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Sven Sager
3294c5e980
Revised various sections across multiple documentation files to reflect updated methods (`run_plc` replacing manual setup with `cycleloop`) and adjust for new default parameters (e.g., `autorefresh`). Enhanced descriptions for timers, Cycletools usage, and new method explanations. Removed outdated or redundant examples and updated system requirements. Signed-off-by: Sven Sager <akira@narux.de>
15 KiB
15 KiB
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Cyclic Programming
Cyclic programming executes a function at regular intervals, similar to PLC programming.
Contents
When to Use Cyclic Programming
Cyclic programming is ideal for:
- Deterministic timing requirements
- Traditional PLC-style logic
- State machines
- Time-critical control
- Continuous monitoring and control
Advantages:
- Predictable timing
- Simple mental model
- Easy to reason about program flow
- Natural for control systems
Considerations:
- Consumes CPU even when idle
- Cycle time affects responsiveness
- Must keep cycle logic fast
Basic Structure
Simple Cycle Loop
import revpimodio2 def main_cycle(ct: revpimodio2.Cycletools): """Execute each cycle.""" if ct.io.start_button.value: ct.io.motor.value = True if ct.io.stop_button.value: ct.io.motor.value = False revpimodio2.run_plc(main_cycle) # .run_plc is a shortcut for: # rpi = revpimodio2.RevPiModIO(autorefresh=True) # rpi.handlesignalend() # rpi.cycleloop(main_cycle)
The main_cycle function is called repeatedly at the configured cycle time (typically 20-50ms).
Info: rpi.handlesignalend()
Understanding Cycle Time
The cycle time determines execution frequency:
- Core 1: 40ms (25 Hz)
- Core3/Connect: 20ms (50 Hz)
- NetIO: 50ms (20 Hz)
Adjust cycle time to match your needs:
revpimodio2.run_plc(main_cycle, cycletime=100) # 100ms = 10 Hz
Important: Faster cycle times consume more CPU but will detect fast changes of input values.
Cycletools Object
The Cycletools object is passed to your cycle function, providing access to:
- ct.io - All IOs
- ct.core - System control
- ct.device - Device access
- ct.var - Persistent variables
- Lifecycle flags (first, last)
- Timing flags (flag5c, flank10c, etc.)
- Timer functions (set_tonc, get_tofc, etc.)
- Change detection (changed)
Initialization and Cleanup
Use ct.first and ct.last for setup and teardown:
def main_cycle(ct: revpimodio2.Cycletools): if ct.first: # Initialize on first cycle ct.var.counter = 0 ct.var.state = "IDLE" print("System started") # Main logic runs every cycle ct.var.counter += 1 if ct.last: # Cleanup before exit ct.io.motor.value = False print(f"Total cycles: {ct.var.counter}") revpimodio2.run_plc(main_cycle)
Persistent Variables
Use ct.var to store variables that persist between cycles:
def main_cycle(ct): if ct.first: ct.var.counter = 0 ct.var.state = "IDLE" ct.var.accumulator = 0.0 # Variables persist between cycles ct.var.counter += 1 ct.var.accumulator += ct.io.sensor.value # Access variables later average = ct.var.accumulator / ct.var.counter
Variables defined in ct.var maintain their values across all cycle executions.
Change Detection
Detect input changes efficiently without storing previous values:
def main_cycle(ct: revpimodio2.Cycletools): # Detect any change if ct.changed(ct.io.sensor): print(f"Sensor changed to: {ct.io.sensor.value}") # Detect rising edge (button press) if ct.changed(ct.io.button, edge=revpimodio2.RISING): print("Button pressed!") # Detect falling edge (button release) if ct.changed(ct.io.button, edge=revpimodio2.FALLING): print("Button released!") rpi = revpimodio2.RevPiModIO(autorefresh=True) rpi.handlesignalend() rpi.cycleloop(main_cycle)
Edge types:
- revpimodio2.RISING - False to True transition
- revpimodio2.FALLING - True to False transition
- revpimodio2.BOTH - Any change (default)
Timing Flags
Built-in timing flags provide periodic execution without manual counting.
