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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