State Transition Diagram

Purpose

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

Example

Here is an example State Transition Diagram.

Description

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 intial 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 components corresponding to input event flows to the STD's control process and in the parent DFD 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:

1. The activation of a named output event flow from the parent control porocess

Logical Operators in Conditions

A condition is used on a transition to indicate that something is being acted on to move the system from one state to another.

While there should be one condition that causes this transition to take place, a number of conditions may be required, or be acted on, at the same time.

There are three logical operators that can be used in the condition to group conditions together. These are the logical operators:

• AND

• OR

• NOT

These can be used to combine conditions together (AND and OR), or the change the test that is made for the conditions (NOT).

In Cradle, the conditions using logical operators are still drawn as separate condition. They are often positioned close to each other to give the appearance of a single condition using logical operators.

It is the use of logical operators in the name of the condition that distinguish this condition from a normal condition. The positioning of the conditions relative to each other has no bearing on the way in which Cradle treats them.

Literal Expressions in Conditions and Actions

In certain cases it can be useful to use literal expressions within the condition used on a transition. For example, when testing an enumerated type for one of its values as the indication of the state which should be transitioned to. In the enumerated set, the literals do not have DD definitions, as they are alternative values for the DD entry.

Assignments are made in literal expressions as follows:

Weaknesses

Do not allow nested states, unlike UML statecharts.

Characteristics

APPEARS IN MODELS	Essential and Implementation
NUMBERING	STDs cannot exist in a hierarchy and so no STD symbol can be expanded into a lower-level diagram. It may, however, be the case that control processes in two DFDs in the DFD hierarchy expand into STDs whose numbers give the illusion of a hierarchy with a DFD-like numbering system. While the decomposition of control into a control hierarchy is a normal element of many systems, such hierarchical numbering of STDs will be purely coincidental and neither should, nor does, have any significance to Cradle.
HIERARCHICAL	No

Linkage