3SL Reference - Extended Functional Flow Block Diagram (eFFBD)

Extended Functional Flow Block Diagram (eFFBD)

eFFBDs show the interaction between system functions by distributing them along a timeline. A timeline consists of a set of directed arrows, connecting functions and nodes to depict a processing sequence. The timeline effectively constitutes a flow of control. No data flows along the timeline, it simply indicates the order in which functions are processed. The timeline may branch into two or more parts at a node, described below, which must subsequently converge. An eFFBD must have a single start point for its timeline and a single end point.

Individual parts of the timeline may be named or unnamed. Named arrows are used to label alternate time line branches, either to represent alternate exit conditions from a function (such as success or failure), or in relation to the action of nodes in the control flow.

Functions represent units of work. A function becomes executable when it is reached in the timeline. It will begin to execute immediately unless it receives triggering input data, when it waits for this input data to be available. A function can consume and/or produce data to/from data items. A function can have one or more exit timelines. If it has more than one, then the alternative exits represent different exit conditions from the work done by the function, such as an OK, Warning or Error exit condition. Other functions can be placed on these exit timelines, to perform corrective or exception handling behaviour. All functions are described in function specifications and can all be decomposed into lower-level diagrams.

There are three types of function:

• Function - a unit of work, as above
• Reference function - function that has its own description (in a function specification), but also has a reference to the number of another function specification. This can be used to have several reference functions all referring to a common function, which in turn can have its own description (in a function specification) and diagram (an eFFBD) and decomposition (lower-level diagram and/or function specifications).
• Shared function - a function that has a single description (in a function specification) and possibly its own decomposition into a hierarchy of lower-level diagrams (as for a function), but unlike functions and reference functions, all shared function symbols with the same name have a single, shared description and decomposition. Therefore, all instances (on any number of diagrams) of a shared function called MyFunction will share the same description and decomposition.

Nodes are used to split the timeline into two or more branches. The processing along each branch is sequential unless that branch also contains nodes which instigate concurrent or other processing.

Nodes do not have a definition: their sole purpose is to define the manner in which eFFBD functions are connected, and their behaviour over time. Some of the nodes will, however, have a label, used either to specify conditional processing, or to link associated nodes or eFFBDs together.

The nodes are intended to be used to create sequential, conditional, concurrent, replicative and iterative processing constructs.

As DFDs and eFFBDs both show functions and data, there are inevitably similarities between them. DFDs are, by definition, inherently concurrent in that all functions without prompts (control flows such as E/D and T or TRIGGER) are concurrently executable. The order in which they execute is determined by:

1. The order in which data and/or control signals arrive on diagram input flows
2. The states and transition logic in the State Transition Diagram (STD) of any control processes on the diagram
3. The order imposed by self-evident truths, such as a function cannot execute without its input data from the output of another function, or a store filled by another function

However, the intrinsic benefit of the eFFBD notation is the way that sequence, iteration, and concurrency are shown on the diagram by the arrangement of functions along a (possibly complex) timeline.

eFFBDs do not have an explicit concept of state as provided by the STD, nor do they provide the same degree of detail in the logic for transitioning between states. The DFD and STD combination therefore offers more expressive power in this respect.

Cradle provides the eFFBD in both Essential and Implementation Domains from two primary perspectives:

1. To allow functionality to be represented completely in terms of eFFBDs: this approach is practicable provided that control signals and states are not significant in the system
2. To allow an alternate view of functionality to be provided by allowing both DFD and eFFBD representations of the system

Where DFDs model the system as it is, eFFBDs model what the system does.

eFFBDs are hierarchical. Their connectivity is:

Symbols

An example eFFBD is:

