4. Draw the general ladder rungs to represent a latch circuit. (N/D 2009)

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2 (Other possible questions) 1. What is meant by PLC? (N/D 2012) A programmable logic controller is a microprocessor based controller that uses a programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting and arithmetic in order to control machines and process. 2. What is an internal relay in a PLC? (N/D 2012) Most PLCs have an area of memory allocated for internal storage that is used to hold data which behave like relays. It is able to switch ON and OFF. But this is only for internal purpose. This will not exist in the real world. 3. What is shift register? What is the data required for a shift register? (N/D 2011) Shift registers can be used where sequence of operations is required for movement or track the flow of parts and information. The data required for the shift register are address of the bit array, address of the control structure, address of the source bit, number of bits in bit array. 4. Draw the general ladder rungs to represent a latch circuit. (N/D 2009) O1 Output IN1 Push button 5. Draw the ladder logic diagram to represent two switches that are normally open and both have to be closed for a motor to operate. (N/D 2008) IN1 and IN2 Input switches M Motor 6. Draw the ladder diagram of ON delay and OFF delay timer. (A/M 2008) ON delay timer: The timer is energized when the input IN1 becomes energized. The timer starts running after some present time.

3 OFF delay timer: When the contact IN1 is closed, the contact will energize the timer T1 and holds the output lamp ON for specified set value of 10 seconds. The action of an OFF delay timer is to delay setting the lamp OFF. 7. Draw a PLC timing circuit that will switch an output on for 10 seconds and then switch it off. (N/D 2007) I1 Input T1 Timer 1 for 1 second T2 Timer 2 for 10 seconds M1, M2 Memory coil O1 Output (light)

4 8. How does PLC differ from relay logic? (N/D 2010) Rewiring should be easily done in PLC. No vertical connections are allowed. In PLC, there must always be one output on each line. 9. What is the use of JUMP control in PLC? (N/D 2010) The JUMP instruction is an output instruction, enabling part of a ladder diagram to be jumped over. With JUMP instruction the processor scan time can be reduced by jumping over instruction not pertinent to the machine operation there by missing intermediate program and can skip instructions when a production fault occurs. 10. Draw the block diagram of PLC. (N/D 2004) 11. What are the logic functions that can be obtained by using switches in series? (N/D 2007) 12. Draw a timing circuit that will switch an output for ON for 1 sec then OFF for 20 seconds, then ON for 1 second, then OFF for 20 seconds and so on. (N/D 2008) I1 Input T1 Timer 1 for 1 second T2 Timer 2 for 20 seconds M1, M2 Memory coil O1 Output (light)

5 13. Draw NOR logic function using ladder diagram. (A/M 2010) 14. What is the main advantage of PLC? PLC s have great advantage that it is possible to modify a control system without having to rewire the connections to the input and output devices. 15. What are the features of PLC as a controller? The features of PLC as a controller are, They are rugged and designed to withstand vibrations, temperature, humidity and noise. The interfacing for inputs and outputs is inside the controller. They are easily programmed and have an easily understood programming language. 16. What is meant by ladder programming? The ladder programming involves each program task being specified as though a rung of a ladder. Thus such a rung could specify that the state of switches A and B, the inputs, be examined and if A and B are both closed then a solenoid, the output is energized.

6 17. What is meant by up counter? An up counter would count up to the preset value. Events are added until the number reaches the set value. When the set value is reached the counters contact changes the state. 18. What is the criteria need for the selection of a PLC? Input / output capacity Types of inputs/outputs Size of memory Speed and power of the CPU 19. What is meant by internal relay in PLC? Most PLCs have an area of memory allocated for internal storage that is used to hold the data, which behave like relays. It can able to switch ON and OFF. But this is for internal purpose. This will not exist in the real world. 20. What is meant by down counter? Down counter counts down from the preset value to zero. Events are subtracted from the preset value. When zero is reached the counters contact changes state.

7 DHANALAKSHMI COLLEGE OF ENGINEERING Department of mechanical engineering ME6702 MECHATRONICS Part B 1. Explain the architecture of a PLC and explain about its elements. (16)(N/D - 16)

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14 2. Write short notes on PLC for the following: (i) Data movement (ii) Data comparison. (16)(N/D - 16)

15 4. Discuss how AND, OR, NAND and NOR systems can be formed with ladder diagram. (16) (A/M-17) We can construct simply logic functions for our hypothetical lamp circuit, using multiple contacts, and document these circuits quite easily and understandably with additional rungs to our original ladder. If we use standard binary notation for the status of the switches and lamp (0 for unactuated or de-energized; 1 for actuated or energized), a truth table can be made to show how the logic works: Now, the lamp will come on if either contact A or contact B is actuated, because all it takes for the lamp to be energized is to have at least one path for current from wire L1 to wire 1. What we have is a simple OR logic function, implemented with nothing more than contacts and a lamp. We can mimic the AND logic function by wiring the two contacts in series instead of parallel: Now, the lamp energizes only if contact A and contact B are simultaneously actuated. A path exists for current from wire L1 to the lamp (wire 2) if and only if both switch contacts are closed.

