ClassicLadder is an open-source Software PLC (Programmable Logic Controller) embedded within LinuxCNC. It bridges traditional industrial automation and modern computer-based machine control.
This guide explains how ClassicLadder works, why LinuxCNC uses it, and how it handles complex machine tasks. What is Ladder Logic?
Ladder logic is a graphical programming language used to program PLCs. It mimics the physical electrical schematics of old relay-based control panels.
The Layout: The code looks like a ladder, featuring two vertical rails (power sources) and horizontal rungs (control circuits).
The Logic: Input conditions (like switches) sit on the left of a rung. Output actions (like motors) sit on the right.
The Flow: If all inputs on a rung are true, power flows across to activate the output. The Role of ClassicLadder in LinuxCNC
LinuxCNC handles high-speed, precise motion control like moving stepper motors or calculating tool paths. However, CNC machines also require “background” sequential automation. ClassicLadder handles these secondary, non-motion tasks.
While LinuxCNC uses HAL (Hardware Abstraction Layer) for basic routing, ClassicLadder manages complex, multi-step logic. It prevents your main motion control from slowing down due to complex auxiliary programming. Common Use Cases
ClassicLadder excels at managing peripheral machine hardware, including:
Automatic Tool Changers (ATC): Sequencing the carousel rotation, tool clamping, and arm movement.
Pallet Changers: Managing safety interlocks during automated stock loading.
Safety Interlocks: Ensuring enclosure doors are locked before the spindle spins.
Lube and Coolant Systems: Triggering a pulse of lubricant every 30 minutes of runtime.
Custom Control Panels: Mapping physical buttons, lamps, and selector switches to specific machine behaviors. Key Features of ClassicLadder
ClassicLadder brings standard industrial PLC capabilities directly into the LinuxCNC environment.
Standard GUI: Features a real-time graphical editor to build, modify, and monitor rungs while the machine runs.
Comprehensive Variables: Supports standard bit memories (%M), words (%W), timers (%TM), counters (%C), and math blocks.
Sequential Function Charts (SFC): Supports Grafcet/SFC programming for step-by-step sequential operations.
Modbus Support: Communicates with external physical I/O blocks, VFDs, and remote PLCs over serial or Ethernet. How ClassicLadder Connects to LinuxCNC
ClassicLadder communicates with the rest of your machine through LinuxCNC’s HAL.
Variables to Pins: ClassicLadder exports its internal memory bits as HAL pins (e.g., classicladder.0.out-00).
HAL Netting: You use HAL commands to connect physical hardware pins (like a breakout board input) to ClassicLadder input pins.
Execution: LinuxCNC runs ClassicLadder as a realtime component, evaluating the entire ladder logic loop hundreds of times per second. Why Use ClassicLadder Instead of Pure HAL?
You can write basic logic using HAL components like and2, or2, or not. However, complex sequences quickly become unreadable in a text file.
ClassicLadder provides visual troubleshooting. When a tool changer jams, you can open the ClassicLadder GUI and instantly see which specific sensor or interlock is breaking the circuit.
To help you get started with your implementation, let me know:
What specific machine function (e.g., tool changer, safety loop) are you trying to program?
What hardware interface (e.g., Mesa board, Modbus VFD) are you connecting to?
I can provide a step-by-step HAL configuration example tailored to your setup.
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