Handling of EVC/EVL

EVC (Exchange Variable Connection) can be used within one or more nonsafe subdevices in order to access (safe and nonsafe) variables of the safety subdevice.

Figure 5. Example: Device tree of a project using a Compound Safety PLC with EVC/EVL

You can add EVC (Exchange Variable Connection) to an application like other "POU" objects: Use the Add Object command and dialog and select EVC.

When you open the EVC object, you get the EVC editor. There you configure which variables of the application of the safe subdevice should be available within the application, to which the EVC object belongs.

Figure 6. Example of an EVC

If the application of the safe subdevice is not up-to-date (if the application was changed after the last successful code generation), then the EVC editor displays a warning similar to the following: Safe application has changed or is not up to date. Execute "Build" command to be able to select variables.

If you want to generate code for the safe application directly, then you can use the Build button. Afterwards, the warning should disappear.

All variables found within the safety application of the safety subdevice are displayed in the EVC editor in a structured way. If you want to access any of these variables individually, then click the corresponding check box. Selected variables are accessible. To remove access privileges to a variable, you clear the corresponding check box.

You can access all accessible variables within the application (where the EVC is located) by means of the namespace "EVC".

IF NOT EVC.GUIDS.xError AND EVC.GUIDS.xGuidValid THEN
  EVC.EVL.nonsafeVAr := 1;
  iVar := EVC.PLC_PRG.iVar;
END_IF

To check the status of EVC, you should use the implicit member GUIDS of the EVC Namespace in your application. The implicit GVL EVC.GUIDS provides two status variables. As a result, you can monitor the consistency of the EVC variables used.