The purpose of a Software Design Specification (SDS) is to define the software that is to meet the functional requirements for the project. It is the stage at which the supplier specifies the detailed design of the software system, produces the program code to realise that design, tests the individual programs and integrates them into the complete software system.
Now, a PCS automation system generally has a collection of standard reusable modules that need to be configured and/or programmed. But unlike a typical IT type system the design of these modules is often part of the standard software of the system and is not needed in detail in the SDS. A good example of this is a PID Controller, where the PID algorithm is not something specifically designed for the project.It may have some documentation in the systems standard manuals but they are not normally considered to be part of the SDS.
There is another class of module that a PCS often contains, these are application library objects rather than standard software
What an Automation SDS should contain includes for example:
Lists of the standard software being used, including version numbers and references to the relevant system documentation
Designs for all the custom software that is produced specifically for the project
Mapping of functional requirements to the software modules
All software modules, including application specific and system standard should have version control
Note - conventional IT SDS's and here is a good example are not really the same as those for Automation system
The Requirements Model can be developed to cover the content of an SDS