3SL Reference - State Transition Diagram (STD)

State Transition Diagram (STD)

State Transition Diagrams (STDs) are used along with specifications to define the functional detail for a system.

Unlike data processes, control processes have a specialised role: enforcing sequencing over environmental control stimuli to the system, and the internal operation of the system. A control process is defined on an STD that represents the definition of the control process as a finite-state-machine. The number of the STD is the same as the number of the control process it defines.

The finite-state-machine enters an initial state when activated, either notionally at system startup or when the control process is activated by an enable or trigger prompt. In entering this initial state, it may perform some actions, defined below.

Once in a state, the finite-state-machine may only enter another state by making a transition to the new state. A transition is only possible when its condition is satisfied, and in moving to the new state the finite-state-machine may perform zero or more actions. Every state must be the destination of at least one transition. A transition may cause the finite-state-machine to return to the same state, i.e. the old and new states resulting from a transition may be the same.

Transition conditions consist of one or more expressions combined with any of the operators AND, OR and NOT. The transition only occurs when the condition is true, that is, when the input event flow(s) is/are present. The finite-state-machine makes transitions between states in response to some internal (to the system) or external (from the environment) event. Environmental control-related events are often modelled as input event flows to a control process, that in turn cause transitions (and consequential actions) in the control process’s STD.

A transition action can take one of three forms:

• The activation of a named output event flow from the parent control process
• The control of a named process that is connected to the STD’s parent control process by a prompt originating from the control process. These actions can take one of the following forms:

• ENABLE process name - enable the named process
• ENABLE: process name - enable the named process
• D: process name - disable the named process
• DISABLE process name - disable the named process
• DISABLE: process name - disable the named process
• T: process name - trigger the named process
• TRIGGER process name - trigger the named process
• TRIGGER: process name - trigger the named process

• The control of a named output event flow from the parent control process:

• R: flow name - raise the named continuous flow
• RAISE flow name - raise the named continuous flow
• RAISE: flow name - raise the named continuous flow
• L: flow name - lower the named continuous flow
• LOWER flow name - lower the named continuous flow
• LOWER: flow name - lower the named continuous flow
• S: flow name - signal the named discrete flow
• SIGNAL flow name - signal the named discrete flow
• SIGNAL: flow name - signal the named discrete flow

You can put algebraic expressions in conditions and actions, e.g.:

I < 10

for a condition and

i++

for an action.

Elements of algebraic expressions are not consistency checked, e.g. we do not look for a data definition called I or i in the above examples. The presence of < > + - * / triggers something being recognised as an algebraic expression.

STDs are not hierarchical. Their connectivity is:

STDs are available in models in both in the Essential and Implementation Domains.

Symbols

An example STD is:

The symbols available in STDs are:

Symbol	Name	Description	Definition	Expansion
	Comment	Makes a note anywhere in the diagram. Are always surrounded by * characters. Note: If you do not want Cradle to automatically add an * go to the Graphics Settings section of Project Setup and turn off the Automatically add asterisks to diagram's comment symbols option.	None	None
	Boundary Point	A connection point for the initial transition to enter the initial state.	None	None
	Condition	The condition which must be true for the associated transition to occur. An event flow name. One or more conditions can be associated with a transition. Keywords AND, OR and NOT can be used to convey a logical expression. The keywords are ignored during STD checks.	Flow data definition	None
	Action	An action performed by the finite-state-machine when making a transition between two states. Either an event flow name or to enable, disable or trigger a named transformation.	Flow data definition (except actions that are the control of a named process)	None
	State	A state which does not need to be named, of the STD’s finite-state-machine.	None	None
	Picture	Allows you to choose the location of a GIF or JPEG image to be displayed as a diagram symbol or to be embedded in an existing diagram symbol.	None	None
	Initial Transition	The transition initially performed by the finite-state-machine when it, or the system, is activated.	None	None
	Transition	A transition between two states.	None	None
	Item	Represents a requirement or system note in the diagram.	None	None
	Context Item	Represents a requirement or system note in a diagram and is a container within which other object symbols can be drawn or attached.	None	None
	Cross Reference Link	Represents a cross reference that exists between a pair of system notes/requirements. It can also represent a cross reference between a system note/requirement and a specification or data definition or system that describes the objects symbols that it connects.	None	None