Basics of the EN/ENO mechanism   

Glossary 

Enabling and disabling the EN/ENO mechanism 

Introduction

Runtime errors that require a program abort can occur during the processing of instructions. You can use the EN/ENO mechanism to avoid such program aborts. This mechanism can be used at two levels:

EN/ENO mechanism for individual instructions

EN/ENO mechanism for block calls

EN/ENO mechanism for instructions in LAD/FBD

In LAD and FBD, certain instructions have an enable input EN and an enable output ENO. 

You can use the enable input EN to make the execution of the instruction dependent on conditions. The instructions are only executed if the signal state is "1" at the enable input EN. 

You can use the enable output ENO to query runtime errors in instructions and react to these.

The enable output ENO returns the signal state "1" if one of the following conditions applies:

No error occurred during processing.

The enable output ENO returns signal state "0" if one of the following conditions applies:

The EN input has signal state "0".

An error occurred during processing.

The EN/ENO mechanism is used for the following basic instructions:

Mathematical functions

Move operations

Conversion operations

Word logic operations

Shift + rotate