The figure below shows three linked faceplates that control four condenser fans.
There is too much to explain in precise detail, but the basics of the control are that the faceplate on the left hand side ( TIC4804 ) provides an output which is used as an input to the middle faceplate. If the PV ( process variable ) gets above the 'NextStartSetpoint' for a specified duration then the next duty pump is to be started. This information is then passed to the final ( RHS ) faceplate which controls which of the four duty pumps is to be the one that is to be started.
( This Duty Control is an automatic version of the Duty Control - where if a pump fails it automatically gets put to the back of the Duty Selection and another not failed pump takes over as the ( Duty ) pump. There are simpler ways of coding the Duty Control and an operator initiated selection procedure is shown in the Arla
section of this website )
It is worth remembering that the picture above is what is known as the 'frontend' display and that all these three separate faceplates are getting their information from code segments in the controller, which controls the plant.
My job at Kelco was not only to devise the graphics, but also the control language and the interface between the two systems.