Software written and run on PC has changed how people work with PLCs. PLC software run on a PLC falls into the following two categories:
PLC software that allows the user to program and gives the user the tools to write a PLC program using ladder logic or other programming language and document or explain the program is as much detail as is necessary.
PLC software that allows the user to monitor and control the process is also called man-machine, or operator, interface. It enables the user to view a process or graphical representation of a process on a CRT, determine how the system is running, trend values, and receives alarm conditions.
PLCs are divided into three major size categories:
Small: It covers units up to 128 I/Os and memories up to 2Kb. It capable of providing simple to advanced levels of machine control.
Medium: It have up to 2048 I/Os and memories up to 32Kb. Special I/Os modules make medium PLCs adaptable to temperature, pressure, flow, weight, position and any type of analog function encountered in process control applications.
Large: The most sophisticated units of PLCs family. They have up to 16,000 I/Os and memories up to 2Mb. It has almost unlimited applications and can control individual production processes or entire plants.
Single-ended: It involves one PLC controlling one process. This would be a stand-alone unit and would not be used for communicating with other computers or PLCs.
Multitask: It usually calls for a medium size PLC and involves one PLC controlling several processes. It can be a subsystem for larger processes and communicating with a central PLC.
Control management: It involves one PLC controlling several others. It requires a large PLC processor designed to communicate with other PLCs and possibly with a computer. The control management PLC supervises several PLCs by downloading programs that tell the other PLCs what has to be done.
Some important characteristics distinguish PLCs from general-purpose computers .
The PLC is designed to operate in the industrial environment with the wide range of ambient temperature and humidity .
A well designed PLC is not affected by the electrical noise inherent in most industrial locations
Hardware and software of PLCs are designed for easy use by plant electricians and technicians
Unlike the computer, the PLC is programmed in relay ladder logic or other easily learned languages
The PLC comes with its program language built into permanent memory, whereas a personal computer requires a disk operating system (DOS). PLC is limited by the language it comes with, unless it is a modular type