PROGRAMMABLE LOGIC CONTROLLERS AND MICROCONTROLLERS 
                  LEARNING OBJECTIVES: 
                          Basics of programmable Logic Controllers, their working and programming 
                          Instructions sets like latch master control self holding relays 
                          Timer and counter instructions sets 
                          Electrical drives which were controlled by relays 
                          Design , modify and troubleshoot control circuit 
                  UNIT 1: INTRODUCTION TO PLC 
                  1.1 What is PLC? 
                          A programmable logic controller (PLC) or programmable controller is an industrial digital 
                           computer which has been ruggedized and adapted for the control of manufacturing processes, 
                           such as assembly lines, or robotic devices, or any activity that requires high reliability control 
                           and ease of programming and process fault diagnosis. 
                          They were first developed in the automobile manufacturing industry to provide flexible, 
                           ruggedized and easily programmable controllers to replace hard-wired relays, timers and 
                           sequencers. Since then they have been widely adopted as high-reliability automation controllers 
                           suitable for harsh environments. A PLC is an example of a "hard" real-time system since output 
                           results must be produced in response to input conditions within a limited time, otherwise 
                           unintended operation will result 
                          PLCs can range from small modular devices with tens of inputs and outputs (I/O), in a housing 
                           integral with the processor, to large rack-mounted modular devices with a count of thousands of 
                           I/O, and which are often networked to other PLC and SCADA systems. 
                          They can be designed for multiple arrangements of digital and analog I/O, extended temperature 
                           ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control 
                           machine operation are typically stored in battery-backed-up or non-volatile memory. 
                          It was from the automotive industry in the USA that the PLC was born. Before the PLC, control, 
                           sequencing, and safety interlock logic for manufacturing automobiles was mainly composed 
                           of relays, cam timers, drum sequencers, and dedicated closed-loop controllers. Since these could 
                           number in the hundreds or even thousands, the process for updating such facilities for the yearly 
                           model change-over was very time consuming and expensive, as electricians needed to 
                           individually rewire the relays to change their operational characteristics. 
                          When digital computers became available, being general-purpose programmable devices, they 
                           were soon applied to control sequential and combinatorial logic in industrial processes. However 
                           these early computers required specialist programmers and stringent operating environmental 
                           control for temperature, cleanliness, and power quality. To meet these challenges the PLC was 
                           developed with several key attributes. It would tolerate the shop-floor environment, it would 
                           support discrete (bit-form) input and output in an easily extensible manner, it would not require 
                           years of training to use, and it would permit its operation to be monitored. Since many industrial 
                           processes have timescales easily addressed by millisecond response times, modern (fast, small, 
                           reliable) electronics greatly facilitate building reliable controllers, and performance could be 
                           traded off for reliability. 
                        
                        
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                       1.2Invention and early development 
                               In 1968 GM Hydramatic (the automatic transmission division of General Motors) 
                                issued a request for proposals for an electronic replacement for hard-wired relay 
                                systems based on a white paper written by engineer Edward R. Clark. The winning 
                                proposal came from Bedford Associates of Bedford, Massachusetts. The first PLC, 
                                designated the 084 because it was Bedford Associates' eighty-fourth project, was the 
                                result Bedford Associates started a new company dedicated to developing, 
                                manufacturing, selling, and servicing this new product: Modicon, which stood for 
                                modular digital controller. One of the people who worked on that project was Dick 
                                Morley, who is considered to be the "father" of the PLC. 
                               The Modicon brand was sold in 1977 to Gould Electronics, later acquired by German 
                                Company AEG, and then by French Schneider Electric, the current oOne of the very 
                                first 084 models built is now on display at Schneider Electric's facility in North 
                                Andover, Massachusetts. It was presented to Modicon by GM, when the unit was 
                                retired after nearly twenty years of uninterrupted service. Modicon used the 84 
                                moniker at the end of its product range until the 984 made its appearance. 
                               Modern PLCs can be programmed in a variety of ways, from the relay-derived ladder 
                                logic to programming languages such as specially adapted dialects of BASIC and C. 
