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.doc  PROGRAMMABLE LOGIC.doc (Size: 233 KB / Downloads: 23)


Simplification of engineering and precise control of manufacturing process can result in significant cost savings. The most cost-effective way, which can pay big dividends in the long run, is flexible automation; a planned approach towards integrated control systems. It requires a conscious effort on the part of plant managers to identify areas where automation can result in better deployment/utilization of human resources and savings in man-hours, down time. Automation need not be high ended and too sophisticated; it is the phased, step-by-step effort to automate, employing control systems tailored to one’s specific requirements that achieves the most attractive results. That is where Industrial electronics has been a breakthrough in the field of automation and control techniques.


A constant demand for better and more efficient manufacturing and process machinery has led to the requirement for higher quality and reliability in control techniques. With the availability of intelligent, compact solid state electronic devices, it has been possible to provide control systems that can reduce maintenance, down time and improve productivity to a great extend. By installing efficient and user friendly industrial electronics systems for manufacturing machinery or processors, one can obtain a precise, reliable and prolific means for generating quality products.
Considering the varied demand and increasing competition, one has to provide for flexible manufacturing process. One of the latest techniques in solid state controls that offers flexible and efficient operation to the user is “PROGRAMMABLE CONTROLLERS”. The basic idea behind these programmable controllers was to provide means to eliminate high cost associated with inflexible, conventional relay controlled systems.


A Programmable controller is a solid state user programmable control system with functions to control logic, sequencing, timing, arithmetic data manipulation and counting capabilities. It can be viewed as an industrial computer that has a central processor unit, memory, input output interface and a programming device. The central processing unit provides the intelligence of the controller. It accepts data, status information from various sensing devices like limit switches, proximity switches, executes the user control program store in the memory and gives appropriate output commands to devices like solenoid valves, switches etc.
Input output interface is the communication link between field devices and the controllers; field devices are wired to the I/O interfaces. Through these interfaces the processor can sense and measure physical quantities regarding a machine or process, such as, proximity, position, motion, level, temperature, pressure, etc. Based on status sensed, the CPU issues command to output devices such as valves, motors, alarms, etc.
Programmer unit provides the man machine interface. It is used to enter the application program, which often uses a simple user-friendly logic.


1. Programmable controllers are made of solid state components and hence provide high reliability.
2. They are flexible and changes in sequence of operation can easily be incorporated due to programmability. They may be modular in nature and thus expandability and easy installation is possible.
3. Use of PLC results in appreciable savings in Hardware and wiring cost.
4. They are compact and occupy less space.
5. Eliminate hardware items like Timers, counters and Auxiliary relays. The presence for timers and counters has easy accessibility.
6. PLC can control a variety of devices and eliminates the need for customized controls.

The Processor

The processor of a PLC holds and executes the user program. In order to carry out this job, the processor must store the most up-to-date input and output conditions.

Input image table:

The input conditions are stored in the input image table, which is a portion of the processor’s memory. That is, every single input module in the I/O section has assigned to it a particular location within the input image table. That particular location is dedicated solely to the task of keeping track of the latest condition of its input terminal. As mentioned in earlier section, if the input terminal has 5v dc power fed to it by its input device, the location within the input image table contains a binary 1(HI); if the input terminal has no 5v dc power fed to it, the location contains a binary 0(LO).
The processor needs to know the latest input conditions because the user program instructions are contingent upon those conditions. In other words, an individual instruction may have one outcome if a particular input is HI and a different outcome if that input is LO.

Output image table:

The output conditions are stored in the output image table, which is another portion of the processor’s memory. The output image table bears the same relation to the output interface of the I/O section that while terminals are analog inputs. You can directly connect any analog input to the processor via these terminals. Analog signal from these terminals is first converted to digital value via programmable peripheral interface (PPI). The I/O section’s output modules are functionally the same as the output amplifiers. They receive a low power digital signal from the processor and convert it into a high power signal capable of driving an industrial load. A modern PLC output module is optically isolated, and uses a triac, power transistor or relay as the series connected load controlling device. Terminal 1 to 8 are these type of O/P terminals whereas terminal D/A is Analog output terminal from processor. Each output device is wired to a particular output terminal on the I/O interface. Thus, for example, if output module 1 receives a digital 1 by applying 5v dc to output terminal 1, thereby illuminating LED is extinguished.

Central processing unit:

The subsection of the processor that actually performs the program execution will be called the central processing unit (CPU) with reference to input and output image table CPU executes the user program and continuously updates the output image table.
The output image table has a dual nature; its first function is to receive immediate information from the CPU and pass if on to the output modules of the I/O section; but secondly, it also must be capable of passing output information “backward” to the CPU, when the user program instruction that the CPU is working on calls for an item of output information. The input image table does not have its dual nature. Its single mission is to acquire information from the input modules and pass that information “forward” to the CPU when the instruction that the CPU is working on calls for an item of input information.

Data Memory:

A PLC is a computer, after all. Therefore, it can perform arithmetic, numeric comparisons, counting, etc. Naturally the numbers and data can change from one scan cycle to the next. Therefore the PLC must have a section of its memory set aside for keeping track of variable data, or numbers, that are involved with the user program. This section of memory we will call data memory.
When the CPU is executing an instruction for which a certain data value must be known, that data value is brought in from data memory. When the CPU executes an instruction that provides a numerical result, that result is put out into data memory. Thus, CPU can read from or write to the data memory. Understand that this relationship is different from the relationship between the CPU and the user program memory. When the user program is executing, the CPU can only reads from the user program memory, never write to it.

Operating System of PLC:

The function of the operating system is to present the user with the equivalent of an extended machine or virtual machine that is easier to program than the underlying hardware.
Due to this operating system, PLC is very easy to program. It can be programmed using electrical schemes with familiar relay symbols so that a plant electrician can easily access the PLC. Even though he does not know the assembly language or even if he may not have any familiarity with computers and electronics, he will be able to program the PLC.

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