A Functional Design Specification also 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.
Let’s go over what we’ve discussed.
A Functional Design Specification or FDS
is a document describing how a process or a control system will operate.
One of the main reasons to have a functional design specification
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.
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.
We hope you enjoyed this video on Function Design Specifications,
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