This something I ve come across today; I ve had learnt how it works during my college days but I had underestimated it when comparing to Programmable Logic Controllers(PLC). The Technolgy in PAC differs a lot from what I came to know. PAC is single device that will control,communicate and process various logics to the field devices in a plant. PAC is something like industrail PC ,where as PLC needs a PC and other communication devices to control various field devices.Other thing, i came to know it supports various industrial communication systems.

So, what I feel, when one PAC is able to handle what several PLCs try to do, its fast gona replace them.
More info about PAC can be found out here @ NI website
https://admin.na3.acrobat.com/_a56821929/p16521534/?launcher=false&fcsContent=true&pbMode=normal

• National Instruments are leading technolgy towards PAC through its programs like LABVIEW.
• U can learn about labview @ here 

http://www.ni.com/academic/lv_training/how_learn_lv.htm