Sysmac Studio TOF Timer (Off-Delay): Complete Programming Guide

The TOF (Off-Delay Timer) instruction in Sysmac Studio delays deactivating an output signal for a preset duration (PT) after its execution condition transitions from TRUE to FALSE. Fully compliant with the IEC 61131-3 standard on Omron NJ/NX series machine automation controllers, TOF is fundamental in industrial machinery for post-cycle lubrication, cabinet cooling fan overrun, conveyor bottle clear-out routines, and pneumatic exhaust delays.

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Technical Specifications and Timer Matrix

Parameter / Feature	Value / Specification	Notes
Controller Families	Omron NJ101, NJ301, NJ501, NX102, NX1P2	All Sysmac MAC platforms
Instruction Standard	IEC 61131-3 Standard Timer	Off-Delay Function Block
Languages Supported	Ladder Diagram (LD), Structured Text (ST)	Deterministic task execution
Input Variable (In)	BOOL	Starts timing on falling edge (TRUE → FALSE)
Preset Time (PT)	TIME (T#0ms to T#106751d_23h_47m_16s_854.775807ms)	Resolution: 1 ns (evaluated per task period)
Output Variable (Q)	BOOL	Stays TRUE during timing; drops to FALSE upon expiration
Elapsed Time (ET)	TIME	Real-time accumulator from T#0s to PT

IEC 61131-3 Timer Comparison Matrix

Timer Type	Output Turn-ON Condition	Output Turn-OFF Condition	Typical Industrial Use Case
TON (On-Delay)	Delays ON by PT after In goes TRUE	Immediately when In goes FALSE	Sensor debouncing, motor star-delta transitions
TOF (Off-Delay)	Immediately when In goes TRUE	Delays OFF by PT after In goes FALSE	Cooling fan overrun, conveyor clearing, vacuum bleed
TP (Pulse Timer)	Immediately on rising edge of In	Fixed duration PT, regardless of In state	Strobe cameras, solenoid pulse ejection

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Programming TOF: Ladder Diagram vs. Structured Text

Sysmac Studio allows instantiating the TOF function block inside graphical ladder networks or Structured Text code blocks:

Ladder Diagram Representation

In a ladder rung, assign a unique instance tag of type TOF (e.g., TOF_Instance):

Structured Text (ST) Syntax

Inside ST programs or function blocks, invoke the instance by passing parameters:

// TOF Timer Invocation in Structured Text
TOF_Instance(
    In := Start_Signal,     // BOOL: Input trigger condition
    PT := T#5s,             // TIME: 5 seconds off-delay duration
    Q  => Motor_Output,     // BOOL: Output delayed state
    ET => Current_Elapsed   // TIME: Elapsed time monitor
);

Parameter Reference Table

Variable	Direction	Data Type	Meaning	Operational Rule
In	Input	BOOL	Timer Input	TRUE keeps Q ON and resets ET. FALSE initiates the timer countdown.
PT	Input	TIME	Preset Time	Target off-delay duration. Format: T#500ms, T#3s, T#1m.
Q	Output	BOOL	Timer Output	Stays TRUE while In = TRUE and throughout the timing cycle. Drops to FALSE when ET = PT.
ET	Output	TIME	Elapsed Time	Tracks duration elapsed since In transitioned to FALSE.

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Operational Modes and Runtime Behavior

Understanding how the internal accumulator responds to input transitions is critical for machine sequence design:

Mode 1: Standard Off-Delay Sequence

When input contact A turns TRUE, output abc energizes immediately. When contact A opens (FALSE), the 5-second preset countdown begins. Output abc remains continuously energized for 5 seconds, after which it drops to FALSE.

Mode 2: Re-Triggering During the Timing Phase

If contact A turns TRUE again while the timer is actively counting down (before ET reaches PT), the timer resets immediately: ET returns to zero, counting halts, and output abc stays continuously TRUE without dipping.

Mode 3: Pulse Triggering via Rising Edge

If input contact A is fed by a rising edge one-shot pulse (@ or R_TRIG), the input stays TRUE for exactly one program scan and then immediately transitions to FALSE. The TOF timer triggers its countdown on scan cycle 2, maintaining output abc for the duration of PT completely independent of contact A.

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Detailed Timing Diagram

The timing chart below illustrates the exact relationship between input In, output Q, and accumulator ET:

Key timing points:

1. When In transitions to TRUE, Q goes TRUE immediately and ET is set to 0.
2. When In transitions to FALSE, ET begins incrementing each task period.
3. When ET reaches PT, Q transitions to FALSE and ET stops incrementing.
4. If In transitions to TRUE while ET < PT, ET resets to 0 and Q remains TRUE.

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Dynamic Preset Time (PT) Modification

You can adjust PT dynamically from an HMI or recipe database while In is FALSE:

Timer State	Output Q State	New PT Value	Controller Action
Timing Complete (ET = PT)	FALSE	Any value	Q remains FALSE. ET remains frozen at original PT.
Actively Timing (ET < PT)	TRUE	PT_new >= ET	Counting continues uninterrupted until reaching PT_new.
Actively Timing (ET < PT)	TRUE	PT_new < ET	Output Q drops to FALSE immediately; ET freezes.

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Field Notes & Safety Practices (“Saha Notu”)

Practical engineering lessons from factory automation:

• Power Cycle & First Scan Behavior: When an NJ/NX controller experiences a power failure, CPU reset, or transition from Program to Run mode, ET initializes to T#0ms. If the logic preceding In does not actively hold it FALSE, or if state flags are uninitialized, output Q may pulse or behave unexpectedly on first scan. Always gate equipment outputs with _IsRunMode or system ready flags.
• Master Control (MC/MCR) Reset: If a TOF instruction is enclosed within a Master Control region and the region is reset, the instruction resets: ET becomes 0 and Q transitions to TRUE if In is active.
• Safety Interlocking: Never rely solely on a standard software TOF timer to maintain personnel safety delays (such as guard locking or deceleration before unlocking a gate). Use safety-rated hardware or an Omron G9SP / NX-SL functional safety controller adhering to ISO 13849-1 (Cat 4 / PL e).
• PLC Diagnostics: In case of task cycle overruns or function block compilation errors, cross-reference the error code using the Sysmac Studio Error Code Lookup.

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Related Guides

• Omron NJ/NX TON Timer: On-Delay Programming and Architecture
• Installing EtherCAT ESI XML Files in Sysmac Studio
• Sysmac Studio & Omron PLC Error Code Lookup Utility