                                Another method is state logic, a very high-level programming language designed to 
                                program PLCs based on state transition diagrams. The majority of PLC systems 
                                today adhere to the IEC 61131/3 control systems programming standard that defines 
                                5 languages: Ladder Diagram (LD), Structured Text (ST), Function Block Diagram 
                                (FBD), Instruction List (IL) and sequential function chart (SFC). 
                               Many early PLCs did not have accompanying programming terminals that were 
                                capable of graphical representation of the logic, and so the logic was instead 
                                represented as a series of logic expressions in some version of Boolean format, 
                                similar to Boolean algebra. As programming terminals evolved, it became more 
                                common for ladder logic to be used, for the aforementioned reasons and because it 
                                was a familiar format used for electro-mechanical control panels. Newer formats 
                                such as state logic and Function Block (which is similar to the way logic is depicted 
                                when using digital integrated logic circuits) exist, but they are still not as popular as 
                                ladder logic. A primary reason for this is that PLCs solve the logic in a predictable 
                                and repeating sequence, and ladder logic allows the programmer (the person writing 
                                the logic) to see any issues with the timing of the logic sequence more easily than 
                                would be possible in other formats. 
                           1.3 Programming 
                                   PLC programs are typically written in a special application on a personal 
                                    computer, then downloaded by a direct-connection cable or over a network to the 
                                    PLC. The program is stored in the PLC either in battery-backed-up RAM or some 
                                    other non-volatile flash memory. Often, a single PLC can be programmed to 
                                    replace thousands of relays. 
                                   Early PLCs, up to the mid-1990s, were programmed using proprietary 
                                    programming panels or special-purpose programming terminals, which often had 
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                                    dedicated function keys representing the various logical elements of PLC 
                                    programs  Some proprietary programming terminals displayed the elements of 
                                    PLC programs as graphic symbols, but plain ASCIIcharacter representations of 
                                    contacts, coils, and wires were common. Programs were stored on cassette tape 
                                    cartridges. Facilities for printing and documentation were minimal due to lack of 
                                    memory capacity. The oldest PLCs used non-volatile magnetic core memory. 
                                   More recently, PLCs are programmed using application software on personal 
                                    computers, which now represent the logic in graphic form instead of character 
                                    symbols. The computer is connected to the PLC through USB, Ethernet, RS-
                                    232, RS-485, or RS-422 cabling. The programming software allows entry and 
                                    editing of the ladder-style logic. In some software packages, it is also possible to 
                                    view and edit the program in function block diagrams, sequence flow charts 
                                    and structured text. Generally the software provides functions for debugging and 
                                    troubleshooting the PLC software, for example, by highlighting portions of the 
                                    logic to show current status during operation or via simulation. The software will 
                                    upload and download the PLC program, for backup and restoration purposes. In 
                                    some models of programmable controller, the program is transferred from a 
                                    personal computer to the PLC through a programming board which writes the 
                                    program into a removable chip such as an EPROM. 
                  UNIT2: Timers and counters 
                  2.1 INTRODUCTION-The main function of a timer is to keep an output on for a specific length of time. 
                       A good example of this is a garage light, where you want power to be cut off after 2 minutes so as to 
                       give someone time to go into the house. The three different types of timers that are commonly used 
                       are a Delay-OFF, a Delay-ON, and a Delay-ON-Retentive. A Delay-OFF timer activates immediately 
                       when turned on, counts down from a programmed time before cutting off, and is cleared when the 
                       enabling input is off. A Delay-ON timer is activated by input and starts accumulating time, counts up 
                       to a programmed time before cutting off, and is cleared when the enabling input is turned off. A 
                       Delay-ON-Retentive timer is activated by input and starts accumulating time, retains the accumulated 
                       value even if the (ladder-logic) rung goes false, and can be reset only by a RESET contact.Counters 
                       are primarily used for counting items such as cans going into a box on an assembly line. This is 
                       important because once something is filled to its max the item needs to be moved on so something else 
                       can be filled. Many companies use counters in PLC's to count boxes, count how many feet of 
                       something is covered, or to count how many pallets are on a truck. There are three types of counters, 
                       Up counters, Down counters, and Up/Down counters. Up counters count up to the preset value, turn 
                       on the CTU (CounT Up output) when the preset value is reached, and are cleared upon receiving a 
                       reset. Down counters count down from a preset value, turns on the CTD (CounT Down output) when 
                       0 is reached, and are cleared upon reset. Up/Down counters count up on CU, count down on CD, 
                       turn on CTUD (CounT Up/Down output) when the preset value is reached, and cleared on reset. 