Toggle Flags
Toggle flags alternate between True/False at regular intervals:
def main_cycle(ct): # Blink LED - flag5c alternates every cycle ct.io.blink_led.value = ct.flag1c # Different blink rates ct.io.fast_blink.value = ct.flag5c # Every 5 cycles ct.io.slow_blink.value = ct.flag20c # Every 20 cycles
Available toggle flags:
- ct.flag1c - Every cycle
- ct.flag5c - Every 5 cycles
- ct.flag10c - Every 10 cycles
- ct.flag20c - Every 20 cycles
Flank Flags
Flank flags are True for exactly one cycle at regular intervals:
def main_cycle(ct): # Execute task every 10 cycles if ct.flank10c: print(f"Runtime: {ct.runtime:.3f}s") # Execute task every 20 cycles if ct.flank20c: temp = ct.io.temperature.value print(f"Temperature: {temp}°C")
Available flank flags:
- ct.flank5c - True every 5 cycles
- ct.flank10c - True every 10 cycles
- ct.flank15c - True every 15 cycles
- ct.flank20c - True every 20 cycles
Timers
RevPiModIO provides three timer types based on PLC standards. All timers are specified in cycle counts or milliseconds.
On-Delay Timer (TON/TONC)
Output becomes True only after input is continuously True for specified cycles (use ton with milliseconds value instead of cycles):
def main_cycle(ct): # Input: sensor value ct.set_tonc("delay", 10) # Output goes high after input is high for 10 cycles if ct.get_tonc("delay"): ct.io.output.value = True else: ct.io.output.value = False
How it works:
- Input goes True
- Timer starts counting
- If input stays True for 10 cycles, output goes True
- If input goes False before 10 cycles, timer resets
Use cases:
- Button debouncing
- Startup delays
- Confirming sustained conditions
Off-Delay Timer (TOF/TOFC)
Output stays True for specified cycles or milliseconds after input goes False (use tof with milliseconds value instead of cycles):
def main_cycle(ct): # Input: button value ct.set_tofc("motor_coast", 20) # Motor continues for 20 cycles after button release ct.io.motor.value = ct.get_tofc("motor_coast")
How it works:
- Input is True, output is True
- Input goes False
- Output stays True for 20 more cycles
- After 20 cycles, output goes False
Use cases:
- Motor coast-down
- Relay hold-in
- Graceful shutdowns
Pulse Timer (TP/TPC)
Generates a one-shot pulse of specified duration (use tp with milliseconds value instead of cycles):
def main_cycle(ct): # Trigger pulse on button press if ct.changed(ct.io.trigger, edge=revpimodio2.RISING): ct.set_tpc("pulse", 5) # Output is True for 5 cycles ct.io.pulse_output.value = ct.get_tpc("pulse")
How it works:
- Call set_tpc to trigger pulse
- Output is True for 5 cycles
- After 5 cycles, output goes False
- Additional triggers during pulse are ignored
Use cases:
- One-shot operations
- Acknowledgment pulses
- Retriggerable delays
State Machines
State machines implement complex control logic with distinct operational modes.
Simple State Machine
def traffic_light(ct: revpimodio2.Cycletools): """Traffic light controller.""" if ct.first: ct.var.state = "GREEN" if ct.var.state == "GREEN": ct.io.green_led.value = True ct.io.yellow_led.value = False ct.io.red_led.value = False # After 2 seconds, go to yellow ct.set_ton("green_time", 2000) if ct.get_ton("green_time"): ct.var.state = "YELLOW" elif ct.var.state == "YELLOW": ct.io.green_led.value = False ct.io.yellow_led.value = True ct.set_ton("yellow_time", 500) if ct.get_ton("yellow_time"): ct.var.state = "RED" elif ct.var.state == "RED": ct.io.yellow_led.value = False ct.io.red_led.value = True ct.set_ton("red_time", 3000) if ct.get_ton("red_time"): ct.var.state = "GREEN" rpi = revpimodio2.RevPiModIO(autorefresh=True) rpi.handlesignalend() rpi.cycleloop(traffic_light)
Complex State Machine
