• Too much work required in connecting wires
• Difficult with changes or replacements
• Difficulty in finding errors; requiring skillful work force
• When a problem occurs, hold-up time is indefinite, usually long.

Control panel with a PLC controller
With invention of programmable controllers, much has changed in how a process control system is designed. Advantage of control panel that is based on PLC controller can be presented in few basic points:

• Compared to a conventional process control system, number of wires needed for connections is reduced by 80%.
• Flexible and can be reapplied to control other systems quickly and easily.
• Needs fewer spare parts.
• Diagnostic functions of a PLC controller allow for fast and easy error detection.
• It is much cheaper compared to a conventional system; especially in cases where a large number of I/O instruments are needed and when operational functions are complex.
• Reliability of a PLC is greater than that of an electro-mechanical relay or a timer.

Systeatic approach in designing a process control system

First, one needs to select an instrument or a system that he wishes to control. Automated system can be a machine or a process and also can be called a process control system. Function of a process control system is constantly watched by I/P device (sensors) that give signals to a PLC controller. In response to this, PLC controller sends a signal to external O/P device (operating instruments) that actually control how system functions in an assigned manner (for simplification it is advisable to draw a block diagram of operations' flow).
Secondly, one need to specify all input and output instrument that will be connected to PLC controller. Input devices are various switches (Limit switches, Selector switches, Proximity switches, Float switches, Pressure switches etc.), sensors and such O/P devices can be solenoids, electromagnetic valves, motors, relays, magnetic starters as well as instruments for sound and light signalization.

Following an identification of all I/P and O/P instruments, corresponding designations are assigned to I/P and O/P lines of PLC controller. Allotment of these designations is in fact an allocation of I/P and O/P on a PLC controller, which correspond to I/Ps and O/Ps of a system being designed.
Third, make a Ladder diagram for a program by following the sequence of operations that was determined in first step.
Finally, program is enterred into the PLC memory. When finished with programming, checkup is done for any existing errors in a program code (using functions for diagnostics) and if possible, an entire operation is simulated. Before this system is started, programmer need to check once again whether all the instruments are connected to correct I/Ps and O/Ps. By bringing supply in, system starts working.


PLC CONTROLLERS

Before PLCs there was always an electrical board for electrical system controls, within this board there were a great number of interconnected electromechanical relays to make the whole system work. By word "connected" it was understood that engineer had to connect all relays manually using wires, an engineer would design logic for a system, and engineers Would receive a schematic outline of logic that they had to implement with relays. These relay schemas often contained hundreds of relays. The plan that engineer was given called "ladder schematic".

Ladder displayed all switches, sensors, motors, valves, relays, etc. found in the system. Engineer's job was to connect them all together. One of the problems with this type of control was that it was based on mechanical relays. Mechanical instruments were usually the weakest connection in the system due to their moveable parts that could wear out. If one relay stopped working, engineer would have to examine an entire system.

The other problem with this type of control was in the system's break period, when a system had to be turned off, so that connections could be made on the electrical board.
It's not hard to imagine an engineer who makes a few small errors during his project. In that case, if system is not perfect with a first try, finding errors was a laborious process.

First programmable controllers
"Generals Motors" were first to recognize need to replace the "wired" control board. Specification required that a new device be based on electronic instead of mechanical parts, to have flexibility of a computer, to function in industrial environment (vibrations, heat, dust, etc.) and have capability of being reprogrammed and used for other tasks. The last criterion was also the most important, the device had to be programmed and maintained easily by engineers.

"Gould Modicon" developed first devices that met these specifications. The key to Success with a new devices was that for its programming one did not have to learn a new programming language. It was programmed so that same language a ladder diagram, already known to supervisors was used. Engineers and supervisors could very easily understand these devices because the logic looked similar to old logic that they were used to working with. Thus they didn't have to learn new programming language which (obviously) proved to be a good move. PLC controllers were initially called PC controllers (programmable controllers). First PLC controllers were simple devices. They were not suitable for complicated controls such as temperature, position, pressure, etc. however, throughout years, makers of PLC controllers added numerous features and improvements.

Today's PLC controllers can handle highly complex tasks such as position control, various regulations and other complex applications. The speed of work and easiness of programming were also improved. Also modules for special purposes were developed, like communication modules for connecting several PLC controllers to the net.

PLC Controller Components
PLC is actually an industrial microcontroller system, which have hardware and software specifically adapted to industrial environment. Block schema with typical components that PLC consists. 

Protection needed in isolating a CPU blocks from damaging influences that industrial environment can bring to a CPU via input lines. Program unit is usually a computer used for writing a program (often in ladder diagram) or a hand-held Programming device.

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