Difference between PLCs and Computers

Programmable Logic Controllers (PLCs) have basic architecture compared to normal general purpose computers. A normal computer can be converted to PLCs by providing a way that the computer can able to receive information or signal from the field devices such as push buttons, switches and valve positions. Computer requires a some software to process the information obtained from the input to generate an output which decide whether to close or open the valve position in the process side.

Some of the important features and characteristics that distinguish between the general purpose computers and Programmable Logic Controllers (PLCs) are given below:

• PLCs are designed to operate under industrial environments (PLCs have to operate under wide range of temperature conditions, humidity and other environmental conditions). They are least affected by the electrical noise and are inherent to electrical noise
• Programming in PLCs is through Relay Ladder Logic or other easily learned language. PLCs comes with program language built in its memory. PLCs do not contain input and output devices such as keyboards, mouse, monitor, CD drives and other hard disks. It is in simple a self contained box with communication ports and set of terminals for input and output devices.
• Unlike computers which performs numerous tasks simultaneously, PLCs execute a single program in an orderly and sequential manner from first instruction to the last instruction
• PLCs have been designed for installation and maintenance by plant electricians. Programming in PLCs is simple (Relay Ladder Programming), it does not include any advanced code. Troubleshooting is simpler and many PLCs are designed to include fault details and written fault details on display screen.

PLCs Advantages or Benefits

Higher Reliability:

Once the program written and tested it can be easily downloaded into other PLC's memory. It requires lesser and simpler wiring compared to conventional hard wired circuits employed. Hence reliability of the system increases significantly with PLCs

More Flexibility:

It is easier to create a new program module or change an existing program in PLC compared to hard wired circuitry system. These software program modules can be changed whenever required. Use can modify the programs in the field and if required, security can be enhanced by hardware interlocks such as key locks and software features such as passwords

Lower Cost:

PLCs were originally designed to replace relay control logic which is not economical and complex especially for large control circuits. With PLCs the cost savings have been so significant that the relay control becomes uneconomical except for some power applications. Generally if the application consists of more than half a dozen control relays, PLCs are least expensive to install

Communication Capability:

Communication capability of PLC with the other controllers and computers in the system is one of the main advantages compared to relay control circuit. Functions such as Supervisory control, data acquisition from the field, monitoring devices and process parameters associated with the field and downloading and uploading of programs can be easily possible with the PLC compared to hardwired circuits

Faster Response:

PLCs are designed for high speed and for the real time applications.Response time for PLCs are much smaller compared to relay logic circuits. The programmable controllers operates in real time i.e, an event taking carrying out at field will result in execution of operation of output

Easy to Troubleshoot:

PLCs have inbuilt diagnostics and override functions that helps the user to easily trance the software and hardware errors.