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: rungs 1, 3, 4, 6, and so on.
The end of the M100 controlled section is indicated by the occurrence of the other MCR,M101. With M101 switched on but M100 off, the sequence is: rungs 2, 4, 5, 6, and so on. The end of this section is indicated by the presence of the reset. This reset has to be used since the rung is not followed immediately by another MCR. Such an arrangement could be used to switch on one set of ladder rungs if one type of input occurs and another set of ladder rungs if a different input occurs.
7.6.1 Examples of Programs
The following looks at a program that illustrates the uses of MCRs. The program is being developed for use with a pneumatic valve system involving the movement of pistons in cylinders to give a particular sequence of piston actions. First, however, we show how latching might be used with such systems to maintain actions.
Consider a pneumatic system with single-solenoid controlled valves and involving two cylinders A and B with limit switches a–, at, b–, bt detecting the limits of the piston rod movements (Figure 7.27), with the requirement to give the sequence At, Bt, A–, B–.Figure 7.28 shows the ladder diagram that can be used.The solenoid At is energized when the start switch and limit switch b– are closed. This provides latching to keep At energized as long as the normally closed contacts for limit switch bt are not activated. When limit switch at is activated, solenoid Bt is energized.This provides latching that keeps Bt energized as long as the normally closed contacts for limit switch a– are not activated. When cylinder B extends, the limit switch bt opens its normally closed contacts and unlatches the solenoid At. Solenoid A thus retracts. When it has retracted and opened the normally closed contacts a–, solenoid Bt becomes unlatched and cylinder B retracts.
Now consider the ladder program that could be used with the pair of single-solenoid-controlled cylinders in Figure 7.27 to give, when and only when the start switch is momentarily triggered, the sequence At, Bt, A–, then a 10 s time delay, B–, and stop at that point until the start switch is triggered again. Figure 7.29 shows how such a program can be devised using a MCR. The MCR is activated by the start switch and remains on until switched off by the rung containing just MCR. (See Chapter 9 for a discussion of timers.)
Summary
In PLCs, elements that are used to hold data, that is, bits, and behave like relays and so are able to switch on or off other devices are termed internal relays (or alternately auxiliary relays, markers, flags, or bit storage elements). With 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. The internal relay switching contacts are designated with the symbol for an input device, namely j j, and given the same address as the internal relay output. Internal relays that are battery-backed are able to retain their setting, even when the power is removed. The relay is said to be retentive.
One 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. This function is termed one-shot. Another function that is often available is the ability to set and reset an internal relay, for which the term flip-flop is used.
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