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This chapter continues from the previous chapters on programming and introduces internal relays. A variety of other terms are often used to describe these elements, such as auxiliary relays, markers, flags, coils, and bit storage. These are one of the elements included among the special built-in functions with PLCs and are very widely used in programming. A small PLC might have a hundred or more internal relays, some of them battery backed so that they can be used in situations where it is necessary to ensure safe shutdown of a plant in the event of power failure. Later chapters consider other common built-in elements.7.1 Internal RelaysIn 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 real-world 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 (Figure 7.1)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 relays can 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. 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 so on. Allen-Bradley uses the term bit storage and notation in the PLC-5 of the form B3/001,B3/002, and so on.7.2 Ladder ProgramsWith ladder programs, an internal relay output is represented using the symbol for an output device, namely , with an address that indicates that it is an internal relay. Thus, with a Mitsubishi PLC, we might have the address M100, the M indicating that it is an internal relay or marker rather than an external device. The internal relay switching contacts are designated with the symbol for an input device, namely , and given the same address as the internal relay output, such as M100.7.2.1 Programs with Multiple Input ConditionsAs an illustration of the use that can be made of internal relays, consider the following situation. A system is to be activated when two different sets of input conditions are realized.We might just program this as an AND logic gate system; however, if a number of inputs have to be checked in order that each of the input conditions can be realized, it may be simpler to use an internal relay. The first input conditions then are used to give an output to an internal relay. This relay has associated contacts that then become part of the input conditions with the second inputFigure 7.2 shows a ladder program for such a task. For the first rung, when input In 1 or input In 3 is closed and input In 2 closed, internal relay IR 1 is activated. This results in the contacts for IR 1 closing. If input In 4 is then activated, there is an output from output Out 1. Such a task might be involved in the automatic lifting of a barrier when someone approaches from either side. Input In 1 and input In 3 are inputs from photoelectric sensors that detect the presence of a person approaching or leaving from either side of the barrier, input In 1 being activated from one side of it and input In 3 from the other. Input In 2 is an enabling switch to enable the system to be closed down. Thus when input In 1 or input In 3, and input In 2, are activated, there is an output from internal relay 1. This will close the internal relay contacts. If input In 4, perhaps a limit switch,detects that the barrier is closed, then it is activated and closes. The result is then an output fromOut 1, a motor that lifts the barrier. If the limit switch detects that the barrier is already open, the person having passed through it, then it opens and so output Out 1 is no longer energized and a counterweight might then close the barrier. The internal relay has enabled two parts of the program to be linked, one part being the detection of the presence of a person and the second part the detection of whether the barrier is already up or down. Figure 7.3a shows how Figure 7.2 would appear in Mitsubishi notation and Figure 7.3b shows how it would appear in Siemens notation.Figure 7.4 is another example of a ladder program involving internal relays. Output 1 is controlled by two input arrangements. The first rung shows the internal relay IR 1, which is energized if input In 1 or In 2 is activated and closed. The second rung shows internal relay IR 2, which is energized if inputs In 3 and In 4 are both energized. The third rung shows that output Out 1 is energized if internal relay IR 1 or IR 2 is activated. Thus there is an output from the system if either of two sets of input conditions is realized.7.2.2 Latching ProgramsAnother use of internal relays is for resetting a latch circuit. Figure 7.5 shows an example of such a ladder program. When the input In 1 contacts are momentarily closed, there is an output at Out 1. This closes the contacts for Out 1 and so maintains the output, even when input In 1 opens. When input In 2 is closed, the internal relay IR 1 is energized and so opens the IR 1 contacts, which are normally closed. Thus the output Out 1 is switched off and so the output is unlatched.Consider a situation requiring latch circuits where there is an automatic machine that can be started or stopped using push-button switches. A latch circuit is used to start and stop the power being applied to the machine. The machine has several outputs that can be turned on if the power has been turned on and are off if the power is off. It would be possible to devise a ladder diagram that has individually latched controls for each such output. However, a simpler method is to use an internal relay. Figure 7.6 shows such a ladder diagram. The first rung has the latch for keeping the internal relay IR 1 on when the start switch gives a momentary input. The second rung will then switch the power on. The third rung will also switch on and give output Out 2 if the input 2 contacts are closed. The third rung will also switch on and give output Out 3 if the input 3 contacts are closed. Thus all the outputs can be switched on when the start push button is activated. All the outputs will be switched off if