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4-3-3 Ladder Instructions
The ladder instructions are those that correspond to the conditions on the
ladder diagram. Ladder instructions, either independently or in combination
with the logic block instructions described next, form the execution conditions
upon which all other instructions are executed.
The first condition that starts any logic block within a ladder diagram corre-
sponds to a LOAD or LOAD NOT instruction.
0000
0000
A LOAD instruction.
A LOAD NOT instruction.
Address Instruction Operands
0000 LD 0000
0001 Instruction
0002 LD NOT 0000
0003 Instruction
When this is the only condition on the instruction line, the execution condition
for the instruction at the right is ON when the condition is ON. For the LOAD
instruction (i.e., a normally open condition), the execution condition would be
ON when IR 0000 was ON; for the LOAD NOT instruction (i.e., an normally
closed condition), it would be ON when IR 0000 was OFF.
When two or more conditions lie in series on the same instruction line, the
first one corresponds to a LOAD or LOAD NOT instruction; the rest of the
conditions, to AND or AND NOT instructions. The following example shows
three conditions which correspond in order from the left to a LOAD, an AND
NOT, and an AND instruction.
0000 0100 HR 000
Instruction
Address Instruction Operands
0000 LD 0000
0001 AND NOT 0100
0002 AND HR 000
0003 Instruction
The instruction at the right would have an ON execution condition only when
all three conditions are ON, i.e., when IR 0000 was ON, IR 0100 was OFF,
and HR 000 was ON.
Actually, AND instructions can be considered individually in series, each of
which would take the logical AND between the execution condition (i.e., the
sum of all conditions up to that point) and the status of the AND instruction’s
operand bit. If both of these were ON, an ON execution condition would be
produced for the next instruction. The execution condition for the first AND
instruction in a series would be the first condition on the instruction line.
Each AND NOT instruction in a series would take the logical AND between
its execution condition and the inverse of its operand bit.
LOAD and LOAD NOT
AND and AND NOT
The Ladder Diagram Section 4-3