PLC Timer Instructions

Chapter

8PLC Timer Instructions

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• Use non-retentive and retentive timer ON-delay instructions to create PLC ladder logic diagrams.

• Use move (MOV) instructions to create variable preset value timer instructions.

• Use OFF-delay timers to create PLC ladder logic diagrams.

Objectives


Objectives

• Connect timer instructions to interlock each other.

• Cascade timer instructions in a PLC ladder logic diagram to increase the timer preset values.


Timers

• Fixed timers• Variable timers• Single-input timers• Double-input timers


Timer Instructions

• In the relay logic diagrams, symbols are used to represent timer coils and their associated timer contacts.


Timer Instructions (Cont.)

• Two types of timer instructions: – ON-delay timers.– OFF-delay timers.


ON-delay Timers

• Start timing when timer coils are energized.• Normally open contacts close and normally

closed contacts open when the timer is finished timing.

• Contacts immediately revert to normal states when the timer accumulated register resets to zero.

• Delay before contacts switch when timer turns on; no delay in switching when timer turns off.


OFF-delay Timers

• Start timing when their timer coils are de-energized.

• Normally open contacts close and normally closed contacts open when the timer is finished timing.

• Contacts revert to their normal states when the timer accumulated register resets to zero.


Timer Instruction Formats

• Coil format:– Uses coils to display the timer instruction.

• Block format:– Uses a box shape to display the timer

instruction.– More common symbol for displaying timer

instructions.


PLC Timer Instruction Registers

• Preset register:– Content is the preset value that the timer is

initialized to hold. – Holds the preset number.

• Accumulated register:– Holds the time that the timer has been timing.

• Status register, also called flag register: – Holds the flag bits that are used by the PLC

processor to monitor the status of the timer.


PLC Timer Instruction Registers (Cont.)

• Timers in Allen-Bradley SLC 500 series PLC:– Data file four (T4) reserved for timer

instructions.– Timers have addresses T4:0 to T4:255.– In theory, can have up to 256 timers.– In practice, number of timers is limited by

PLC memory capacity.


Timer Base Number

• Multiplied by the preset number. • Defines unit of time as a fraction of a

second or a second.• 0.01 seconds, 0.1 seconds, and 1

second most commonly used in PLC timers.


Timing Bits

• Enable bit (EN)• Timer timing bit (TT)• Done bit (DN)


PLC Timer Instruction Timers

• Three different types of timers for Allen-Bradley SLC 500 series PLCs:– Non-retentive timer ON-delay.– Retentive timer ON-delay.– Non-retentive timer OFF-delay.


Fixed Preset Value Non-retentive Timer ON-Delay Instructions

• Block diagram of an SLC 500 non-retentive timer ON-delay with address T4:0.


Variable Preset Value Non-retentive Timer ON-Delay Instructions

• Move (MOV) instruction:– Used for copying the content of one

register into another or for loading a number into a register.

– Can be used to create a variable preset timer.


PLC Ladder Logic Diagram Using MOV and TON


Retentive Timer ON-Delay Instructions

• Reset (RES) instruction:– Used to reset a retentive timer.


Timer OFF-Delay Instructions

• PLC ladder logic diagram using timer off-delay.


Interlocking Timers

• Interlocked timers:– Output turns on and off sequentially.

• Common method of flashing lights or ringing bells in Allen-Bradley SLC 500 series programmable logic control systems.


Interlocking Timers (Cont.)

• PLC ladder logic diagram using two timers.


Cascading Timers

• In some industrial applications:– Timers must have a large preset value.– PLC ladder diagrams that contain several

timers can be created.• Cascading links multiple timers

together.


Cascading Timers (Cont.)

• Timers are cascaded together such that when one timer is finished timing, the second one starts.

• When the second timer is done, the third one starts, and so forth.


PLC Ladder Logic Diagram Cascading Timers

PLC Timer Instructions

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