This function realizes the initial step of the sequence control. It is a special form of the Step function block.

At the beginning of runtime, the Start block automatically gets a token. A Start block is used to initiate a flow control and normally cannot get a token later on. Only by a preset, the Start block can get a token at run time. With the Preset function block, the flow control is set to a predefined state and so a Start block can get a token again.

This function block contains the outputs SO and SM. The output SM is a BIT data type. It shows if this block has a token or not. SO has to be connected with a transition. Over this transition the token will be transferred.

For general information about sequence function and its realization see introduction.


Name I/O Type(s) Function
SO O TOTRANS Sequence output
SM O BIT State (token)


If this initial step is currently active, the symbol will be depicted in green. If the function block is connected in a way, which is potentially harmful for the control flow (e.g. aggregation of tokens), it will be depicted in yellow.


All flow controls within a structure block are part of the same flow control net. A macro block can be a part of a flow control. For connecting a macro with a flow control block, use the connectors of the type TOTRANS or TOSTEP.