What is a Functional Design Specification (FDS)?

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A Functional Design Specification also is known as FDS is a document that describes how a process or a control system will operate.
It does not contain any highly technical detail, rather, it describes how the proposed system will operate, how people will interact with it and what to expect when different operational scenarios occur.

For example, if we were writing a Functional Design Specification for this RealPars video we may include a statement such as:
When the user clicks the Like button, which is indicated by the thumbs up icon, the Number of Likes counter will increment by a value of 1 and the thumbs up icon will change from grey to blue as a visual indicator to the user that this has been selected.

Note how this statement describes the action and effect of clicking the Like button but does not discuss how the coding for this button operation works.

There are many reasons to have a functional design specification.
One of the main reasons is to have some form of documented consensus on what the system is to achieve before making the more time-consuming effort of producing drawings or writing PLC code.

The Functional Design Specification can be passed between relevant team members, customers, and stakeholders for comment and review until the final document is agreed and signed off. This process of review and change is crucial to ensuring the final design will be fit for purpose and meet the needs of the stakeholders.

When the team of stakeholders agrees that a functional design specification consensus is reached, the document is typically declared complete or signed off.

After this, the document is handed to the engineering teams for technical design and programming, using the functional specification as the reference.

A completed Functional Design Specification will:
– Let the Engineers know what to design.
– Let the Programmers know what the code should do.
– Let the stakeholders know what will be delivered.

In a typical industrial software engineering life-cycle, the functional design specification describes what has to be implemented.
The next document after this may be a Systems architecture document, describing how the functions will be realized using the chosen PLC Controller, this, in turn, will help produce the IO layouts and electrical drawings.

Finally, when the testing phase is performed, the actual behavior of the final system is compared against the expected behavior as defined in the functional specification.

A Functional Design Specification can have many types of layouts but will typically contain a basic overview of each main part of the system, its function and how it will operate.

Depending on the system being discussed, a Functional Design Specification may contain mock-ups of operator interface screens and show the operation of each button and part of the screen.

The cause and effect of alarms and information messages on the system will also be discussed, as will safety devices and their effects on the system.

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