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Examples of the equivalent ladder logic and IL pro grams are shown in Figure 18.4 and Figure 18.5. The programs in Figure 18.4 show differ ent variations when there is only a single output. Multiple IL programs are given where available. When looking at these examples recall the stack concept. When a LD or LDN instruction is encountered it will put a value on the top of the stack. The ANB and ORB instructions will remove the top two values from the stack, and replace them with a single value that is the result of an Boolean operation. The AND and OR functions take one value off the top of the stack, perform a Boolean operation and put the result on the top of the stack. The equivalent programs (to the right) are shorter and will run faster.

plc il -18.5

Ladder

Instruction List (IL)

LD A ST X

LDN A

X ST X

LD A

X LD B ANB ST X

LD A

X LDN B ANB ST X

LD A

X LD B ORB LD C ANB ST X