Configuring Data Variables for an M580 BMEH58•040(S) Hot Standby Application

Introduction

BMEH58•040 Hot Standby and BMEH58•040S safety Hot Standby CPUs support the following data attributes:

Attribute	CPU
Attribute	BMEH582040, BMEH582040S, BMEH584040, BMEH584040S	BMEH586040, BMEH586040S
Exchange On STBY	X	X
Retain	–	X
X: Supports the attribute. –: Does not include the attribute, because all data is retained.

For a safety CPU, each variable set to Exchange On STBY is associated with a task (MAST, FAST, or SAFE). The amount of data that can be exchanged from the primary to the standby safety CPU depends on the task:

MAST & FAST: up to 4 MB of data can be exchanged.
SAFE: up to 1 MB of data can be exchanged.

For information on how to use the Control Expert Data Editor, and display the Retain and Exchange On STBY attributes, refer to the Unity Pro Operating Modes manual.

Retain

All BME•586040 CPUs present the Retain variable attribute. This attribute determines whether the variable value will persist after a warm start of the CPU. If the attribute is:

Selected: Variable data persists and is applied to the variable after a warm start.
De-selected: Variable data is lost after a warm start; the variable value is reset.

For non-safety standalone Modicon M580 CPUs, this attribute is read-only. It is selected by default and cannot be de-selected.

For both standalone and Hot Standby safety CPUs, the Retain variable attribute is not included for variables created in the safe area. All safe data is not retained, because the SAFE task executes a cold start.

NOTE: In the event of a cold start of the CPU, both retained and non-retained data is reset.

The amount of maximum configurable retained data varies, depending on the CPU.

For the BME•586040 CPUs, you cannot edit the Retain attribute for a variable that existed at CPU start-up. When a variable is created online as part of a CCOTF change, you can edit the Retain attribute which remains modifiable until the first build change is performed.

NOTE: The amount of retained data is presented as saved data in the Memory Usage window.

Exchange On STBY

Before each scan in a Hot Standby system, the primary Hot Standby CPU exchanges data with the standby CPU. It exchanges only that data with the Exchange On STBY attribute set to YES.

NOTE:

When a reference is initialized inside the Data Editor, the initialization variable needs to be part of the same task as the reference. Otherwise, a detected error message is included in the Output Window when the project is analyzed.
The Exchange On STBY attribute is not editable for all variables.
In a Hot Standby system, if you have configured explicit messaging using a communication function, exclude the communication function block Management_Param from the data to be transferred from primary to standby. To do this, de-select the Exchange on STBY attribute for the Management_Param parameter in Control Expert.

You cannot edit the Exchange On STBY attribute for a variable that existed at CPU start-up. When a variable is created online as part of a CCOTF change, you can edit the Exchange On STBY attribute which remains modifiable until the first build change is performed.

The amount of maximum configurable Hot Standby transfer data varies, depending on the CPU.

Each variable that is included in the Hot Standby exchange also presents a read-only Task attribute. The setting of the Task attribute is auto-generated by Control Expert for each variable included in the Hot Standby exchange.