Vision 230/260/280 User Guide
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1
Reads data from inputs
2
Processes data according to
program
3
Sends data to outputs
Figure 16. PLC Scan
First, the input data is read at the beginning of each scan. The data has two sources: the PLC’s
physical inputs, and data that are entered via the PLC’s keypad.
Next, the program is executed. The user creates the PLC control program. The program is
composed of instructions that are written in the Ladder language, and is written using the
PLC’s proprietary software. All program instructions are executed in each scan cycle.
Last, the outputs are updated with the new data.
The sample program below causes an alarm, connected to output #1, to actuate whenever a
gate, connected to input #1, opens.
The command
means that the status of the gate is checked at the beginning of each scan.
When the gate is open, the value in the operand is 1 or on. When the gate is closed, the value
in the operand is 0 or off.
The command
controls the alarm. When the value in contact 1 is found to be 1, the
alarm is switched on. When the value is 0, the alarm is off.
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