16 If we take our OR function and invert each input through the use of normally-closed contacts, we will end up with a NAND function. In a special branch of mathematics known as Boolean algebra, this effect of gate function identity changing with the inversion of input signals is described by DeMorgan s Theorem, a subject to be explored in more detail in a later chapter. The lamp will be energized if either contact is unactuated. It will go out only if both contacts are actuated simultaneously. Likewise, if we take our AND function and invert each input through the use of normally-closed contacts, we will end up with a NOR function:

17 A pattern quickly reveals itself when ladder circuits are compared with their logic gate counterparts: Parallel contacts are equivalent to an OR gate. Series contacts are equivalent to an AND gate. Normally-closed contacts are equivalent to a NOT gate (inverter). 5. Explain the basics of ladder programming used in PLC. (APR/MAY 2008) Ladder logic is widely used to program PLCs, where sequential control of a process or manufacturing operation is required. Ladder logic is useful for simple but critical control systems or for reworking old hardwired relay circuits. As programmable logic controllers became more sophisticated it has also been used in very complex automation systems. Often the ladder logic program is used in conjunction with an HMI program operating on a computer workstation. The motivation for representing sequential control logic in a ladder diagram was to allow factory engineers and technicians to develop software without additional training to learn a language such as FORTRAN or other general purpose computer language. Development, and maintenance, was simplified because of the resemblance to familiar relay hardware systems. Implementations of ladder logic have characteristics, such as sequential execution and support for control flow features that make the analogy to hardware somewhat inaccurate. This argument has become less relevant given that most ladder logic programmers have a software background in more conventional programming languages. Ladder logic can be thought of as a rule-based language rather than a procedural language. A "rung" in the ladder represents a rule. When implemented with relays and other electromechanical devices, the various rules "execute" simultaneously and immediately. When implemented in a programmable logic controller, the rules are typically executed sequentially by software, in a continuous loop (scan). By executing the loop fast enough, typically many times per second, the effect of simultaneous and immediate execution is achieved, if considering intervals greater than the "scan time" required executing all the rungs of the program. Proper use of programmable controllers requires understanding the limitations of the execution order of rungs. Ladder logic uses graphic symbols similar to relay schematic circuit diagrams. Ladder diagram consists of two vertical lines representing the power rails. Circuits are connected as horizontal lines between these two verticals. Ladder diagram features Power flows from left to right. Output on right side cannot be connected directly with left side. Contact cannot be placed on the right of output. Each rung contains one output at least. Each output can be used only once in the program. A particular input a/o output can appear in more than one rung of a ladder. The inputs a/o outputs are all identified by their addresses, the notation used depending on the PLC manufacturer. Example of a simple ladder logic program The language itself can be seen as a set of connections between logical checkers (contacts) and actuators (coils). If a path can be traced between the left side of the rung and the output, through asserted (true or "closed") contacts, the rung is true and the output coil storage bit is asserted (1) or true. If no path can be traced, then the output is false (0) and the "coil" by analogy to electromechanical relays is considered "de-energized". The analogy between logical propositions and relay contact status is due to Claude Shannon. Ladder logic has contacts that make or break circuits to control coils. Each coil or contact corresponds to the status of a single bit in the programmable controller's memory. Unlike electromechanical relays, a ladder program can refer any number of times to the status of a single bit, equivalent to a relay with an indefinitely large number of contacts.

18 So-called "contacts" may refer to physical ("hard") inputs to the programmable controller from physical devices such as pushbuttons and limit switches via an integrated or external input module, or may represent the status of internal storage bits which may be generated elsewhere in the program. Each rung of ladder language typically has one coil at the far right. Some manufacturers may allow more than one output coil on a rung. Rung Input : Checkers (contacts) [ ] Normally open contact, closed whenever its corresponding coil or an input which controls it is energized. (Open contact at rest) [\] Normally closed ("not") contact, closed whenever its corresponding coil or an input which controls it is not energized. (Closed contact at rest) Rung Output: Actuators (coils) ( ) Normally inactive coil, energized whenever its rung is closed. (Inactive at rest) (\) Normally active ("not") coil, energized whenever its rung is open. (Active at rest) The "coil" (output of a rung) may represent a physical output which operates some device connected to the programmable controller, or may represent an internal storage bit for use elsewhere in the program. A way to recall these is to imagine the checkers (contacts) as a push button input, and the actuators (coils) as a light bulb output. The presence of a slash within the checkers or actuators would indicate the default state of the device at rest. 6. Write short notes on Jump control used in PLC using a ladder diagram. (NOV/DEC 2009), (NOV/DEC 2014) (A/M-17) This type of operation is most widely used in hazardous area where some action must be taken immediately despite of the status of currently running process. Use JMP instruction to perform this task. This Jump instruction does not differ from any Microcontroller Jump instruction, this Jump instruction also must be used with LBL (Label) instruction. Simply explaining, when JMP is activated, all the outputs between JMP and LBL are disabled until this is active. By saying all outputs, it does not include latch (L) outputs. Here is PLC program to Jump to Other Process, along with program explanation and run time test cases. List of Inputs and Outputs I:1/0 = Input to Jump to Label (Input) I:1/1 = Input A (Input) I:1/2 = Input B (Input) I:1/3 = Input C (Input) I:1/4 = Sensor Input (Input) O:2/0 = Output 0 (Output) O:2/1 = Output 1 (Output) O:2/2 = Output 2 (Output) O:2/3 = Output 3 (Output) Q2:0 = Jump (JMP and LBL) (I/O)

