You will find that there are many ways to program, but the UPCM is the "unit" controller that is where all the "meat" of the programming for your AHU operation is at,CPL lives in the BCU and is used for averaging and other high level functions that rely on a lot of information like VAV temps and valve position that the UPCM cannot manage it is then used to manipulate the AHU to acheive a comfort level based on averaging of the "area" and so on, it is a bit confusing at first but after you get a basic understanding you should find that it is a great way to control a system if it is done right. The UPCM and PCM are old and going fast, but it depends on the person that programmed it, You can take 10 programmers and get 100 different programs all to do the same thing. You will find that the summit variables are used as place holders in the code of the UPCM, that is how you pass the analog and binary points from the summit level to the "unit" level(UPCM), you have to look at it as Master and slave, ie, BCU is the master, UPCM is a slave but has some control over its operation but looks to summit to provide certain information, ie... static setpoint, discharge air setpoint and so on.
Most of the time if summit is not passing valid information then the UPCM should default to what was programmed in as the default. There are a lot of people who can help you if you can post a little at a time as you find something you do not understand. Hope that helps a bit.
