PSIM …. The Original PLC Simulator / Emulator – Proffessor Bill’s Free PLC Simulator! – Allen Bradley PLC Simulator, Hands-On Programmable Logic Controller Training Without the PLC. The PLC tutors are nice, Basic Lessons and Quiz Exercise can brush up your Fundamentals.
A Standalone PLC or PLC based System or SCADA form the automation systems of today. Allen Bradley, Siemens and GE-Fanuc were PLC manufacturers. The firm which i had started many years back, also did PLC related work as a System House for GE-Fanuc. This is a modular software configured/programmable embedded system.
A Basic PLC was a system built around an 8051 type device. It would have a EEPROM or UV PROM. EEPROM the ancestor of flash had just made an entry. SEEQ was one of the makers. It needed a little additional circuitry for programming. A 286/386 PC would have a ladder logic program, which could be loaded via serial port to PLC. The Ladder Logic was a Electrician Friendly GUI to create the 8031 firmware. The PLC would have Logic inputs and Relay outputs. Analog I/O Modules were more expensive Modules.
Programmable Logic Controllers (PLC) and Programmable Automation Controllers (PAC). “Acquiring data from sensors involves precision measurement and the processing of very low values or small changes in analog voltages/currents.”
So, if you want to specialize in Instrumentation like T&M or Process Control, you should be good at analog. PLC is the Modular or Component based approach to Industrial Automation. That means Configurable, Scalable, Serviceable/Maintainable. Most important fact is it is Programmable.