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OR and OR NOT When two or more conditions lie on separate instruction lines which run in paral-
lel and then join together, the first condition corresponds to a LOAD or LOAD
NOT instruction; the other conditions correspond to OR or OR NOT instructions.
The following example shows three conditions which correspond (in order from
the top) to a LOAD NOT, an OR NOT, and an OR instruction. Again, each of
these instructions requires one line of mnemonic code.
Instruction
00100
LR 0000
00000
Address Instruction Operands
00000 LD 00000
00001 OR NOT 00100
00002 OR LR 0000
00003 Instruction
The instruction will have an ON execution condition when any one of the three
conditions is ON, i.e., when IR 00000 is OFF, when IR 00100 is OFF, or when LR
0000 is ON.
OR and OR NOT instructions can be considered individually, each taking the
logical OR between its execution condition and the status of the OR instruction’s
operand bit. If either one of these were ON, an ON execution condition will be
produced for the next instruction.
When AND and OR instructions are combined in more complicated diagrams,
they can sometimes be considered individually, with each instruction performing
a logic operation on the execution condition and the status of the operand bit.
The following is one example. Study this example until you are convinced that
the mnemonic code follows the same logic flow as the ladder diagram.
Instruction
00002 0000300000 00001
00200
Address Instruction Operands
00000 LD 00000
00001 AND 00001
00002 OR 00200
00003 AND 00002
00004 AND NOT 00003
00005 Instruction
Here, an AND is taken between the status of IR 00000 and that of IR 00001 to
determine the execution condition for an OR with the status of IR 00200. The
result of this operation determines the execution condition for an AND with the
status of IR 00002, which in turn determines the execution condition for an AND
with the inverse (i.e., and AND NOT) of the status of IR 00003.
In more complicated diagrams, however, it is necessary to consider logic blocks
before an execution condition can be determined for the final instruction, and
that’s where AND LOAD and OR LOAD instructions are used. Before we consid-
er more complicated diagrams, however, we’ll look at the instructions required to
complete a simple “input-output” program.
Combining AND and OR
Instructions
Basic Ladder Diagrams Section 4-4