Figure 18. Plc Scan - Unitronics Vision 120 OPLC User Manual

Vision120 OPLC™ User Guide
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
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
alarm is switched on. When the value is 0, the alarm is off.
38
Reads data from inputs
Processes data according to
Sends data to outputs

Figure 18. PLC Scan

means that the status of the gate is checked at the beginning of each scan.
controls the alarm. When the value in
1
2
program
3
contact 1 is found to be 1, the