the stop switch is opened. Thus all the outputs are latched by IR 1.7.2.3 Response TimeThe time taken between an input occurring and an output changing depends on such factors as the electrical response time of the input circuit, the mechanical response of the output device, and the scan time of the program. A ladder program is read from left to right and from top to bottom. Thus if an output device, such as an internal relay, is set in one scan cycle and the output has to be fed back to earlier in the program, it will require a second scan of the program before it can be activated. Figure 7.7 illustrates this concept.7.3 Battery-Backed RelaysIf the power supply is cut off from a PLC while it is being used, all the output relays and internal relays will be turned off. Thus when the power is restored, all the contacts associated with those relays will be set differently from when the power was on. Therefore, if the PLC was in the middle of some sequence of control actions, it would resume at a different point in the sequence. To overcome this problem, some internal relays have battery backup so that they can be used in circuits to ensure a safe shutdown of a plant in the event of a power failure and so enable it to restart in an appropriate manner. Such battery-backed relays retain their state of activation, even when the power supply is off. The relay is said to have been made retentive.The term retentive memory coil is frequently used for such elements. Figure 7.8a shows the IEC 1131-3 standard symbol for such elements. With Mitsubishi PLCs, battery-backed internal relay circuits use M300 to M377 as addresses for such relays. Other manufacturers use different addresses and methods of achieving retentive memory. The Allen-Bradley PLC-5 uses latch and unlatch rungs. If the relay is latched, it remains latched if power is lost and is unlatched when the unlatch relay is activated. (See Section 7.5 for a discussion of such relays in the context of set and reset coils.)As an example of the use of such a relay, Figure 7.8b shows a ladder diagram for a system designed to cope with a power failure. IR 1 is a battery-backed internal relay. When input In 1 contacts close, output IR 1 is energized. This closes the IR 1 contacts, latching so that IR 1 remains on even if input In 1 opens. The result is an output from Out 1. If there is a power failure, IR 1 still remains energized and so the IR 1 contacts remain closed and there is an output from Out 1.7.4 One-Shot OperationOne of the functions provided by some PLC manufacturers is the ability to program an internal relay so that its contacts are activated for just one cycle, that is, one scan through the ladder program. Hence when operated, the internal relay provides a fixed duration pulse at its contacts. This function is often termed one-shot. Though some PLCs have such a function as part of their programs, such a function can also easily be developed with just two rungs of a ladder program. Figure 7.9 shows such a pair of rungs.For Figure 7.9a, when the trigger input occurs, it gives a trigger output in rung 1. In rung 2 it gives a cycle control output on an internal relay. Because rung 2 occurs after rung 1, the effect of the cycle control is not felt until the next cycle of the PLC program, when it opens the cycle control contacts in rung 1 and stops the trigger output. The trigger output then remains off, despite there being a trigger input. The trigger output can only occur again when the trigger output is switched off and then switched on again.Figures 7.9b and 7.9c show the built-in facilities with Allen-Bradley and Mitsubishi PLCs.With the Mitsubishi PLC (Figure 7.9c), the output internal relaysay, M100is activated when the trigger inputsay, X400contacts close. Under normal circumstances, M100 would remain on for as long as the X400 contacts were closed. However, if M100 has been programmed for pulse operation, M100 only remains on for a fixed period of timeone program cycle. It then goes off, regardless of X400 being on. The programming instructions that would be used are LD X400, PLS M100. The preceding represents pulse operation when the input goes from off to on, that is, is positive-going. If, in Figure 7.9c, the trigger input is made normally closed rather than normally open, the pulse occurs when the input goes from on to offin other words, is negative-going.The IEC 1131-3 gives standards for the symbols for positive transition-sensing and negative transition-sensing coils (Figure 7.10).With the positive transition-sensing coil, if the power flow to it changes from off to on, the output is set on for one ladder rung evaluation. With the negative transition-sensing coil, if the power to it changes from off to on, the output is set on for one ladder rung evaluation. Thus, for the ladder rung of Figure 7.11, with the input off there is no output. When the input switches on, there is an output from the coil. However, the next and successive cycles of the program do not give outputs from the coil even though the switch remains on. The coil only gives an output the first time the switch is on.7.5 Set and ResetAnother function that is often available is the ability to set and reset an internal relay. The set instruction causes the relay to self-hold, that is, latch. It then remains in that condition until the reset instruction is received. The term flip-flop is often used. Figure 7.12 shows the IEC 1131-3 standards for such coils. The SET coil is switched on when power is supplied to it and remains set until it is RESET. The RESET coil is reset to the off state when power is supplied to it and remains off until it is SET.Figure 7.13 shows an example of a ladder diagram involving such a function. Activation of the first input, X400, causes the output Y430 to be turned on and set, that is, latched. Thus if the first input is turned