Ok a short history of controls they were first electro mechanical a bit like a Sangamo timeclock and who is good at making clocks yep the Swiss so 30 years ago th e majority of controls companies as they were known was Swiss ie Sauter, Staffa,Landis&Gyre and so on then electronic controls cam out ie Proportional,Proportional Integral and so on then came DDC Direct Digital Control ie Sauters Micos system then someone put a computer terminal on the front and you got BMS Building Management System great but that meant the Control companies had what was known as proprietory software in other words but a couple of system companies started up ie JEL controls and Trend controls who used IBM based PCs or basically a windows Intel based computer.
Only problem was the big control companies were way behind on using IBM computers so the system companies got the jump on them why because control companies brought out new BMS systems but the system companies stuck to IBM so all their stuff was backword compatable in other words the 1st generation outstation worked on a 4th generation BMS
Got that ? Good
So what do we have now well we got BMS outstations or controllers that have 2 inputs 1 digital 1 analogue 2 outputs 1 digital 1 analogue
Digital either on or off ie input could be an airflow switch on an AHU or output ie BMS switching a pump on
Analogue in simple terms a variable output or input output being say a 0-10v (4-20mA) from a humidity sensor or output 0-10vdc to a invertor drive where you can set the speed of an electric motor.
Great so hold on you say we have PLC control well yep but a BMS is a poor mans PLC and thats how you need to look at it