def machine_controller(ct: revpimodio2.Cycletools): """Multi-state machine controller.""" if ct.first: ct.var.state = "IDLE" ct.var.production_count = 0 # State: IDLE - Ready to start if ct.var.state == "IDLE": ct.io.motor.value = False ct.io.green_led.value = True ct.io.red_led.value = False if ct.changed(ct.io.start_button, edge=revpimodio2.RISING): ct.var.state = "STARTING" print("Starting...") # State: STARTING - Startup sequence elif ct.var.state == "STARTING": ct.io.yellow_led.value = True # 2-second startup delay ct.set_ton("startup", 2000) if ct.get_ton("startup"): ct.var.state = "RUNNING" print("Running") # State: RUNNING - Normal operation elif ct.var.state == "RUNNING": ct.io.motor.value = True ct.io.yellow_led.value = False ct.io.green_led.value = ct.flag5c # Blink # Count production if ct.changed(ct.io.sensor, edge=revpimodio2.RISING): ct.var.production_count += 1 # Check for stop if ct.io.stop_button.value: ct.var.state = "STOPPING" # Check for error if ct.io.error_sensor.value: ct.var.state = "ERROR" # State: STOPPING - Controlled shutdown elif ct.var.state == "STOPPING": # Coast motor for 1 second ct.set_tof("coast", 1000) ct.io.motor.value = ct.get_tof("coast") if not ct.io.motor.value: ct.var.state = "IDLE" print("Stopped") # State: ERROR - Fault condition elif ct.var.state == "ERROR": ct.io.motor.value = False ct.io.red_led.value = ct.flag5c # Blink red if ct.changed(ct.io.ack_button, edge=revpimodio2.RISING): if not ct.io.error_sensor.value: ct.var.state = "IDLE" print("Error cleared") if ct.last: print(f"Total production: {ct.var.production_count}") revpimodio.run_plc(machine_controller)
Practical Examples
Temperature Control
Temperature monitoring with hysteresis control:
def temperature_monitor(ct: revpimodio2.Cycletools): """Monitor temperature and control cooling.""" if ct.first: ct.var.cooling_active = False temp = ct.io.temperature.value # Hysteresis: ON at 75°C, OFF at 65°C if temp > 75 and not ct.var.cooling_active: ct.io.cooling_fan.value = True ct.var.cooling_active = True print(f"Cooling ON: {temp}°C") elif temp < 65 and ct.var.cooling_active: ct.io.cooling_fan.value = False ct.var.cooling_active = False print(f"Cooling OFF: {temp}°C") # Warning if too hot if temp > 85: ct.core.a1green.value = False ct.core.a1red.value = ct.flag5c # Blink else: ct.core.a1green.value = ct.flag5c # Blink ct.core.a1red.value = False # Emergency shutdown if temp > 95: ct.io.emergency_shutdown.value = True revpimodio2.run_plc(temperature_monitor)
Production Counter
Count production items with start/stop control:
def production_counter(ct: revpimodio2.Cycletools): """Track production count.""" if ct.first: ct.var.total_count = 0 ct.var.running = False # Start/stop control if ct.changed(ct.io.start_button, edge=revpimodio2.RISING): ct.var.running = True if ct.changed(ct.io.stop_button, edge=revpimodio2.RISING): ct.var.running = False # Count items if ct.var.running: if ct.changed(ct.io.item_sensor, edge=revpimodio2.RISING): ct.var.total_count += 1 ct.set_tpc("count_pulse", 5) # Pulse LED print(f"Item #{ct.var.total_count}") ct.io.count_led.value = ct.get_tpc("count_pulse") # Reset counter if ct.changed(ct.io.reset_button, edge=revpimodio2.RISING): print(f"Final count: {ct.var.total_count}") ct.var.total_count = 0 revpimodio2.run_plc(production_counter)
Best Practices
Keep Cycle Logic Fast
Minimize processing time in each cycle:
def optimized_cycle(ct): # Heavy work only when needed if ct.flank100c: heavy_calculation() # Keep cycle logic minimal ct.io.output.value = ct.io.input.value
Guidelines:
- Avoid blocking operations (network, file I/O)
- Use flank flags for expensive operations or even Threads
- Keep cycle time ≥20ms for stability
Use Appropriate Cycle Time
Match cycle time to application requirements:
# Fast control (motion, high-speed counting) rpi.cycletime = 20 # 50 Hz # Normal control (most applications) rpi.cycletime = 50 # 20 Hz # Slow monitoring (temperature, status) rpi.cycletime = 100 # 10 Hz
Handle Errors Safely
Always implement safe failure modes:
def safe_cycle(ct): try: value = ct.io.sensor.value ct.io.output.value = process(value) except Exception as e: print(f"Error: {e}") ct.io.output.value = False # Safe state
Initialize Properly
Use ct.first for all initialization:
def main_cycle(ct): if ct.first: # Initialize all variables ct.var.counter = 0 ct.var.state = "IDLE" ct.var.last_value = 0 # Set initial outputs ct.io.motor.value = False
See Also
:doc:`basics` - Core concepts and configuration
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:doc:`event_programming` - Event-driven programming
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:doc:`advanced` - Advanced topics and examples
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:doc:`api/helper` - Cycletools API reference
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