In CNC automation, everything runs on logic—and that logic depends on FANUC bits. These tiny digital signals tell the machine when it’s safe to move, start, stop, or stay still. If even one required bit isn’t set to ON, your machine might freeze or throw an error. That’s why knowing what FANUC bits need to be on can save time, reduce downtime, and keep production flowing.
Whether you’re just starting out or learning how to troubleshoot, this guide breaks down which bits to watch, how to check them, and what to do when things don’t go as planned.
Understanding FANUC Bits and Their Functions
What Are FANUC Control Bits?
FANUC bits are digital ON/OFF switches used inside CNC machines. These can be input bits (from external sensors or buttons), output bits (to turn on relays or motors), or internal bits used by the PMC ladder logic.
- Input bits: Signals from real-world actions like pressing a button.
- Output bits: Send signals to devices like solenoids or indicator lights.
- Internal bits: Used by the Programmable Machine Control (PMC) to manage processes.
Each bit has a specific address like X100.0 (input) or Y020.2 (output), and these addresses control specific machine actions.
Why Do Certain Bits Need to Be ON?
Machines won’t move unless the right safety and control bits are ON. These bits signal that all systems are go—like doors are closed, emergency stops are clear, and power is stable.
If any required bit is OFF, the machine locks out. It’s a built-in safety mechanism.
Common FANUC Bits Required for Startup
Power-On Ready Bits
Before anything starts, the machine must be powered on correctly. A common bit for this is the Machine Ready Bit, often labeled something like R100.0.
It checks:
- Power supply stability
- Battery status
- System boot completion
If this bit is OFF, no movement happens—even with a green light on the panel.
Emergency Stop and Reset Bits
These are crucial. Emergency Stop (E-Stop) bits must be OFF (which means the E-Stop is not pressed), and Reset Bits must be ON to restart the system.
Bit examples:
- E-Stop signal: X002.0 OFF
- Reset signal: X003.1 ON
Servo and Spindle Enable Bits
Servos and spindles need their own “OK” signals.
- Servo ready bit: R200.1
- Spindle ready bit: R300.4
If any servo is not ready, movement won’t start. These bits tell the control that all motors are in the correct state.
Bit Monitoring in the PMC Ladder
Navigating the Diagnostic Screen
To check FANUC bits, go to the Diagnostic (DGNOS/PAGE) screen on the CNC display. This shows all bits in real time—ON or OFF.
Use this screen to:
- Track startup issues
- Monitor ladder sequences
- Watch automation logic as it runs
Identifying Input vs Output Bits
Use the address format to identify the type:
- X: External input (switches, sensors)
- Y: External output (lights, relays)
- R: Internal relay (used for logic control)
- K: Constant or fixed values
Knowing which is which helps trace signal issues quickly.
Using Keep Relays and Data Bits Correctly
Keep Relays (K-bits) are memory bits that store settings. They’re used to keep a value ON even after a power cycle. For example, auto mode settings or alarm overrides.
Be careful with these—accidentally keeping the wrong bit ON can stop the machine from responding correctly.
Troubleshooting Machine Not Starting
Bit Not Setting Issue
Sometimes a bit that should be ON stays OFF. Reasons include:
- Loose wire from a switch
- Damaged relay
- Faulty sensor
Use the ladder diagram to trace the logic path and find out which condition isn’t met.
Safety Door Interlocks and Bit Dependencies
Most CNCs won’t start if safety doors are open. This is usually controlled by a door switch bit like X010.0.
If this bit stays OFF:
- Check the physical door switch
- Inspect ladder logic for overrides
- Confirm it’s not a soft alarm holding the state
Ladder Logic Hold Conditions
Some bits rely on a sequence of other bits. If a previous bit isn’t ON, the next one won’t turn ON either. This chain of bits is what makes up ladder logic.
For example:
- Bit A must be ON for Bit B to turn ON
- Bit B enables Bit C, which starts a motor
Ensuring Correct Bit Sequences for Automation
Robot or Conveyor Integration via I/O Bits
FANUC machines often connect with robots or conveyors through I/O bits. You’ll need to confirm handshake bits like:
- Robot Ready: X020.3
- Conveyor Start: Y015.1
- Part Present Sensor: X005.4
Getting the order right ensures smooth transfer and operation.
Using M-Codes That Rely on Bit States
Some M-codes (like M06 for tool change) require certain bits to be ON first.
Example:
- M06 won’t run unless Tool Clamp Bit is ON
- M30 (program end) might wait for a Bit Confirming Finish
These checks prevent mechanical damage or missed operations.
Solutions and Preventive Measures
Using Diagnostic Parameters to Confirm Bit Status
In FANUC Diagnostic Parameters, each bit has a description. Learn to read these values to understand:
- Which bit failed
- What it’s waiting on
- If a signal never reached the ladder
Creating a Bit Checklist for Startup
Keep a printed checklist of required startup bits. Include:
- Emergency Reset OK
- Servo ON
- Spindle Ready
- Door Closed
- Power OK
Use this during startup or after power loss to save troubleshooting time.
Training Teams on Bit Logic and PMC Basics
Even basic ladder training can help operators:
- Identify which bits to check
- Reset faults quicker
- Avoid incorrect troubleshooting
Many issues are fixed faster when everyone understands FANUC logic at a basic level.
Conclusion: Turn the Right Bits ON, Every Time
Knowing what FANUC bits need to be on is key to running your CNC machine safely and efficiently. Whether it’s startup signals, safety checks, or PMC ladder logic, every bit matters.
By learning how to check, trace, and understand these digital signals, you’ll reduce downtime, fix problems faster, and run smoother operations.
From power ready to spindle enable, bit knowledge isn’t just for engineers—it’s for anyone serious about automation and CNC machining.