Ward and Mellor Extensions

12.4.3 Ward and Mellor Extensions

Ward and Mellor [WAR85] extend basic structured analysis notation to accommodate the following demands imposed by a real-time system:

• Information flow is gathered or produced on a time-continuous basis. • Control information is passed throughout the system and associated control

processing.

F I G U R E 12.12

Monitored

Input

Time- temperature

data flow

“continuous”

Monitor and adjust

Temperature set point

• Multiple instances of the same transformation are sometimes encountered in multitasking situations.

• Systems have states and a mechanism causes transition between states. In a significant percentage of real-time applications, the system must monitor time-

continuous information generated by some real-world process. For example, a real- To adequately model a time test monitoring system for gas turbine engines might be required to monitor

real-time system, turbine speed, combustor temperature, and a variety of pressure probes on a con- structured analysis

tinuous basis. Conventional data flow notation does not make a distinction between notation must be

available for time- discrete data and time-continuous data. One extension to basic structured analysis continuous data and

notation, shown in Figure 12.12, provides a mechanism for representing time-con- event processing.

tinuous data flow. The double headed arrow is used to represent time-continuous flow while a single headed arrow is used to indicate discrete data flow. In the figure, mon-

itored temperature is measured continuously while a single value for tempera-

ture set point is also provided. The process shown in the figure produces a time-continuous output, corrected value.

The distinction between discrete and time-continuous data flow has important implications for both the system engineer and the software designer. During the cre- ation of the system model, a system engineer will be better able to isolate those processes that may be performance critical (it is often likely that the input and out- put of time-continuous data will be performance sensitive). As the physical or imple- mentation model is created, the designer must establish a mechanism for collection of time-continuous data. Obviously, the digital system collects data in a quasi-con- tinuous fashion using techniques such as high-speed polling. The notation indicates where analog-to-digital hardware will be required and which transforms are likely to demand high-performance software.

In conventional data flow diagrams, control or event flows are not represented explicitly. In fact, the software engineer is cautioned to specifically exclude the

PA R T T H R E E C O N V E N T I O N A L M E T H O D S F O R S O F T WA R E E N G I N E E R I N G

F I G U R E 12.13

Movement

Data and

alarm

control flows Status of each

using Ward fixture

and Mellor Parts status buffer

Start/stop

control

Bit string

operator interface

Operator Position

robot commands

Robot movement record Robot command file

representation of control flow from the data flow diagram. This exclusion is overly restrictive when real-time applications are considered, and for this reason, a spe- cialized notation for representing event flows and control processing has been devel- oped. Continuing the convention established for data flow diagrams, data flow is represented using a solid arrow. Control flow, however, is represented using a dashed or shaded arrow. A process that handles only control flows, called a control process, is similarly represented using a dashed bubble.

Control flow can be input directly to a conventional process or into a control process. Figure 12.13 illustrates control flow and processing as it would be represented using