Function block for controller redundancy

Available with AXC F 2152 and AXC F 3152 

This topic describes the redundancy feature for use with PLCnext Control AXC F 2152 or AXC F 3152 on an applicative base. 

Note: For hardware-based redundancy with a pair of PLCnext Control RFC 4072R, see System redundancy with RFC 4072R.

General concept

ASR Controller redundancy

The communication between the two PLCs necessary for controller redundancy is realized via UDP/IP communication protocol.

For this purpose, the following rules for the assignment of the IP addresses must be considered:

  • The ID addresses of the First PLC and Second PLC must use the same Net IDs.
  • The difference between the IP addresses of the First PLC and the Second PLC must be limited to the fourth octet only.

Example:

 

First PLC

Second PLC

IP Address

Subnet Mask

192.168.0.91

255.255.255.0

192.168.0.92

255.255.255.0

Prerequisites

From the PLCnext Store, download the ASR AXC F 2152 or the ASR AXC F 3152 function block library (free of charge) and import it into PLCnext Engineer on your computer. In the following 

Function block ASR_ContrRed_2152

On the two PLCs the same program is in operation. Therefore, during run time of the program it is necessary to determine which PLC is the First controller and which PLC is the Second controller. For this purpose, the fourth octet of the IP addresses (host ID) of the First and of the Second controller needs to be stored as a constant at the corresponding input parameters of this function block in the PLCnext Engineer program.

During the start-up of the program, the own IP address is read out by executing a firmware service. As a result of the comparison of the own IP address with these constants, it can be determined whether the controller is the First or the Second PLC.

Only one controller (either First or Second) controls the process. The PLC which controls the process is the Primary, the other PLC has the role of the Backup. The determination which PLC is the Primary respectively the Backup is done by a comparison of the value of its own current state signals with the current state signals of the partner PLC.

Therefore, this function block compares cyclically the current state signals of the partner PLC (xPartnerSignal...) with its own current state signals (xSignal...). The values of the signals are compared in ascending order beginning with 0.

If a difference of the values at a determined bit number x is perceived the values of the current state signals with higher bit numbers (> x) are not compared anymore. Therefore, the priority of a switch-over condition depends on the bit number used for this current state signal.

Possible switch-over conditions are:

  • Switching over the state of a PLC from Backup to Primary is carried out if the value of its own current state signal is TRUE and the value of the corresponding current state signal from the partner PLC is FALSE.
  • Switching over the state of a PLC from Primary to Backup is carried out if the value of its own current state signal is FALSEand the value of the corresponding current state signal from the partner PLC is TRUE.

function block ASR_ContrRed_2152

 

Description of inputs and outputs

Name

Data type

Data direction

Description

iIP4_First

INT

Input

Fourth octet of the IP address of the First PLC.

iIP4_Second

INT

Input

Fourth octet of the IP address of the Second PLC.

tSwitchOverDelay

TIME

Input

Delay time for switching over the PLCs in case a switch-over condition occurs.[1]

xResetSignalDelayMax

BOOL

Input

A rising edge at this input signal resets the uiSignalDelayMax output signal.

xInhibitPrimaryRole

BOOL

Input

With TRUE, the takeover of the Primary role is inhibited, e.g. if the PLC has no connection to the network at all.

xSignal...

BOOL

Input

Current state signals:
0: highest priority to
7: lowest priority

arrAR_DevList

ASR_ARR_STR_1_50

Input

Name of station list of all PROFINET® IO devices.

xFirst

BOOL

Output

Indicates whether the PLC is the First-PLC.

xSecond

BOOL

Output

Indicates whether the PLC is the Second-PLC.

xPrimary

BOOL

Output

Indicates whether the PLC has the redundancy role Primary.

xBackup

BOOL

Output

Indicates whether the PLC has the redundancy role Backup.

xPartnerIsFirst

BOOL

Output

Indicates whether the partner PLC is the First PLC.

xPartnerIsSecond

BOOL

Output

Indicates whether the partner PLC is the Second PLC.

xPartnerIsPrimary

BOOL

Output

Indicates whether the partner PLC has the Primary redundancy role.

xPartnerIsBackup

BOOL

Output

Indicates whether the partner PLC has the Backup redundancy role.

xPartnerSignal...

BOOL

Output

Current state signals of the partner PLC:
0: highest priority to
7: lowest priority

strOwnIP

STRING

Output

Own IP address.

strPartnerIP

STRING

Output

IP address of the partner PLC.

strLastToPrimary

STRING

Output

Indicates time and date for the last switch-over of the PLC from Backup to Primary.

bLastToPrimarySig

BYTE

Output

Indicates the state of the signal when the last switch-over from Backup to Primary was performed.

bLastToPrimaryPartnerSig

BYTE

Output

Indicates the state of the signal from the partner PLC when the last switch-over from Backup to Primary was performed.

strLastToBackup

STRING

Output

Indicates time and date for the last switch-over of the PLC from Primary to Backup.

bLastToSlaveSig

BYTE

Output

Indicates the state of the signals, when the last switch-over from Primary to Backup was performed.

bLastToSlavePartnerSig

BYTE

Output

Indicates the state of the signals from the partner PLC when the last switch-over from Primary to Backup was performed.

diSignalDelayMax

DINT

Output

Maximum measured delay time for the transmission of the current state signals. This output parameter can be reset by a rising edge at the input signal xResetSignalDelayMax.

xError

BOOL

Output

This output indicates with a rising edge that an error has occurred. You can read the corresponding error codes at the output signals wDiagCode and wAddDiagCode during the cycle in which the rising edge occurs.

wDiagCode

WORD

Output

Indicates the cause of the error.

wAddDiagCode

WORD

Output

Indicates additional information to the cause of the error.

iRole

INT

Output

Indicates the current role of the PLC:

0: Undefined

1: Primary

2: Backup

iSyncLink

INT

Output

Indicates the status for the transmission of the current state signals:

0: Undefined

1: Transmission of the signals is OK

2: Transmission of the signals is not OK

iSwitchCount

INT

Output

Number of switch-overs of the PLC from Backup to Primary.

