OATs
OATs - IEC 61131 - Sequential Function Chart

Sequential Function Chart (SFC) is the IEC 61131 -3 method to program sequential controls.

  • Actions may contain Boolean variables or entire networks,
  • networks are programmed in Ladder Diagram or Function Block Diagram,
  • true action associations from several steps,
  • several concurrent SFC networks in one diagram,
  • textual SFC for ST and IL,
  • online viewing,
  • steps contain X and T attribute variables,
  • actions contain Q and A attribute variables,
  • SFCs can be reset,
  • "one final execution" of actions optionally switched off.

SFC is often regarded the fifth language of the standard, but it is a common element for all other four languages. Because of it's powerful features it is worth to mention separately. And SFC is the only IEC 61131 -3 language to program sequential controls.

Sequential Function Chart (SFC)

Usually Sequential Function Chart is regarded as a graphical programming method. But the standard also defines a textual representation which OATs supports for Structured Text (ST) and Instruction List (IL.)

OATs supports all 11 action association attributes, including the new P0 and P1 attributes of the IEC 61131 -3 second edition.

Attribute Behaviour

N non-stored
action is activated as long as the step is activated
R reset
action is deactivated regardless of any other activation
S set/stored
the action is activated and remains activated when the step is deactivated, usually requires deactivation by an action association with R-attribute (reset) anywhere else
L time limited
action is activated for a specified time or until the step is deactivated
D time delayed
the action is activated after a specified time has elapsed while the step is still activated
P pulse
action is activated for a single execution when the step gets activated and possibly once again when the step is deactivated
P0 falling edge pulse
action is activated for a single execution when the step gets deactivated
P1 raising edge pulse
action is activated for a single execution when the step gets activated
SD stored, time delayed
the action is activated after a specified time regardless if the step gets deactivated, usually requires deactivation by an action association with R-attribute (reset) anywhere else
DS time delayed, stored
the action is activated after a specified time if the step is still activated, usually requires deactivation by an action association with R-attribute (reset) anywhere else
SL stored, time limited
action is activated for a specified time regardless if the step gets deactivated

Additionally to the IEC 61131 -3 standard it is possible to reset a SFC to it's initial condition. There is no way in IEC 61131 -3 to accomplish the same behaviour. Within OATs it is always possible to either continue a SFC or to restart it.

SFC is by far the easiest way to program time related behaviour. Often, there is no need to declare timers. Just use the T duration attribute of a step.

Sequential Function Chart (SFC)


OATs also correctly handles multiple action associations for action which have been programmed in ladder diagram or function block diagram. A typical example is starting a control in one of the very first steps of a SFC and stopping it near the end of the SFC. This requires the same action has been attached to two different steps using the S- and the R-attributes for the action associations.


You can define if a SFC uses a final execution when a step gets inactive or not for each SFC module.


Copyright 2003-2010 Ing.-Büro Dr. Friedrich Haase