7.5 Set and Reset

Another 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 Y430 Other program rungs are:

LD X401 R Y430

With a Telemecanique PLC, the ladder diagram would be as shown in Figure 7.14 and the

programming 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 1?4 Q2.0

Toshiba 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