19 Ladder Diagram to perform Jumping Program Description RUNG000 comprises of a Manual input which may be a toggle button or sensor output by which operation is performed. Until I:1/0 is pressed, JMP does not energize, that means program is being scanned rung by rung and it does not affect program or process at all. When I:1/0 is pressed, program is directly jumped to the LBL and scans RUNG004. If Sensor Input I:1/4 is true, it energizes O:2/3 turning ON Output 3. As long as JMP is energized, all the output between JMP and LBL rungs are skipped and Input I:1/1 to I:1/3 do not affect O:2/0 to O:2/2 at all. Important thing to note here is that if Latch Output is used between JMP-LBL rungs and is active then it is not unlatched when JMP energizes. Runtime Test Cases Fig.1 -Output O:2/0 is latched even if JMP is energized. Fig.2 -When Sensor Input is TRUE and JMP is energized.

20 7. Explain the factors to be considered while selecting a PLC. (NOV/DEC 2007), (NOV/DEC 2009), (NOV/DEC 2014), (APR/MAY 2014) (NOV/DEC 2015) (MAY/JUNE 2016) PLC selection criteria consists of: * System (task) requirements. * Application requirements. * What input/output capacity is required? * What type of inputs/outputs is required? * What size of memory is required? * What speed is required of the CPU? * Electrical requirements. * Speed of operation. * Communication requirements. * Software. * Operator interface. * Physical environments. System requirements * The starting point in determining any solution must be to understand what is to be achieved. * The program design starts with breaking down the task into a number of simple understandable elements, each of which can be easily described. Application requirements * Input and output device requirements. After determining the operation of the system, the next step is to determine what input and output devices the system requires. * List the function required and identifies a specific type of device. * The need for special operations in addition to discrete (On/Off) logic. * List the advanced functions required beside simple discreet logic. Electrical Requirements The electrical requirements for inputs, outputs, and system power; When determining the electrical requirements of a system, consider three items: Incoming power (power for the control system); Input device voltage; and Output voltage and current. Speed of Operation How fast the control system must operate (speed of operation). When determining speed of operation, consider these points: How fast does the process occur or machine operate? Are there time critical operations or events that must be detected? In what time frame must the fastest action occur (input device detection to output device activation)? Does the control system need to count pulses from an encoder or flow-meter and respond quickly? Communication If the application requires sharing data outside the process, i.e. communication. Communication involves sharing application data or status with another electronic device, such as a computer or a monitor in an operator s station. Communication can take place locally through a twisted-pair wire, or remotely via telephone or radio modem. Operator Interface If the system needs operator control or interaction. In order to convey information about machine or process status, or to allow an operator to input data, many applications require operator interfaces. Traditional operator interfaces

