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2-2. Logic operations for instructions
2-2-1. Basic operation of the programmable
The basic operation of the programmable
controller is shown in the figure on the right.
This operation is the same as for a computer
of the stored program method, and the hard-
ware composition also is similar, so that this is
called
a
programmable controller of the
quasicom put er type.
@First, the contents of the program counter
showing the program step number are given
to the memory address, and the contents
are read.
@
The read memory contents are translated,
and discrimination is executed for
AND,
OR, and other instructions as well as input
and output numbers.
@
According to the discriminated source
(logic input signal to be included) and the
instructions, AND, OR, and other logic
operations as well as output operation are
executed.
controller
@
1 is added to the program counter contents, and specification of the next step number is ex-
Fig. 2-3. Basic logic operations
ecuted.
The above
4
operations are executed for each
step number, Le. each instruction in the
Memory
Step number
programmable controller, and sequential
execution is executed in the sequence of the
numbers for all step numbers stored in the
memory. This is called scanning.
The figure on the right shows the concept for
scanning. The above 4 operations are ex-
ecuted in the sequence of the program
numbers according to the program counter
contents, return to number 0 is executed when
the
END
instruction is reached, and then the
sequence is repeated. This operation is called
scanning, and with one scanning, all inputs
written into the memory are taken in, logic
operations are executed, and the logic
operation results are put out to all outputs.
'
I
- - I _ - - - _ _ _
- - - - . - -
Fig. 2-4. Scanning operation
The scanning speed about corresponds to the electromagnetic relay response speed, and this is
called the logic operation speed.
As
shown in the above, serial arithmetic operations are executed in the programmable controller,
but as the logic operation speed is high, all inputs are taken in, and logic operation results are put
out to all outputs, the operation from the outside appears to be the same parallel arithmetic
operation as for electromagnetic relays.
- 4 -
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