off, the output remains on. Activation of the second input, X401,causes the output Y430 to be reset, that is, turned off and latched off. Thus the output Y430 is on for the time between X400 being momentarily switched on and X401 being momentarily switched on. Between the two rungs indicated for the set and reset operations, there could be other rungs for other activities to be carried out, with the set rung switching on an output at the beginning of the sequence and off at the end.The programming instructions for the ladder rungs in the program for Figure 7.13 are: LD X400 S Y430Other program rungs are: LD X401 R Y430With a Telemecanique PLC, the ladder diagram would be as shown in Figure 7.14 and theprogramming instructions would be: L I0,0 S O0,0 L I0,1 R O0,0 With an Allen-Bradley PLC, the terms latch and unlatch are used. Figure 7.15 shows the ladder diagram.The SET and RESET coil symbols are often combined in a single box symbol. Figure 7.16 shows the equivalent ladder diagram for the set-reset function in the preceding figures with a Siemens PLC. The term memory box is used by them for the SET/RESET box, and the box shown is termed an SR or reset priority memory function in that reset has priority.With set priority (RS memory box), the arrangement is as shown in Figure 7.17.The programming instructions (F indicates an internal relay) for reset priority are: A I0.0 S F0.0 A I0.1 R F0.0 A F0.0 14 Q2.0Toshiba uses the term flip-flop, and Figure 7.18 shows the ladder diagram.Figure 7.19 shows how the set-reset function can be used to build the pulse (one-shot)function described in the previous section. Figure 7.19a shows it for a Siemens PLC(F indicates internal relay) and Figure 7.19b for a Telemecanique PLC (B indicates internal relay). In Figures 7.19a and 7.19b, an input (I0.0, I0,0) causes the internal relay (B0, F0.0)in the first rung to be activated. This results in the second rung, in the set/reset internal relay being set. This setting action results in the internal relay (F0.1, B1) in the first rung opening,and so, despite there being an input in the first rung, the internal relay (BO, F0.0) opens.However, because the rungs are scanned in sequence from top to bottom, a full cycle mustelapse before the internal relay in the first rung opens. A pulse of duration one cycle has thus been produced. The system is reset when the input (I0.0, I0,0) ceases.7.5.1 Program ExamplesAn example of the basic elements of a simple program for use with a fire alarm system is shown in Figure 7.20. Fire sensors provide inputs to a SET/RESET function block so that if one of the sensors is activated, the alarm is set and remains set until it is cleared by being reset. When set it sets off the alarm.Another program showing the basic elements of a program is shown in Figure 7.21. This could be used with a system designed to detect when a workpiece has been loaded into the correct position for some further operation. When the start contacts are closed, the output causes the workpiece to move. This continues until a light beam is interrupted and resets,causing the output to cease. A stop button is available to stop the movement at any time. 7.6 Master Control RelayWhen large numbers of outputs have to be controlled, it is sometimes necessary for whole sections of ladder diagrams to be turned on or off when certain criteria are realized. This could be achieved by including the contacts of the same internal relay in each of the rungs so that its operation affects all of them. An alternative is to use a master control relay.Figure 7.22 illustrates the use of such a relay to control a section of a ladder program.With no input to input In 1, the output internal relay MC 1 is not energized, and so its contacts are open. This means that all the rungs between where it is designated to operate and the rung on which its reset MCR or another master control relay is located are switched off.Assuming that it is designated to operate from its own rung, we can imagine it to be located in the power line in the position shown, and so rungs 2 and 3 are off. When input In 1 contacts close, the master relay MC 1 is energized. When this happens, all the rungs between it and the rung with its reset MCR 1 are switched on. Thus outputs Out 1 and Out 2 cannot be switched on by inputs In 2 and In 3 until the master control relay has been switched on. The master control relay MC 1 acts only over the region between the rung it is designated to operate from and the rung on which MCR 1 is located.With a Mitsubishi PLC, an internal relay can be designated as a master control relay by programming it accordingly. Thus to program an internal relay M100 to act as a master control relay, the program instruction is: MC M100To program the resetting of that relay, the program instruction is: MCR M100Thus for the ladder diagram shown in Figure 7.23, which is Figure 7.22 with Mitsubishiaddresses, the program instructions are: LD X400 OUT M100 MC M100 LD X401 OUT Y430 LD X402 OUT Y431 MC M100 Figure 7.24 shows the format used by Allen-Bradley. To end the control of one master control relay (MCR), a second master control relay (MCR) is used with no contacts or logic preceding it. It is said to be programmed unconditionally.The representation used for MCRs in Siemens ladder programs is shown in Figure 7.25.An area in which an MCR is to operate is defined by the activate master control area and deactivate master control relay functions. Within that area, the MCR is enabled when the MCR coil is activated and disabled when the MCR coil is enabled.A program might use a number of MCRs, enabling various sections of a ladder program to be switched in or out. Figure 7.26 shows a ladder program in Mitsubishi format involving two MCRs. With M100 switched on but M101 off, the sequence is:
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