21 include pushbuttons, pilot lights and LED numeric display. Electronic operator interface devices display messages about machine status in descriptive text, display part count and track alarms. Also, they can be used for data input. Physical Environment The physical environment in which the control system will be located. Consider the environment where the control system will be located. In harsh environments, house the control system in an appropriate IP-rated enclosure. Remember to consider accessibility for maintenance, troubleshooting or reprogramming. 8. Explain the timers, counters, internal relays. (NOV/DEC 2013), (APR/MAY 2014) (A/M-17) Timers Timer is an instruction that waits a set amount of time before doing something (control time). Timers count fractions of seconds or seconds using the internal CPU clock. The time duration for which a timer has been set is termed the preset and is set in multiples of the time base used. Most manufacturers consider timers to behave like relays with coils which when energized result in the closure or opening of contacts after some preset time. The timer is thus treated as an output for a rung with control being exercised over pairs of contacts elsewhere. Others treat a timer as a delay block which when inserted in a rung delays signals in that rung reaching the output. Timers Types On-Delay timer- simply delays turning on. It is called TON, TIM or TMR. Off-Delay timer- simply delays turning off. It is called TOF and is less common than the on-dellay type. The on/off delay timers above would be reset if the input sensor wasn t on/off for the complete timer duration. Retentive or Accumulating timer- holds or retains the current elapsed time when the sensor turns off in mid-stream. It is called RTO or TMRA. This type of timer needs 2 inputs. We need to know 2 things when using timers: 1. What will enable the timer? Typically this is one of the inputs (a sensor connected to one input). 2. How long we want to delay before we react? Wait x seconds before we turn on a load. When the instructions before the timer symbol are true the timer starts ticking. When the time elapses the timer will automatically close its contacts. When the program is running on the plc the program typically displays the current value. Typically timers can tick from 0 to 9999 (16-bit BCD) or 0 to times (16-bit binary). Counters A counter is set to some preset value and, when this value of input pulses has been received, it will operate its contacts. The counter accumulated value ONLY changes at the off to on transition of the pulse input. Typically counters can count from 0 tto 9999, -32,768 to +32,767 or 0 to The normal counters are typically software counters they don t physically exist in the plc but rather they are simulated in software. A good rule of thumb is simply to always use the normal (software) counters unless the pulses you are counting will arive faster than 2X the scan time.

22 Counter Types Up-counters counts from zero up to the preset value. These are called CTU, CNT, C, or CTR. Down-counters count down from the preset value to zero. These are calllled CTD. Up-down counters count up and/or down. These are called CTUD. For CTU or CTD counter we need 2 inputs, but in CTUD we need 3 (up, down and preset). To use counters we must know 3 things: 1. Where the pulses that we want to count are coming from. Typically this is from one of the inputs. 2. How many pulses we want to count before we react. 3. When/how we will reset the counter so it can count again. Counter Formats Some manufacturers consider the counter as a relay and consist of two basic elements: One relay coil to count input pulses and one to reset the counter, and the associated contacts of the counter being used in other rungs. Others (Siemens for example) treat the counter as an intermediate block in a rung from which signals emanate when the count is attained. High Speed Counter Most manufacturers also include a limited number of high-speed counters (HSC). Typically a high-speed counter is a hardware device. Hardware counters are not dependent on scan time. Internal Relays In PLCs there are elements that are used to hold data, that is, bits, and behave like relays, being able to be switched on or off and to switch other devices on or off. Hence the term internal relay. Such internal relays do not exist as realworld switching devices but are merely bits in the storage memory that behave in the same way as relays. For programming, they can be treated in the same way as an external relay output and input. Thus inputs to external switches can be used to give an output from an internal relay. This then results in the internal relay contacts being used, in conjunction with other external input switches, to give an output, such as activating a motor. Thus we might have: On one rung of the program: Inputs to external inputs activate the internal relay output. On a later rung of the program: As a consequence of the internal relay output, internal relay contacts are activated and so control some output. In using an internal relay, it has to be activated on one rung of a program and then its output used to operate switching contacts on another rung, or rungs, of the program. Internal relay scan be programmed with as many sets of associated contacts as desired. To distinguish internal relay outputs from external relay outputs, they are given different types of addresses. Different manufacturers tend to use different terms for internal relays and have different ways of expressing their addresses. For example, Mitsubishi uses the term auxiliary relay or marker and the notation M100, M101, and so on.

23 Siemens uses the term flag and the notation F0.0, F0.1, and so on. Telemecanique uses the term bit and the notation B0, B1, and so on. Toshiba uses the term internal relay and the notation R000, R001, and soon. Allen-Bradley uses the term bit storage and notation in the PLC-5 of the form B3/001, B3/002, and so on. 9. Explain the data handling operation in a PLC. (NOV/DEC 2012) (NOV/DEC 2015) Data Handling The following are examples of data-handling instructions to be found with PLCs. Data Movement For moving data from one location or register to another, Figure illustrates a common practice of using one rung of a ladder program for each move operation, showing the form used by three manufacturers: Mitsubishi, Allen-Bradley, and Siemens. For the rung shown, when there is an input to in the rung, the move occurs from the designated source address to the designated destination address. For data handling with these PLCs, the typical ladder program data-handling instruction contains the data-handling instruction, the source (S) address from where the data is to be obtained, and the destination (D) address to where it is to be moved. The approach that is used by some manufacturers, such as Siemens, is to regard data movement as two separate instructions, loading data from the source into an accumulator and then transferring the data from the accumulator to the destination. Figure shows the Siemens symbol for the MOVE function. The data is moved fromthe IN input to the OUT output when EN is enabled. Data transfers might be to move a preset value to a timer or counter, or a time or counter value to some register for storage, or data from an input to a register or a register to output.

