PLC Ladder Logic Software V&V

A recent question I received after one of my webinars:

Ques:  Software Val as part of the DHF- Do lab equipment processes controlled by Programmable Logic Controllers (PLCs) such as for valves to open a door or for temperature control count as software? Or would they be verified / validated as part of the function they control?

Ans:  You could do it either way, and usually software V&V as I outlined in the webinar addresses both, as I do "black box" software V&V (check software operation by the hardware's operation) for all software validations, to see the software's operation by validating the hardware's operation.

With PLC ladder logic, I do both, "white" and "black box" V&V , i.e., a "code" review by a "white box" review of the ladder logic, often by annotating each "rung" as to what that "rung" says versus what tt actually does, and also by running the process / hardware controlled by the ladder logic ("black box").

As mentioned, this isn't the only way to do it. But it's the approach I have taken and has passed numerous US FDA and EU N-B inspections.  

--   jel@jelincoln.com