Execution Sequence of the FFBs

Introduction

The execution sequence is determined by the position of the FFBs within the section (executed from left to right and from top to bottom). If the FFBs are then linked graphically, the execution sequence is determined by the signal flow.

The execution sequence is indicated by the execution number (number in the top right corner of the FFB frame).

Execution Sequence on Networks

For network execution sequences, the following rules apply:

Executing a section is completed network by network based on the FFB links from above and below.
Links may not be used to create loops since the sequence of execution in this case cannot be clearly determined. Loops must be created using actual parameters (see Loop Planning).
The execution sequence for networks that are not linked is determined by the graphic sequence (from top-right to bottom-left). This execution sequence can be influenced (see Change Execution Sequence).
Processing on a network is ended completely before the processing begins on another network for which outputs are used on the previous network.
No element of a network is deemed to be processed as long as the status of all inputs of this element are not calculated.
Processing on a network is only ended if all outputs on this network have been processed.

Signal Flow within a Network

For execution sequences within a network, the following rules apply:

An FFB is only processed if all elements (FFB outputs etc.) with which its inputs are linked are processed.
The execution sequence of FFBs that are linked with various outputs of the same FFB runs from top to bottom.
The execution sequence of FFBs is not influenced by the location within the network.

This does not apply if more than one FFB is linked to the same output of the "calling" FFB. In this case, the execution sequence is determined by the graphic sequence (from top to bottom).

Priorities

Priorities in Defining the Signal Flow Within a Section.

Priority	Rule	Description
1	Link	Links have the highest priorities in defining the signal flow within a FBD section.
2	User Definition	User Access to Execution Sequence.
3	Network by Network	Processing on a network is ended completely before the processing begins on another network.
4	Output Sequence	FFBs that are linked to the outputs of the same "calling" FFB are processed from top to bottom.
5	Rung by Rung	Lowest priority. (Only applies if none of the other rules apply).

Example