32
Programming Concepts Section 1-1
Mnemonic Execution Order
PLCs execute ladder programs in the order the mnemonics are entered so
instructions may not operate as expected, depending on the way rungs are
written. Always consider mnemonic execution order when writing ladder dia-
grams.
Example: CIO 102.10 in the above diagram cannot be output. By rewriting the
rung, as shown above, CIO 102.10 can be turned ON for one cycle.
Rungs Requiring Rewriting
PLCs execute instructions in the order the mnemonics are entered so the sig-
nal flow (power flow) is from left to right in the ladder diagram. Power flows
from right to left cannot be programmed.
Example: The program can be written as shown in the diagram at the left
where TR0 receives the branch. The same value is obtained, however, by the
rungs at the right, which are easier to understand. It is recommended, there-
fore, that the rungs at the left be rewritten to the rungs at the right.
Rewrite the rungs on the left below. They cannot be executed.
The arrows show signal flow (power flow) when the rungs consist of control
relays.
0.00 110.00
0.00
0.00
102.10
102.10
110.00
110.00
110.00
0.00 0.03 102.11
0.01 102.11
TR0
102.12
0.01 0.02
102.120.04
0.02 0.03
0.00
0.040.01
A
C
B
D
R1
R2
E
A
C
B
D
E
A
C
E
R1
R2
To Next Page
Loading...