                  2.2 Programmable logic relay (PLR) -In more recent years, small products called PLRs 
                       (programmable logic relays), and also by similar names, have become more common and accepted. 
                       These are much like PLCs, and are used in light industry where only a few points of I/O (i.e. a few 
                       signals coming in from the real world and a few going out) are needed, and low cost is desired. These 
                       small devices are typically made in a common physical size and shape by several manufacturers, and 
                       branded by the makers of larger PLCs to fill out their low end product range. Popular names include 
                       PICO Controller, NANO PLC, and other names implying very small controllers. Most of these have 8 
                       to 12 discrete inputs, 4 to 8 discrete outputs, and up to 2 analog inputs. Size is usually about 4" wide, 
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         3" high, and 3" deep. Most such devices include a tiny postage-stamp-sized LCD screen for viewing 
         simplified ladder logic (only a very small portion of the program being visible at a given time) and 
         status of I/O points, and typically these screens are accompanied by a 4-way rocker push-button plus 
         four more separate push-buttons, similar to the key buttons on a VCR remote control, and used to 
         navigate and edit the logic. Most have a small plug for connecting via RS-232 or RS-485 to a 
         personal computer so that programmers can use simple Windows applications for programming 
         instead of being forced to use the tiny LCD and push-button set for this purpose. Unlike regular PLCs 
         that are usually modular and greatly expandable, the PLRs are usually not modular or expandable, 
         but their price can be two orders of magnitude less than a PLC, and they still offer robust design and 
         deterministic execution of the logics. 
       2.3  Scan time: A PLC program generally loops i.e. executes repeatedly, as long as the controlled 
         system is running. At the start of each execution loop, the status of all physical inputs are copied to 
         an area of memory, sometimes called the "I/O Image Table", which is accessible to the processor. 
         The program then runs from its first instruction rung down to the last rung. It takes some time for the 
         processor of the PLC to evaluate all the rungs and update the I/O image table with the status of 
         outputs.[14] Scan times of a few milliseconds may be encountered for small programs and fast 
         processors, but for older processors and very large programs much longer scan times (on the order 
         of 100 ms) may be encountered. Excessively long scan times may mean the response of the PLC to 
         changing inputs or process conditions is too slow to be useful .As PLCs became more advanced, 
         methods were developed to change the sequence of ladder execution, and subroutines were 
         implemented. This simplified programming could be used to save scan time for high-speed processes; 
         for example, parts of the program used only for setting up the machine could be segregated from 
         those parts required to operate at higher speed. Newer PLCs now have the option to run the logic 
         program synchronously with the IO scanning. This means that IO is updated in the background and 
         the logic reads and writes values as it's required during the logic scanning. 
       UNIT 3: WORKING OF PLC 
       3.1Process of a scan cycle -There are 5 main steps in a scan cycle: 
       Reading inputs 
       Executing the program 
       Processing communication requests 
       Executing CPU diagnostics 
       Writing outputs 
       3.2User interface-User interface and List of human-computer interaction topics.PLCs may need to 
       interact with people for the purpose of configuration, alarm reporting, or everyday control. A human-
       machine interface (HMI) is employed for this purpose. HMIs are also referred to as man-machine 
       interfaces (MMIs) and graphical user interfaces (GUIs). A simple system may use buttons and lights to 
       interact with the user. Text displays are available as well as graphical touch screens. More complex 
       systems use programming and monitoring software installed on a computer, with the PLC connected via 
       a communication interface. 
       3.2 Communications-Many models of PLCs have built-in communications ports, using RS-232, RS-
         422, RS-485, or Ethernet. Various protocols are usually included. Many of these protocols are vendor 
         specific.Most modern PLCs can communicate over a network to some other system, such as a 
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