.

NOTE: Ensure that the standby switched to primary.

6

Open file 2.

7

Connect to the new primary controller.

8

Access the Command Register system bit used in step 4.

9

Set bit to 1.

NOTE: Ensure that the standby controller is now online.

10

Ensure both primary and standby controllers are in Run Primary and in Run Standby mode.