A functional design specification, also known as FDS, is a document that describes how a process/control system will operate. It does not contain any highly technical details detail. It describes how the proposed system will operate, how people will interact with it, and what to expect when different operational scenarios occur. Note that FDS does not discuss how the coding works and the SFC style documents for PLC code is not part of FDS documents. Also, We can attach PFD and P&ID to FDS but they are chemical/process engineering drawings.

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 upon 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 agree 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. The next document after this may be a systems architecture document like IAB for Rockwell Automation (integrated architecture builder Rockell) or TIA selection tool for Siemens. This will help reduce the IO layouts and electrical drawings.

Finally, when the testing phase is performed, the actual behaviour of the final system is compared against the expected behaviour 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 mockups 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 well. Safety devices and their Effects on the System