This program controls how the motor speed value (MW10) is initialized and updated during PLC operation. It uses two special inputs—First Scan Cycle (I0.0) and Initialization Button (I0.1)—to determine when the system should load a default motor speed value. Under normal operation, motor speed is taken from the operator display (MW8).
Inputs and Memory Words Used
| Signal / Memory | Description |
|---|---|
| I0.0 | First Scan Cycle input (activates only once when PLC transitions to RUN mode) |
| I0.1 | Initialization push-button (allows operator to reset motor speed) |
| MW8 | Motor speed value coming from HMI or display |
| MW10 | Final Motor Speed Word used by the PLC to control the motor |
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| https://plcscadaacademy.blogspot.com/ |
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| https://plcscadaacademy.blogspot.com/ |
FBD diagram for the first scan cycle.
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| https://plcscadaacademy.blogspot.com/ |
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| https://plcscadaacademy.blogspot.com/ |
Program Description
The main function of this program is to determine what value should be written into MW10 (Motor Speed) under different conditions:
A. Initialization Conditions (I0.0 or I0.1 active)
If the First Scan Cycle (I0.0) turns ON
ORIf the Initialization Button (I0.1) is pressed
→ The PLC will load a default motor speed value of 10 into MW10.
This ensures that:
On PLC startup, the motor always begins with a known safe speed.
On operator demand, the motor speed can be reset to the same safe/default value.
B. Normal Operation (None of the initialization conditions active)
When neither I0.0 nor I0.1 is TRUE:
→ The motor speed is read from MW8 (HMI/Display Input) and written to MW10.
This allows the operator to modify motor speed from the HMI during normal running conditions.
Why This Logic Is Important
✔ Prevents unpredictable motor start-up values
✔ Ensures safety by assigning a known speed after startup or reset
✔ Allows operator control during normal running conditions
✔ Provides priority for first scan and reset operations
Final Explanation (Compact Version for Documentation)
This program ensures that MW10 (Motor Speed) is properly initialized and updated. During PLC startup or when the initialization button is pressed, the value 10 is moved into MW10. This sets a safe, predefined motor speed. If neither I0.0 nor I0.1 is active, the motor speed is taken from the operator display (MW8) and written into MW10. Thus, the logic provides automatic startup initialization and operator-controlled speed during normal operation.
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