If you suspect a conflict, follow this procedure on the FANUC control:
To work with Keep Relays, you need access to the PMC maintenance mode. Here is the standard method on a Fanuc 31i/0i-D control.
Warning: Only modify Keep Relays if you have the exclusive documentation from the machine builder. Changing the wrong bit can disable emergency stop circuits or cause physical damage.
In the world of CNC manufacturing, Fanuc controls are the undisputed industry standard. From the reliable Series 0i to the powerful 31i, these systems are built on a bedrock of parameters, macros, and ladder logic. Among the most misunderstood—yet most critical—elements of this ecosystem are the Keep Relay parameters.
For maintenance engineers, setup technicians, and integrators, mastering the phrase "Fanuc Keep Relay parameters exclusive" is not just technical jargon; it is the key to maintaining machine integrity, preventing unauthorized changes, and ensuring production uptime.
This article explores what Keep Relays are, why their "exclusive" nature is vital, how to access them, and the best practices for managing these hidden switches.
Ensure specific relay (keep-relay) parameters persistently remain enabled/unchanged and are not overwritten by automatic parameter loads, backups, or other user changes on FANUC CNC controls.
Rating: 9/10 for Necessity, 6/10 for User Safety.
The FANUC Keep Relay "Exclusive" concept is a fundamental requirement for modular machine building. It allows a single standard CNC control to adapt to thousands of different machine configurations.
However, FANUC’s interface for these parameters is raw. It lacks a user-friendly GUI or distinct "Read-Only" flags for critical addresses. It puts the burden of integrity squarely on the programmer and the maintenance team.
Recommendation: If you are a machine builder, use exclusive Keep Relays to define hardware existence. If you are a maintenance tech, back up your Keep Relay states before touching them, and treat them like physical DIP switches—do not flip them unless you know exactly what they do.
Keep Relays (K-parameters) are software switches in the Programmable Machine Control (PMC) that maintain their state (ON/OFF) even after a power cycle. Unlike standard parameters, Keep Relays are "exclusive" because their functions are defined by the Machine Tool Builder (MTB) , not Fanuc itself. Custom Functions:
They typically control machine-specific options like auto-door behavior, chip conveyor timers, or enabling a 4th axis. Documentation: Because they are builder-defined, the meanings of cap K 99.9 fanuc keep relay parameters exclusive
vary wildly between a Haas, a Doosan, or a Matsuura. You must refer to the Ladder Diagram
or the machine's electrical manual to find their specific definitions. The Ghost in the Ladder: A Short Story The shop floor was quiet, except for the rhythmic
of the cooling fans on the old Fanuc 0i-MC. Elias stared at the screen. The machine was "dead"—no alarms, no errors, just a stubborn refusal to start the spindle.
He’d checked the proximity sensors and the air pressure. Everything was green. On a whim, he pulled the dusty, yellowed manual from the back cabinet—the one the previous maintenance lead said "contained the machine’s soul."
He flipped to the PMC section and found a handwritten note scrawled in the margin: “Keep Relay K12.3 – The Apprentice’s Lock.” Elias navigated to the PMC parameters. There it was: cap K 0012 was set to
In the world of Fanuc, Keep Relays are the machine’s permanent memory. Unlike the volatile bits that reset when the breaker flips, these are the "Exclusive" secrets of the builder. K12.3 wasn’t a Fanuc standard; it was a custom safety latch the builder had programmed into the ladder logic years ago. He changed the bit to The contactor in the cabinet clicked—a sharp, mechanical
that sounded like a heavy bolt sliding home. The "Cycle Start" lamp flickered to life. Elias didn't know who "The Apprentice" was or why they needed a lock, but he’d just found the one bit of data that refused to be forgotten.
He closed the cabinet, the spindle beginning its low, rising howl. Some parameters are just numbers; Keep Relays are history.
