PLC (Programmable Logic controller) is believed by a lot of professional control people to be a very old apparatus, which is correct, they are in use for about 35 years, with a prehistoric operating system and a programming language, the Relay Ladder Logic. According to these adolescent conclusions they search for better control platforms like the very accepted personal computers with extra hardware and latest programming languages. The reality shows the opposite. PLC used by almost any aspect fit the newest concepts in the up to date computerized world.
The first main misunderstanding of the concept behind PLC is philosophy of it as a "Relay Ladder Logic" mechanism rather than a "Scan based" mechanism. The scan is the PLC operating system’s heart the more than any thing moreover. The most modern of operating systems are the multi-tasking ones such as, UNIX, WINDOWS etc. As most of the present computers are still depend on one CPU, the "time sharing" is the only method these machines can do more than one job.
The PLC SCAN is precisely time sharing operating system, and should be believed as such. The PLC checks all the jobs under its control every 15 - 25 ms, which is think as one "Scan" The tasks order in the scan and the scan length are simply controlled in most of nowadays PLCs. All of the I/Os and other resources such as counters, data registers, timers, etc. are simply distributed among all the jobs, and each job can use others tasks' results. As each line of code in the PLC program can compact with an independent job, the tasks number in this multi-task platform is nearly unlimited. Everything in the PLC platform is monitored, controlled, and calculated every scan. In other platforms it gets a complex break off system with a lot of interrupt priorities and interrupts masking to do the similar. If you evaluate it with other multi-tasking operating systems you'll be induced that PLCs still exist.
Graphical Operator Interface
Some companies try to induce the Community of Control Engineers' that it's time to move to "Plain English Control Language" and create history of those "strange relay symbols". Graphics with windows, icons, graphic switches, etc., isn't symbols of PLC's relay just another graphic method to present logic combinations? The symbol of RLL is a very instinctive and simple method to represent complex logic combination. Try to explain the combination of logic below using simple English and you will be influenced. In mode of "on line monitor" it suits even more clear that a "Plain text" is very complex to monitor and the symbols of relay are brilliant method to monitor, see, and understand.