Ladder diagram and mnemonic program

There are other methods for programming PLCs. Ladder diagram involved mnemonic instructions. These instructions can be derived directly from the ladder logic diagrams and entered into the PLC through a simple programming terminal. An example of mnemonics is shown in Figure below. In this example the instructions are read one line at a time from top to bottom. The first line 00000 has the instruction LDN (input load and not) for input 00001. This will examine the input to the PLC and if it is off it will remember a 1 (or true), if it is on it will remember a 0 (or false). The next line uses an LD (input load) statement to look at the input. If the input is off it remembers a 0, if the input is on it remembers a 1 (note: this is the reverse of the LD).

 The AND statement recalls the last two numbers remembered and if the are both true the result is a 1, otherwise the result is a 0. This result now replaces the two numbers that were recalled, and there is only one number remembered. The process is repeated for lines 00003 and 00004, but when these are done there are now three numbers remembered. The oldest number is from the AND, the newer numbers are from the two LD instructions. The AND in line 00005 combines the results from the last LD instructions and now there are two numbers remembered. The OR instruction takes the two numbers now remaining and if either one is a 1 the result is a 1, otherwise the result is a 0. This result replaces the two numbers, and there is now a single number there. The last instruction is the ST (store output) that will look at the last value stored and if it is 1, the output will be turned on, if it is 0 the output will be turned off.

An Example of a Mnemonic Program and Equivalent Ladder Logic

0000	LDN	0001
0001	LD	0002
0002	AND
0004	LD	0003
0005	LD	0004
0006	AND
0007	OR	0007
0008	ST
0009	END

The ladder logic program in Figure above, is equivalent to the mnemonic program. Even a PLC proggrame with ladder logic, it will be converted to mnemonic form before being used by the PLC. In the past mnemonic programming was the most common, but now it is uncommon for users to even see mnemonic programs.

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