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fanuc-io-marker-m-instruction.md

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title FANUC IO Marker M[] Instruction — Background Logic Alternative
domain fanuc
subdomain tp-programming
tags
marker
m-register
io
background-logic
handling-tool
vass
source robot-forum.com
status published
confidence 0.85
created 2026-07-01
verified_date
domain_expert cattmampbell

Problem

The M[] option appears in the I/O replace menu but is undocumented in standard TP programming guides. Users don't know what it does or when to use it.

Root Cause

M[] is a Marker register — a background-logic-like flag system built into FANUC HandlingTool. It evaluates Boolean expressions continuously without BG Logic. Disabled by default: $MIX_LOGIC.$USE_MKR=FALSE.

Solution

Enable Markers

Set $MIX_LOGIC.$USE_MKR = TRUE

A new Marker option appears in MENU → I/O.

Usage

M[1] = (DI[1] AND DI[2])

This expression runs in the background continuously until cleared or overwritten. No BG Logic task needed.

VASS Standard Use Cases (VW/Audi/SEAT/Škoda)

Markers are used extensively in VASS (Volkswagen Automotive Standard) for:

Category Examples
Docking Docking Stand 1 Occupied/Empty, Tool 1 Docked/Undocked
Handling Clamp 1 Open/Close, Magnet 1 On/Off, Vacuum 1 Blow/Suck
Machine Safety Air OK, Water OK, Spot Welding Gun OK

Used with TIMER BEFORE, POINT LOGIC (P-SPS), and TC ONLINE instructions.

Practical Example: Analog Sensor to Digital Input

-- Map analog sensor to marker for MH valve input
M[1] = (AI[1] > R[1])  -- R[1] = closed threshold per part type
-- Use M[1] as input in MH valve config instead of DI

This is useful when "closed" distance varies by part type — set threshold in R[1], MH valve config stays constant.

Verification

  1. Set $MIX_LOGIC.$USE_MKR = TRUE
  2. Create M[1] = (DI[1] AND DI[2]) in a program
  3. Toggle DI[1] and DI[2] — M[1] updates automatically in background
  4. Check MENU → I/O → Marker shows the correct state

Notes

  • Markers are specific to HandlingTool option — not available on all controllers
  • Advantage over BG Logic: no separate task, no PR/register restrictions
  • Disadvantage: limited to Boolean expressions, no motion control
  • Originally from KUKA, adopted by VW into FANUC standard
  • Source: https://www.robot-forum.com/robotforum/thread/54934/