Glossary · Requirements and interfaces
Functional design specification (FDS)
Also known as: Functional specification (FS)
German: Funktionale Designspezifikation (FDS)
In automation projects, a functional design specification (FDS) describes how the control system will implement the required functions: sequences, operating modes, interlocks, alarms, HMI behavior and interfaces, written so that the customer can review and approve it before programming starts.
- Systems engineering
- Technical documentation
In one sentence
A functional design specification (FDS) describes how an automation system will implement required functions, for review before programming starts.
Example
The FDS for a CIP skid describes each cleaning step, its setpoints and timeouts, the interlocks with the filling machine and the alarms shown on the HMI.
How it applies
- Engineering: The FDS translates the customer's requirements into control behavior and becomes the reference for software design and the test plan.
- Validation: In regulated industries (for example under GAMP 5) test protocols such as OQ are written against the functional specification.
- Documentation: The FDS is one of the best sources for the operating manual: sequences, modes and alarm texts can be reused if the documentation team works from the approved revision.
FDS vs. SRS
A Software requirements specification (SRS) says what the software must do. The FDS describes how the functions will behave from a user and process perspective, and often covers hardware and HMI as well. Many projects merge the two; if so, requirements and design statements should still be distinguishable.