24 Figure shows the rung, in the Allen-Bradley format that might be used to transfer a number held at address N7:0 to the preset of timer T4:6 when the input conditions for that rung are met. A data transfer from the accumulated value in a counter to a register would have a source address of the form C5:18.ACC and a destination address of the form N7:0. A data transfer from an input to a register might have a source address of the form I:012 and a destination address of the form N7:0. A data transfer from a register to an output might have a source address of the form N7:0 and a destination address of the form O:030. Data Comparison The data comparison instruction gets the PLC to compare two data values. Thus it might be to compare a digital value read from some input device with a second value contained in a register. For example, we might want some action to be initiated when the input from a temperature sensor gives a digital value that is less than a set value stored in a data register in the PLC. PLCs generally can make comparisons for less than (< or LT or LES), equalto (¼ or ¼ ¼ or EQ or EQU), less than or equal to ( or or GT or GRT), greater than or equal to (! or >¼ or GE or GEQ), and not equal to (6¼ or or NE or NEQ). The parentheses alongside each of the terms indicates common abbreviations used in programming. As an illustration, in structured text we might have: (*Check that boiler pressure P2 is less than pressure P1*)Output :¼ P2 < P1; With ladder programs, for data comparison the typical instruction will contain the data transfer instruction to compare data, the source (S) address from which the data is to be obtained for the comparison, and the destination (D) address of the data against which it is to be compared. The instructions commonly used for the comparison are the terms indicated in the preceding parentheses. Figure shows the type of formats used by three manufacturers using the greater-than form of comparison. Similar forms apply to the other forms of comparison. In Figure the format is that used by Mitsubishi, S indicating the source of the data value for the comparison and D the destination or value against which the comparison is to be made. Thus if the source value is greater than the destination value, the output is 1. In Figure the Allen-Bradley format has been used. Here the source of the data being compared is given as the accumulated value in timer 4.0 and the data against which it is being compared is the number 400. Figure shows the Siemens format. The values to be compared are at inputs IN1 and IN2 and the result of the comparison is at the output: 1 if the comparison is successful, otherwise 0. The R is used to indicate real numbers that is, floating point numbers, I being

25 used for integers, that is, fixed-point numbers involving 16 bits, and D for fixed-point numbers involving 32 bits. Both the inputs need to be of the same data type, such as REAL. As an illustration of the use of such a comparison, consider the task of sounding an alarm if a sensor indicates that a temperature has risen above some value, say, 100oC. The alarm is to remain sounding until the temperature falls below 90oC. Figure shows the ladder diagram that might be used. When the temperature rises to become equal to or greater than 100oC, the greater-than comparison element gives a 1 output and so sets an internal relay. There is then an output. This output latches the greater-than comparison element, so the output remains on, even when the temperature falls below 100oC. The output is not switched off until the less-than 90oC element gives an output and resets the internal relay. Another example of the use of comparison is when, say, four outputs need to be started in sequence, that is, output 1 starts when the initial switch is closed, followed sometime later by output 2, sometime later by output 3, and sometime later by output 4. Though this could be done using three timers, another possibility is to use one timer with greaterthan or equal elements. Figure 12.6 shows a possible ladder diagram. When the X401 contacts close, the output Y430 starts. The timer is also started. When the timer-accumulated value reaches 5 s,the greater-than or equal-to element switches on Y431. When the timer-accumulated value reaches 15 s, the greater-than or equal-to element switches on Y432. When the timer reaches 25 s, its contacts switch on Y433. Data Selection There are a number of selection function blocks available with PLCs. Figure shows the standard IEC symbols.

