106 Programmable Logic Controllers: Hardware and Programming
Rung comments:
Comments that describe
the operation of the rung
or rung that follow.
Rule 3. All contacts must be placed horizontally. No vertical con-
tacts are allowed. Compare the incorrect diagram in Figure 6-4 with
the correct diagram in Figure 6-5.
Rule 4. Outputs cannot be connected in series with other outputs.
They can only be connected in parallel. See the incorrect ladder dia-
gram in Figure 6-6.
Rule 5. Program execution flow must be from left to right. (See
Figure 6-7).
Rule 6. Place the rung numbers on the left side of each rung.
The instruction address and device descriptions are placed above
the instruction. Rung comments are placed on the top of each rung
and typically describe the major function of the following rungs. You
can also specify the location of each contact associated with a coil by
placing next to the coil the number of the rung the associated contact
is on. For example in Figure 6-8, the numbers 1, 2, and 3 are placed
next to the coil for motor #1 to indicate that rungs 1 and 3 contain the
related normally open contacts and rung 2 contains the related nor-
mally closed contacts.
I:0/0 I:0/1 I:0/2
L1 L2
O:0/0
I:0/0 I:0/3 I:0/5
I:0/4
O:0/1
I:0/0 I:0/1 I:0/2
L1 L2
O:0/0
I:0/0 I:0/3
I:0/5 I:0/4 O:0/1
Figure 6-4. Incorrect ladder diagram. All contacts must be placed horizontally.
Figure 6-5. Corrected version of diagram from Figure 6-4.