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Basic Concepts
Basic Ladder Program Concepts
1,2,3...
46
1. When ladder diagrams are executed by PLCs, the signal flow (power flow)
is always from left to right. Programming that requires power flow from right
to left cannot be used. Thus, flow is different from when circuits are made
up of hard-wired control relays. For example, when the circuit "a" is imple-
mented in a PLC program, power flows as though the diodes in brackets
were inserted and coil R2 cannot be driven with contact D included. The
actual order of execution is indicated on the right with mnemonics. To
achieve operation without these imaginary diodes, the circuit must be re-
written. Also, circuit "b" power flow cannot be programmed directly and
must be rewritten.
Circuit "a"
(1)
A
Signal flow
(2) ((3))
(4)
C
D
((8))
(9)
E
Circuit "b"
A
B
E
C
D
In circuit "a," coil R2 cannot be driven with contact D included.
In circuit "b," contact E included cannot be written in a ladder diagram. The
program must be rewritten.
2. There is no limit to the number of I/O bits, work bits, timers, and other input
bits that can be used. Rungs, however, should be kept as clear and simple
as possible even if it means using more input bits to make them easier to
understand and maintain.
3. There is no limit to the number of input bits that can be connected in series
or in parallel in series or parallel rungs.
4. Two or more output bits can be connected in parallel.
0000
0000
00
05
(6)
B
(7)
((5))
R1
(10)
R2
R1
R2
TIM
Section 2-1
Order of execution (mnemonic)
(1) LD A
(6) AND B
(2) LD C
(7) OUT R1
(3) OUT TR0
(8) LD TR0
(4) AND D
(9) AND E
(5) OR LD
(10) OUT R2
0000
#0100
0002
00
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