26 10. Explain how the shift register can be used to sequence the event with a neat diagram. (NOV/DEC 2010) A register is a number of internal relays grouped together, normally 8, 16, or 32. Each internal relay is either effectively open or closed, these states being designated 0 and 1. The term bit is used for each such binary digit. Therefore, if we have eight internal relays in the register, we can store eight 0/1 states. Thus we might have, for internal relays: and each relay might store an on/off signal such that the state of the register at some instant is: that is, relay 1 is on, relay 2 is off, relay 3 is on, relay 4 is on, relay 5 is off, and so on. Such an arrangement is termed an 8-bit register. Registers can be used for storing data that originate from input sources other than just simple, single on/off devices such as switches. With the shift register it is possible to shift stored bits. Shift registers require three inputs: One to load data into the first location of the register, one as the command to shift data along by one location, and one to reset or clear the register of data. To illustrate this idea, consider the following situation where we start with an 8-bit register in the following state: Suppose we now receive the input signal 0. This is an input signal to the first internal relay. Input 0! If we also receive the shift signal, the input signal enters the first location in the register, and all the bits shift along one location. The last bit overflows and is lost. Overflow ! Thus a set of internal relays that were initially on, off, on, on, off, off, on, off are now off, on, off, on, on, off, off, on. The grouping together of internal relays to form a shift register is done automatically by a PLC when the shift register function is selected. With the Mitsubishi PLC, this is done using the programming code SFT (shift) against the internal relay number that is to be the first in the register array. This then causes a block of relays, starting from that initial number, to be reserved for the shift register. Consider a 4-bit shift register and how it can be represented in a ladder program. The input In 3 is used to reset the shift register, that is, put all the values at 0.The input In 1 is used to input to the first internal relay in the register. The input In 2 is used to shift the states of the internal relays along by one. Each of the internal relays in the register, that is, IR 1, IR 2, IR 3, and IR 4, is connected to an output, these being Out 1, Out 2, Out 3, and Out 4. Suppose we start by supplying a momentary input to In 3. All the internal relays are then set to 0 and so the states of the four internal relays IR 1, IR 2, IR 3, and IR 4 are 0, 0, 0, 0.When In 1 is momentarily closed, there is a 1 input into the first relay. Thus the states of the internal relays IR 1, IR 2, IR 3, and IR 4 are now 1, 0, 0, 0. The IR 1 contacts close and we thus end up with an output from Out 1. If we now supply a momentary input to In 2, the 1 is shifted from the first relay to the second. The states of the internal relays are now 0, 1, 0, 0. We now have no input from Out 1 but an output from Out 2. If we supply another momentary input to In 2, we shift the states of the relays along by one location to give 0, 0, 1, 0. Outputs Out 1 and Out 2 are now off, but Out 3 is on. If we supply another momentary input to In 2, we again shift the states of the relays along by one and have 0, 0, 0, 1. Thus now Out 1, Out 2, and Out 3 are off and Out 4 has been switched on. When another momentary input is applied to In 2, we shift the states of the relays along by one and have 0, 0, 0, 0, with the 1 overflowing and being lost. All the outputs are then off. Thus the effect of the sequence of inputs to In 2 has been to give a sequence of outputs Out 1, followed by Out 2, followed by Out 3, followed by Out 4. Figure shows the sequence of signals.

27 11. Explain latching with ladder diagram. (NOV/DEC 2014) Latching There are often situations in which it is necessary to hold an output energized, even when the input ceases. A simple example of such a situation is a motor that is started by pressing a push-button switch. Though the switch contacts do not remain closed, the motor is required to continue running until a stop push-button switch is pressed. The term latch circuit is used for the circuit that carries out such an operation. It is a self-maintaining circuit in that, after being energized, it maintains that state until another input is received. An example of a latch circuit is shown in Figure. When the input A contacts close, there is an output. However, when there is an output, another set of contacts associated with the output closes. These contacts form an OR logic gate system with the input contacts. Thus, even if input A opens, the circuit will still maintain the output energized. The only way to release the output is by operating the normally closed contact B. As an illustration of the application of a latching circuit, consider a motor controlled by stop and start push-button switches and for which one signal light must be illuminated when the power is applied to the motor and another when it is not applied. Figure shows a ladder diagram with Mitsubishi notation for the addresses. X401 is closed when the program is started. When X400 is momentarily closed, Y430 is energized and its contacts close. This results in latching as well as the switching off of Y431 and the switching on of Y432. To switch the motor off, X401 is pressed and opens. Y430 contacts open in the top rung and third rung but close in the second rung. Thus Y431 comes on and Y432 goes off. Latching is widely used with startups so that the initial switching on of an application becomes latched on.

28 Multiple Outputs With ladder diagrams, there can be more than one output connected to a contact. Figure shows a ladder program with two output coils. When the input contacts close, both the coils give outputs. For the ladder rung shown in Figure, output A occurs when input A occurs. Output B occurs only when both input A and input B occur.

29 Such an arrangement enables a sequence of outputs to be produced, the sequence being in the sequence in which contacts are closed. Figure illustrates this idea with the same ladder program in Mitsubishi and Siemens notations. Outputs A, B, and C are switched on as the contacts in the sequence given by the contacts A, B, and C are being closed. Until input A is closed, none of the other outputs can be switched on. When input A is closed, output A is switched on. Then, when input B is closed, output B is switched on. Finally, when input C is closed, output C is switched on. 12. Explain about Mnemonics with examples.(8) (NOV/DEC 2015) 1) In general, a mnemonic (from Greek mnemon or mindful; pronounced neh-mahn-ik ) is a memory aid, such as an abbreviation, rhyme or mental image that helps to remember something. The technique of developing these remembering devices is called "mnemonics." Mnemonics can be used to remember phone numbers, all your new department colleagues' names or the years of the reigns of the Kings and Queens of England. A number of approaches are used. Here's a mnemonic device for remembering a list of unrelated items in order: Start at the top of the list and make up an outlandish story connecting the first item to the next, continue by connecting the second item to the third, and so on. When your story is done and the list is removed, you'll have a mental picture of a story that, as you recall its progression, will lead you from one remembered item to the next. 2) In computer assembler (or assembly) language, a mnemonic is an abbreviation for an operation. It's entered in the operation code field of each assembler program instruction. For example, on an Intel microprocessor, inc ("increase by one") is a mnemonic. On an IBM System/370 series computer, BAL is a mnemonic for "branch-and-link."

