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Now you have all of the instructions required to write simple input-output pro-
grams. Before we finish with ladder diagram basic and go onto inputting the pro-
gram into the PC, let’s look at logic block instruction (AND LOAD and OR LOAD),
which are sometimes necessary even with simple diagrams.
4-4-6 Logic Block Instructions
Logic block instructions do not correspond to specific conditions on the ladder
diagram; rather, they describe relationships between logic blocks. The AND
LOAD instruction logically ANDs the execution conditions produced by two logic
blocks. The OR LOAD instruction logically ORs the execution conditions pro-
duced by two logic blocks.
AND LOAD Although simple in appearance, the diagram below requires an AND LOAD
instruction.
Address Instruction Operands
00000 LD 00000
00001 OR 00001
00002 LD 00002
00003 OR NOT 00003
00004 AND LD ---
00005 Instruction
Instruction
00002
00003
00000
00001
The two logic blocks are indicated by dotted lines. Studying this example shows
that an ON execution condition will be produced when: either of the conditions in
the left logic block is ON (i.e., when either IR 00000 or IR 00001 is ON), and
when either of the conditions in the right logic block is ON (i.e., when either IR
00002 is ON or IR 00003 is OFF).
The above ladder diagram cannot, however, be converted to mnemonic code
using AND and OR instructions alone. If an AND between IR 00002 and the re-
sults of an OR between IR 00000 and IR 00001 is attempted, the OR NOT be-
tween IR 00002 and IR 00003 is lost and the OR NOT ends up being an OR NOT
between just IR 00003 and the result of an AND between IR 00002 and the first
OR. What we need is a way to do the OR (NOT)’s independently and then com-
bine the results.
To do this, we can use the LOAD or LOAD NOT instruction in the middle of an
instruction line. When LOAD or LOAD NOT is executed in this way, the current
execution condition is saved in a special buffer and the logic process is re-
started. To combine the results of the current execution condition with that of a
previous “unused” execution condition, an AND LOAD or an OR LOAD instruc-
tion is used. Here “LOAD” refers to loading the last unused execution condition.
An unused execution condition is produced by using the LOAD or LOAD NOT
instruction for any but the first condition on an instruction line.
Basic Ladder Diagrams Section 4-4