I maybe way off here, but I think the significant differences between a PLC (Programable Logic Controller) and DCS (Distributive Contol System) are:
1. How the proccessors scan the I/O.
PLC-in scan top to bottom
DCS-scan all I/O ussually based on change.
2. The refresh speed of the data or scan time.
PLC-Depends on the length of the program, loops, Sub routines, etc...
DCS-Typically more real time data.
3. Cost
PLC-More cost efective on a per I/O basis than DCS.
If I am going to apply a new control system to, for example, a chemical proccess that would tend to be easily upset, I would choose the DCS.
If on the other hand I am adding a package conveying system, a PLC would be more that sufficient. Of course this is an over simplification of the problem, but I think you see my point...
Opinions are like bosses, everybody has one....
)
Daniel Camacho