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Programming Concepts Section 1-1
Mnemonics A mnemonic program is a series of ladder diagram instructions given in their
mnemonic form. It has program addresses, and one program address is
equivalent to one instruction.
Basic Ladder Program Concepts
1,2,3... 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
Input bit
Connecting line
Output bit
Rungs
Left bus bar
Special
instruction
Right bus bar
Instruction blocks
Program Address Instruction (Mnemonic) Operand
0LD0.00
1AND0.01
2LD0.02
3 AND NOT 0.03
4 LD NOT 1.00
5AND1.01
6OR LD
7 AND LD
8 OUT 102.00
9END
0.00 0.01 0.02 0.03 102.00
1.00 1.01
Example
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