Chapter 8 PLC Timer Instructions Objectives Use nonretentive

Chapter 8 PLC Timer Instructions

Objectives • 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. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Objectives • Connect timer instructions to interlock each other. • Cascade timer instructions in a PLC ladder logic diagram to increase the timer preset values. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Timers • • Fixed timers Variable timers Single-input timers Double-input timers © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Timer Instructions • In the relay logic diagrams, symbols are used to represent timer coils and their associated timer contacts. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Timer Instructions (Cont. ) • Two types of timer instructions: – ON-delay timers. – OFF-delay timers. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

PLC Timer Instruction Registers (Cont. ) • Timers in Allen-Bradley SLC 500 series PLC: – Data file four (T 4) reserved for timer instructions. – Timers have addresses T 4: 0 to T 4: 255. – In theory, can have up to 256 timers. – In practice, number of timers is limited by PLC memory capacity. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Timing Bits • Enable bit (EN) • Timer timing bit (TT) • Done bit (DN) © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Fixed Preset Value Non-retentive Timer ON-Delay Instructions • Block diagram of an SLC 500 non-retentive timer ON-delay with address T 4: 0. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

PLC Ladder Logic Diagram Using MOV and TON © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Retentive Timer ON-Delay Instructions • Reset (RES) instruction: – Used to reset a retentive timer. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Timer OFF-Delay Instructions • PLC ladder logic diagram using timer offdelay. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Interlocking Timers (Cont. ) • PLC ladder logic diagram using two timers. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

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. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

PLC Ladder Logic Diagram Cascading Timers © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.