In the beginning there was the timer and the relay, There was the hardwired controller and in the process industries. Some decades ago, digital electronics made the hardwired relay and controller obsolete in favor of the PLC, the programmable logic controller. Have limited control functions and made to be fully deterministic, PLCs were able to sweep hardwiring into the dustbin because they were easy to program in the "ladder" style of electrical wiring diagram, and they were easily reprogrammable so that they can accommodate changes on assembly lines and in batch processes.
But PLCs are limited. Ladder-logic programming can not be used for complex formula of mathematical, such as the basic PID algorithm found in every single loop controller in a process plant. Along came the PC. Inexpensive computing power became easily and ubiquitous affordable. PCs were tried early on in industrial control, but early operating systems and hardware were not up to the standards and stresses of the industrial workplace.
The biggest issues were determinism. In many industrial automation environments, especially in other discrete automation applications and motion control, it is absolutely required that an instruction get where it is supposed to go, when it is supposed to get there. PC operating systems are non-deterministic and have variable latency, depending on processor loading or the requirements of the operating program.
Several companies have produced a powerful and useful hybrid of the programmable controller and the PC. This device is usually called a PAC, or Programmable Automation Controller. ARC Advisory Group is generally credited with coining the name PAC, and analyst Craig Resnick defines a PAC as having these characteristics:
• Multi-domain functionality, including motion, logic, drives and process on a single platform
• Common tagging and a single database
• Software tools that allow design by process flow across several machines or process units
• Mirror industry applications in open modular architectures that from machine layouts in factories to unit operations in process plants
• De-facto standards for languages and network interfaces, etc. allowing data exchange as part multi-vendor systems networked