Ladder logic

Discrete logic. Sequence logic can be implemented via ladder logic, and this approach is common when sequence logic is implemented in programmable logic controllers (PLCs). 

Modern RTUs typically use a ladder-logic approach to programming due to its similarity to standard electrical circuits. An RTU that employs this ladder-logic programming is called a programmable logic controller (PLC). 

Figure 7.109 shows a simple ladder-logic diagram 601. This represents the logic statement (or pseudocode) ... 

One such construct is for a "one-shot. Some PLCs provide this as a built-in function, but here it will be presented in terms of separate components. The one-shot is generated by the third rung of ladder logic in Fig. 8-60a. But first examine the fourth rung. The input LL drives the output coil LL1. This coil provides the state of level switch LL on the previous scan of ladder logic. This is used in the third rung to produce the one-shot. Output coil OneShot is energized if... 

LL is not actuated on this scan of ladder logic (note the use of the normally closed contact for LL)... 

When LL deactuates, coil OneShot is energized for one scan of ladder logic. OneShot does not energize when LL actuates (a slight modification of the circuit would give a one-shot when LL actuates). 

The one-shot is used in the fifth rung of ladder logic to toggle the lead pump. The output coil LeadlsB is energized provided that... 

FIG. 8-60 (a) Ladder logic, (b) Sequence logic for effluent tank sump pumps. 

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