30 13. Discuss on input/output Processing.(6) (MAY/JUNE 2016) Input signals from sensors and outputs required for actuating devices can be: Analog. A signal for which the size is related to the size of the quantity being sensed.discrete. Essentially just an on/off signal.digital. A sequence of pulses. The CPU, however, must have an input of digital signals of a particular size, normally 0 to5 V. The output from the CPU is digital, normally 0 to 5 V. Thus there is generally a need to manipulate input and output signals so that they are in the required form. The input/output (I/O) units of PLCs are designed so that a range of input signals can be changed into 5 V digital signals and so that a range of outputs are available to drive external devices. It is this built-in facility to enable a range of inputs and outputs to be handled that makes PLCs so easy to use. The following is a brief indication of the basic circuits used for input and output units. In the case of rack instruments, they are mounted on cards that can be plugged into the racks, and so the input/output characteristics of the PLC can thus be changed by changing the cards. A single box form of PLC has input/output units incorporatedby the manufacturer. Input Units The terms sourcing and sinking refer to the manner in which DC devices are interfaced with the PLC. For a PLC input unit with sourcing, it is the source of the current supply for the input device connected to it. With sinking, the input device provides the current to the input unit.figures show the basic input unit circuits for DC and AC inputs. Optoisolatorsare used to provide protection. With the AC input unit, a rectifier bridge network is used to rectify the AC so that the resulting DC signal can provide the signal for use by the optoisolator to give the input signals to the CPU of the PLC. Individual statuslights are provided for each input to indicate when the input device is providing a signal. When analog signals are inputted to a PLC, the input channel needs to convert the signal to adigital signal using an analog-to-digital converter. With a rack-mounted system this may beachieved by mounting a suitable analog input card in the rack. So that one analog card is not required for each analog input, multiplexing is generally used. This involvesmore than one analog input being connected to the card and then electronic switches used to select each input in turn. Cards are typically available containing 4, 8, or 16 analog inputs.

31 Figure illustrates the function of an analog-to-digital converter (ADC). A single analog input signal gives rise to on/off output signals along perhaps eight separate wires. The eight signals then constitute the so-termed digital word corresponding to the analog input signal level. With such an 8-bit converter there are 28 ¼ 256 different digital values possible; these are to , that is, 0 to 255. The digital output goes up in steps and the analog voltages required to produce each digital output are termed quantization levels. If the binary output is to change, the analog voltage has to change by the difference in analog voltage between successive levels. The term resolution is used for the smallest change in analog voltage that will give rise to a change in 1 bit in the digital output. With an 8-bit ADC,if, say, the full-scale analog input signal varies between 0 and 10 V, a step of one digital bit involves an analog input change of 10/255 V or about 0.04 V. This means that a change of 0.03 V in the input will produce no change in the digital output. The number of bits in the output from an analog-todigital converter thus determines the resolution, and hence accuracy, that is possible. If a 10-bit ADC is used, then 210 ¼ 1024 different digital values are possible and, for the full-scale analog input of 0 to 10 V, a step of one digital bit involves an analog input change of 10/1023 V, or about 0.01 V. If a 12-bit ADC is used,then 212 ¼ 4096 different digital values are possible and, for the full-scale analog input of0 to 10 V, a step of one digital bit involves an analog

32 input change of 10/4095 V, or about 2.4 mv. In general, the resolution of an n-bit ADC is 1/(2n 1); this is some times approximated to 2 n. The following illustrates the analog-to-digital conversion for an 8-bit converter when the analog input is in the range 0 to 10 V: To illustrate this idea, consider a thermocouple used as a sensor with a PLC and giving an output of 0.5 mv per C. What will be the accuracy with which the PLC will activate the output device if the thermocouple is connected to an analog input with a range of 0 to 10 VDC and using a 10-bit analog-to-digital converter? With a 10-bit converter, there are 210 ¼1024 bits covering the 0 to 10 V range. Thus a change of 1 bit corresponds to 10/1023 V or about 0.01 V, that is, 10 mv. Hence the accuracy with which the PLC recognizes the input from the thermocouple is Æ5 mv or Æ10 C. Conversion from analog to digital takes time and, in addition, the use of a multiplexer means that an analog input card of a PLC only takes snapshot samples of input signals. For most industrial systems, signals from a plant rarely vary so fast that this presents a problem.conversion times are typically a few milliseconds. Output Units With a PLC output unit, when it provides the current for the output device it is said to be sourcing, and when the output device provides the current to the output unit, it is said to be sinking. Quite often, sinking input units are used for inter facing with electronic equipment and sourcing output units for interfacing with solenoids.