The symbols available in an eFFBD 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
	Time Function	A series of functions occurring over time in a predefined order. A Time Function may contain lower level Time Functions in a lower level eFFBD.	Function specification	BD DFD IDEF0
	Shared Function	A function that may be reused in this, or other, eFFBD and therefore considered to be off-the-shelf functionality. All uses of the shared function X on any eFFBD, reference the same definition and/or eFFBD for X.	Function specification	eFFBD
	Discrete Item	An item at the lowest level of detail of interest.	Store data definition	None
	Time Line Start	Represents the start of a time sequence.	None	None
	Time Line End	Represents the end of a time sequence.	None	None
	Parallel Node	Acts as the starting point for a set of concurrent operations. Has two or more outgoing branches, none with a label. The set of operations distributed along each branch are, collectively, considered to occur in parallel with the operations along all other branches. Within a branch, the processing of individual operations is sequential, unless that branch itself contains other nodes. All branches converge at a matching Parallel Node. The processing along all branches is considered to be synchronised by this node, so that the processing along some branches may be held until the processing along other branches has completed.	None	None
	Selection Node	Acts as an exclusive-OR operation. Has two or more outgoing Time Line branches, each with a label. The node effectively routes the flow of control along one of these outgoing branches, depending on the exit condition of the upstream operation. All the Time Line branches must subsequently converge at a matching destination Selection Node. As an alternative, an operation may have multiple, labelled, outgoing branches, which converge at a Selection Node. This effectively subsumes the first Selection Node within the operation.	None	None
	Iteration Node	Causes its outgoing branch to be processed iteratively. At the end of the iterative processing section of Time Line, there is another Iteration Node which may or may not have a label; if it does, this label must match that of the Iteration Node at the start of the loop. From this second Iteration Node, a Time Line Loop branches back to the first Iteration Node. This Time Line Loop contains the iteration condition, either at its start or at its end, depending on the type of loop that is required.	None	None
	Replicate Node	Creates a set of replicants of those of its outgoing branches that have a label. The number of these replicants is specified in the Replicate Node’s label. There must be one unlabelled outgoing branch containing operations that control the replicants. All the outgoing branches converge into a second Replicate Node that marks the end of the replication.	None	None
	End Loop Node	Causes a jump in the sequence of control to the outgoing branch of the loop node with the specified label, such that control continues after the specified loop.	None	None
	Goto Node	Causes a jump in the sequence of control along the Time Line to the Label Node in the same eFFBD that has the matching label. Can be many Goto Nodes with the same label.	None	None
	Label Node	Marks the destination of a Goto Node with a matching label. There may not be more than one Label Node with the same label.	None	None
	Exit Diagram Node	Causes all processing in the diagram to stop, and returns the flow of control to the exit of the eFFBD’s parent function with the same name as the node’s incoming branch.	None	None
	Event (Run Diag) Node	Causes the flow of control to begin at the start of the Time Line in the eFFBD specified in the node’s label.	None	None
	Validation Node	Serves simply as a means of reporting timing statistics from the Time Line (when timing information is available for eFFBDs) where the information reported is identified by the node’s label.	None	None
	Join Node	Rejoins two or more Time Lines that have been created by either an AND, OR or multiple exit node earlier in the processing.	None	None
	Environment	Represents an external system or environment to the system being modelled.	Environment	None
	Reference Function	A function that references a user-specified function specification as well as having its own specification definition. This is to provide an alternative method to shared functions for providing reuse of functions within a model.	Function specification	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
	Timeline	Represents the flow of time from object to object.	None	None
	Timeline Loop	Used in a loop or iteration structure to return to a Loop or Iteration Node and begin another repetition.	None	None
	Data Link	Represents the flow of data both into and out of functions.	None	None
	Trigger Data Link	Represents the flow of data into a function, that triggers the function to execute.	Flow data definition	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

Functions can have multiple exits for alternate conditions. Shared functions are identified by the name, not the number. They have a common definition and a common child diagram.

Trigger data links can be named, when they can link functions together, or link functions to the environment. This reduces the need for discrete items, reducing the diagrams’ complexity.