53
Basic Concepts Section 2-1
5. Rungs Requiring Caution or Rewriting
OR Instructions
With an OR/OR NOT instruction, an OR is taken with current execution condition, i.e., the results of
ladder logic up to the OR/OR NOT instruction.
In the example at the left, an OR LD instruction will be needed if the rungs are programmed as
shown without modification. A few steps can be eliminated by rewriting the rungs as shown.
Output Instruction Branches
A TR bit will be needed if there is a branch before an AND/AND NOT instruction. The TR bit will
not be needed if the branch comes a point that is connected directly to the first output instruction.
After the first output instruction, an AND/AND NOT instruction and the second output instruction
can be connected without modification.
In the example at the left, a temporary storage bit TR0 output instruction and load (LD) instruction
are needed at a branch point if the rungs are programmed without modification. A few steps can
be eliminated by rewriting the rungs. See the following pages for more information on TR bits.
In the example below, use TR0 to store the execution condition at the branch point or rewrite the
rungs.
0000
00
0000
01
0002
07
0002
07
0000
01
0000
00
0002
07
0002
07
0000
00
0000
01
0002
08
0000
00
0002
09
0002
09
TR0
0002
08
0000
01
0000
00
0000
03
0002
11
0000
01
0002
11
TR0
0002
12
0000
01
0000
02
0002
12
0000
04
0000
02
0000
03
0000
00
0000
04
0000
01
0000
00
0010
00
0000
00
0000
00
0002
10
0002
10
0010
00
0010
00
0010
00
Mnemonic Execution Order
CIO 000210 shown below will never turn ON because the PLC executes instructions in mnemonic
order. By rewriting the rung, CIO 000201 can be turned ON for one cycle.
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