33 Output units can be relay, transistor, or triac. Figure shows the basic form of a relayout put unit, Figure that of a transistor output unit, and Figure that of a triacoutput unit. Analog outputs are frequently required and can be provided by digital-to-analog converters(dacs) at the output channel. The input to the converter is a sequence of bits with each bit along a parallel line. Figure shows the basic function of the converter.

34 A bit in the 0 line gives rise to a certain size output pulse. A bit in the 1 line gives rise to an output pulse of twice the size of the 0 line pulse. A bit in the 2 line gives rise to an output pulse of twice the size of the 1 line pulse. A bit in the 3 line gives rise to an output pulse of twice the size of the 2 line pulse, and so on. All the outputs add together to give the analog version of the digital input. When the digital input changes, the analog output changes in a stepped manner, the voltage changing by the voltage changes associated with each bit. For example, if we have an 8-bit converter, the output is made up of voltage values of 28 ¼ 256 analog steps. Suppose the output range is set to 10 V DC. One bit then gives a change of 10/255 V or about 0.04 V. Thus we have:

35 Analog output modules are usually provided in a number of outputs, such as 4 to 20 ma, 0 toþ5 V DC, and 0 to þ10 V DC, and the appropriate output is selected by switches on the module. Modules generally have outputs in two forms, one for which all the outputs from that module have a common voltage supply and one that drives outputs with their own individual voltage supplies. Figure shows the basic principles of these two forms of output. 14. Discuss in detail about cylinder sequencing with PLC and its programming.(10) (MAY/JUNE 2016) Pneumatic circuits are built of these basic elements to create the logic of a required set of motions. The two positions of the piston in a cylinder are usually shown by +/- signs. The sequence of desired positions of cylinders is shown as a list of symbols as in figure 5.4, which shows a simple pneumatic circuit.

36 The Cascade method The cascade method is a simple procedure to create pneumatic circuits involving many cylinders to generate a sequence of actions. We illustrate the method by the use of a simple example. Example: A small punch press is operated as follows. The part is clamped in position; the press punches the part; the clamp is released; and the part is removed from the table. The operations are achieved using three pneumatic cylinders, A, B, C. The operation sequence can be described as: START, A+, B+, B-, A-, C+, C- What do cylinders A, B, and C do? We describe the cascade method first, and then build the circuit for the example. (1) The cylinder action sequence is listed. (2) The sequence is partitioned into groups, such that no letter is repeated in any group. The aim is to minimize the number of groups. (3) If the last group has no letters in common with the first, it can be merged into the first group. (4) Each cylinder is double-acting. (5) Each cylinder is controlled by a 5/2 valve, actuated on both ends pneumatically (pneumatic valve actuation lines are also called pilot lines). (6) Each cylinder is associated with two limit valves, one each at the + and - positions. (7) Each group is assigned a manifold line. A manifold line is simply a tube with multiple outlets. When the group is active, the manifold line associated with it is pressurized. At all other times, it is open to the atmosphere. The manifold line connects to the limit valves associated with the cylinders. This ensures that the pilot valves (the 5/2 valves) never get contradictory signals (i.e. both the pilot lines of the valve are never at the same pressure.) (8) The air pressure in the manifolds is controlled by 5/2 valves called group valves. The total number of group valves is one less than the total number of groups. We now return to our example, where we need the sequence: START, A+, B+, B-, A-, C+, C- Break it down into groups: START, A+, B+ / B-, A-, C+ / C- GRP 1 GRP 2 GRP 3 Since GRP 3 has no letter in common with GRP 1, we can include it in GRP 1: START, A+, B+ / B-, A-, C+ / C- GRP 1 GRP 2 GRP 1 Locate the three cylinders, and draw all the required valves: Two limit valves (3/2) for each, one 5/2 actuating valve for each, one start valve (3/2), and since there are 2 groups, 2 manifold lines, and (2-1 = 1) group valve. Note the following: Limit valves a2, b2 and c1 get their air supply from manifold 1, while a1, b1 and c2 get their air supply from manifold 2 (according to step 7 above). Pilot line 1 of the group valve will be activated by c2, after cylinder action C+, which marks the transition

37 from GRP 2 to GRP 1. Pilot line 2 of the group valve is activated by b2, after cylinder action B+ Why? Cylinder X is supplied actuation power via valve VX ( X = A, B, C). Note the configuration of the discharge lines in each of these valves. Pilot line VA- is actuated via b1 (after B-). Why? The various connections are now established to create the required logic. The final circuit is shown in figure 5.5 on the following page.