 

  1. The valid range for the value of the tSwitchOverDelay input parameter is between 300 ms and 1000 ms. The achievable switch-over time of the system depends on the following parameters of the overall application and therefore cannot be generalized:
    1. the task interval in which this function block is used
    2. the overall CPU load of the PLC
    3. the Network Redundancy protocol which is used for the application specific network layout
    However, there is a recommended minimum switch-over time which can be adjusted depending on the task interval in which the function block is used:
    For a task interval of 20 ms, the minimum value for the tSwitchOverDelay input variable should be 300 ms.

 

The switch-over time of the system must also be parameterized for the PROFINET® IO devices. This is done with PLCnext Engineer at the settings of the DAP node via the System redundancy data hold time. The value entered here must not be lower than the switch-over time set at the function block.

Grafik 26

 

Error Codes

 

Error Code

Description

C401

Indicates that an error while trying to stop the PROFINET® ARs.
The wAddDiagCode output parameter indicates the relevant error code from the responding function block.

C402

Indicates that an error while trying to start the PROFINET® ARs.
The wAddDiagCode output parameter indicates the relevant error code from the responding function block.

C403

Indicates that an error occurred while reading the adjustments of the IP address.

The wAddDiagCode output parameter indicates the relevant error code from the responding function block.

C404

Indicates that this PLC is neither the First nor the Second PLC because the fourth octet of the IP address is neither equal with the value of the input parameter iIP4_First nor with iIP4_Second.

The wAddDiagCode output parameter contains the fourth octet of the adjusted IP address.

C405

Indicates that during the start-up of the internal state machine a timeout occurred.

The output parameter indicates the relevant step at which the timeout was detected.

 

Additional program instances in the ESM

For the controller redundancy to work properly, the following program instances must be created in addition to the cyclic call of the function block:

  • Program instance for the Cold Start system event task:
  • Program instance for the Warm Start system event task:
  • Program instance for the Stop system event task :

    Note: The watchdog of this system event task must be set to 2000 ms.

 

The ASR_ContrRed_2152 function block internally uses some firmware services. To make these services available, it is necessary to include the PLCnextBaseServices library into the PLCnext Engineer project, and to create a program instance of the ServiceProvider program into one cyclic task:

 

 

Recommendations for the configuration of the function block

arrAR_DevList InOut parameter

 

At start-up, the function block performs some PROFINET® specific services on the PROFINET® IO devices and therefore the function block needs to know the names of all connected IO devices. For this purpose, the “Name of Station” of all PROFINET IO-devices (bus couplers) must be stored at the arrAR_DevList InOut parameter:

 

In this example program, two PROFINET® IO devices with the names “bk1” and “bk2” are used. If there are more IO devices connected their names must also be added to this list.

xSignal0 to xSignal7 input parameters

The switch-over conditions are defined by logical operations which are assigned to the xSignal0 to xSignal7 input parameters.

If there are no special requirements related to the switch-over conditions the following configuration is recommended in order to trigger a switch-over in case of “standard failure” situations, like switching off the power supply of a PLC, setting a PLC into Stop operating mode, or disconnecting the Ethernet connection:

xSignal0 to xSignal7 input parameters

 

The I_BKx_PN_DEVICE_STATE variables are to be defined as External Variables and the responding Global Variables needs to be connected to the PN_DEVICE_STATE process data item of the PROFINET® bus couplers. 

In this example program, two bus couplers are used. If there are more bus couplers in the system, the AND and the OR function needs to be extended accordingly.

xInhibitPrimaryRole input parameter

This input parameter is used to prevent the PLC from taking over the Primary role. This is usually useful when the PLC has no connection to an Ethernet network at all. In this case the PLC doesn’t know the current status of the partner PLC and therefore it should not takeover the Primary role.

In the following example two bus couplers and two Ethernet switches are used in the system. Thus, if the controller has neither a connection to one of the bus couplers nor to one of the switches, it can be assumed that the controller is not connected to the network, and therefore the takeover of the Primary role would not make sense:

xInhibitPrimaryRole input parameter

OPC UA variables

To make it possible to display the current status of the redundant controllers on a visualization system, the function block provides corresponding information in the form of OPC UA variables via the embedded OPC UA server on the PLCs.

Name

Data type

Description

iRole

INT

Indicates the current role of the PLC:

0: Undefined

1: Primary

2: Backup

iSwitchCount

INT

Number of switch-overs of the PLC from Backup to Primary.

iSyncLink

INT

Indicates the status for the transmission of the Current-State-Signals:

0: Undefined

1: Transmission of the signals is OK

2: Transmission of the signals is not OK

uiPlcInRun

UINT

Cycle counter for the “PLC in Run” monitoring.
The value of this counter is incremented each program cycle by 1 as soon as the application program is in operating mode RUN.

siFirstPlcCurrentRole

SINT

Indicates the current role of the First PLC:

0: Undefined

1: Primary

2: Backup

siFirstPlcState

SINT

Indicates the current state of the First PLC:

0: Undefined

1: Error

2: Run

siSecondPlcCurrentRole

SINT

Indicates the current role of the Second PLC:

0: Undefined

1: Primary

2: Backup

siSecondPlcState

SINT

Indicates the current state of the Second PLC:

0: Undefined

1: Error

2: Run

 

 

 


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• Published/reviewed: 2022-09-14 • Revision 046 •