Implicit Variables - SFC Flags

Overview

Each SFC step and IEC action provides implicitly generated variables for watching the status of steps and IEC actions during runtime. Also, you can define variables for watching and controlling the execution of an SFC (timeouts, reset, tip mode). These variables can also be generated implicitly by the SFC object.

Basically, for each step and each IEC action, an implicit variable is generated. A structure instance, named for the element, for example, step1 for a step with step name step1. You can define in the element properties whether for this flag a symbol definition should be exported to the symbol configuration and how this symbol should be accessible in the controller.

The data types for those implicit variables are defined in library IecSFC.library. This library will automatically be included in the project as soon as an SFC object is added.

Step and Action Status and Step Time

Basically, for each step and each IEC action, an implicit structure variable of type SFCStepType or SFCActionType is created. The structure components (flags) describe the status of a step or action or the currently processed time of an active step.

The syntax for the implicitly done variable declaration is:

<stepname>: SFCStepType;

or

_<actionname>:SFCActionType;

NOTE: Implicit variables for IEC actions are preceded by an underscore.

The following boolean flags for step or action states are available:

Boolean flags for steps:

If <stepname>.x = TRUE, the step will be executed in the current cycle.

If <stepname>._x = TRUE and <stepname>.x = FALSE, the step will be executed in the following cycle. This means that <stepname>._x is copied to <stepname>.x at the beginning of a cycle.

Boolean flags for actions:

Symbol generation

In the element properties of a step or an action, you can define if a symbol definition should be added to a possibly created and downloaded symbol configuration for the step or action name flag. For this purpose, make an entry for the desired access right in column Symbol of the element properties view.

NOTE: If you use the boolean flag <stepname>.x to force a certain status value for a step (for setting a step active), be aware that this will affect uncontrolled states within the SFC.

Time via TIME variables:

The flag t gives the current time span which has passed since the step had become active. This is only for steps, no matter whether there is a minimum time configured in the step attributes or not (see below: SFCError).

For steps:

<stepname>.t (<stepname>._t not usable for external purposes)

For actions:

The implicit time variables are not used.

Control of SFC Execution (Timeouts, Reset, Tip Mode)

You can use some implicitly available variables, also named SFC flags (see table below) to control the operation of an SFC. For example, for indicating time overflows or enabling tip mode for switching transitions.

In order to be able to access these flags you have to declare and activate them. Do this in the SFC Settings dialog box. This is a subdialog box of the object Properties dialog box.

Manual declaration, as it was needed in SoMachine / SoMachine Motion V3.1, is only necessary to enable write access from another POU (refer to the paragraph Accessing Flags).

In this case, consider the following:

If you declare the flag globally, you have to deactivate the Declare option in the SFC Settings dialog box.Otherwise this leads to an implicitly declared local flag, which then would be used instead of the global one. Keep in mind, that the SFC settings for an SFC POU initially are determined by the definitions set in the Options > SFC dialog box.

Consider that a declaration of a flag variable solely done via the SFC Settings dialog box will only be visible in the online view of the SFC POU.

The following implicit variables (flags) can be used. For this purpose, you have to declare and activate them in the SFC Settings dialog box.

The following figure provides an example of some SFC detected error flags in online mode of the editor.

A timeout has been detected in step s1 in SFC object POU by flag SFCError.

 

 

Accessing Flags

For enabling access on the flags for the control of SFC execution (timeouts, reset, tip mode), declare and activate the flag variables as described above (Control of SFC Execution).

Syntax for accessing from an action or transition within the SFC POU:

<stepname>.<flag>

or

_<actionname>.<flag>

Examples:

status:=step1._x;

checkerror:=SFCerror;

Syntax for accessing from another POU:

<SFC POU>.<stepname>.<flag>

or

<SFC POU>._<actionname>.<flag>

Examples:

status:=SFC_prog.step1._x;

checkerror:=SFC_prog.SFCerror;

Consider the following in case of write access from another POU:

oThe implicit variable additionally has to be declared explicitly as a VAR_INPUT variable of the SFC POU

oor it has to be declared globally in a GVL (global variable list).

Example: Local declaration

PROGRAM SFC_prog
VAR_INPUT
  SFCinit:BOOL;
END_VAR

Example: Global declaration in a GVL

VAR_GLOBAL
  SFCinit:BOOL;
END_VAR

Accessing the flag in PLC_PRG:

PROGRAM PLC_PRG
VAR
  setinit: BOOL;
END_VAR
SFC_prog.SFCinit:=setinit;  //Write access to SFCinit in SFC_prog