Fanuc o m series 4 th axis enable parameters pls... - Facebook
In the context of (Programmable Machine Controller) maintenance, Keep Relay parameters
act as non-volatile memory bits that retain their state even after power is turned off. These are primarily used by machine tool builders to enable or disable specific machine options, such as chip conveyors or tool management features. "exclusive"
in the context of these parameters typically refers to two scenarios: exclusive-OR (XOR) logic applied to PMC addresses or exclusive access/protection of parameters. 1. Functional Roles of Keep Relays If you suspect a conflict, follow this procedure
Keep relays (K-parameters) serve several critical roles in machine automation and maintenance: Feature Toggling: Enabling or disabling optional equipment like a chip conveyor high-pressure coolant Bypassing Sensors:
Temporarily shielding external alarms or bypassing faulty sensor inputs (e.g., conveyor rotation detection) to keep the machine running. Safety & Interlocks: Managing interlocks such as chuck clamp signals or ATC (Automatic Tool Changer) safety conditions. System Configuration:
Certain "System K" parameters (like K900+) are reserved for internal CNC functions and protected against general modification. 2. Exclusive Logic and Protection
The concept of "exclusive" keep relay parameters appears in technical maintenance documentation in two ways: A. Exclusive-OR (XOR) Operations
In PMC ladder logic maintenance, specific addresses may be processed using Exclusive-OR
operations. For example, a diagnostic address (like R90) might be XOR'ed with a decimal value (like 255) to flip all bits for specific logic evaluations. B. Exclusive Protection (Parameter Hiding)
Modern FANUC controls allow machine builders to set keep relays to "exclusive" or protected states to prevent unauthorized changes: K902.6 / K19.6 (HIDE PMC PARAM):
When set to 1, this "hides" the Timer, Counter, Keep Relay, and Data Table screens entirely. K902.7 / K19.7 (PROTECT PMC PARAM):
When set to 1, it becomes impossible to modify these screens, even if they are visible. 3. How to Access and Modify Keep Relays
To change a keep relay (e.g., turning a feature ON or OFF), follow these standard steps: How to Enable Parameter Write Enable (PWE) on a Fanuc CNC
Fanuc Keep Relays, also known as K parameters, are a subset of PMC parameters used to control machine-specific functions and maintenance operations. Unlike standard parameters that might reset, Keep Relays are stored in non-volatile memory, meaning they retain their ON/OFF state even after the machine is powered down. Core Functionality
Keep Relays act as soft switches within the machine's ladder logic. Because their assignments are determined by the Machine Tool Builder (MTB) rather than Fanuc, their functions vary significantly between different machine brands and models. Common uses for Keep Relays include: Changing the wrong bit can disable emergency stop
Enabling/Disabling Hardware: Turning optional equipment like chip conveyors or parts catchers on and off.
Safety & Interlock Bypassing: Temporarily shielding external alarms or bypassing sensors (like a faulty conveyor rotation sensor) during troubleshooting.
Operational Modes: Switching between proximity sensor-based clamping or timer-based clamping for chucks.
Cycle Optimization: Modifying tool change requirements, such as allowing a turret to rotate before axes have reached their home position to save time. Parameter Structure & Addresses
Keep Relays are typically organized into an 8-bit octal system (bits 0 through 7).
User Keep Relays (K0–K100+): Assigned by the MTB for custom machine functions like tool management, coolant control, or interlocks.
System Keep Relays (K900–K999): Reserved for core system functions, such as the safety circuit interface (e.g., K928.7 or K929.0). How to Access and Modify
Modifying these parameters requires specific steps to prevent accidental machine damage:
Enable Parameter Write: In MDI Mode, go to the Setting screen and change "Parameter Write" (PWE) from 0 to 1.
Navigate to PMC Maintenance: Press the System key, then use the soft keys to find PMC Maintenance > Keep Relay.
Find the Address: Use the cursor to select the specific K-address. Addresses are read from right to left (Bit 0 is the far right, Bit 7 is the far left). Enter Value: Type 1 for ON or 0 for OFF and press Input.
Restore Safety: Change the "Parameter Write" setting back to 0 once finished.
Note: For System Keep Relays (K900+), the values may not be visible or changeable unless the machine is in an Emergency Stop state. Get to know how to change the Keep Relay!