PLC Timer Types: A Comprehensive Guide
Programmable Logic Controllers (PLCs) use various types of timers to control time-based operations in industrial automation. This guide explains the different timer types, their characteristics, and practical applications.
Basic Timer Types
1. On-Delay Timer (TON)
The On-Delay Timer starts timing when the input condition becomes true and turns on the output after the preset time has elapsed.
**Characteristics:**
– Starts timing when input is ON
– Output turns ON after the preset time
– Resets when input turns OFF
– Common applications: Delayed start of motors, sequential operations
**Example Ladder Logic:**
“`
Input —[TON Timer1]— Output
Preset: 5s
“`
2. Off-Delay Timer (TOF)
The Off-Delay Timer turns on the output immediately when the input becomes true and starts timing when the input turns off.
**Characteristics:**
– Output turns ON immediately with input
– Starts timing when input turns OFF
– Output turns OFF after preset time
– Common applications: Motor cooling time, light delay circuits
**Example Ladder Logic:**
“`
Input —[TOF Timer1]— Output
Preset: 3s
“`
3. Retentive On-Delay Timer (RTO)
The Retentive On-Delay Timer accumulates time even when the input turns off, maintaining its current value.
**Characteristics:**
– Accumulates time even when input is OFF
– Requires explicit reset
– Maintains the timer value during power loss
– Common applications: Total running time, maintenance scheduling
**Example Ladder Logic:**
“`
Input —[RTO Timer1]— Output
Preset: 10s
Reset —[RES Timer1]
“`
Advanced Timer Types
4. Pulse Timer (TP)
The Pulse Timer generates a fixed-duration output pulse when triggered, regardless of input duration.
**Characteristics:**
– Generates a fixed-width pulse
– Output duration equals preset time
– Independent of input duration
– Common applications: Signal debouncing, short-duration outputs
**Example Ladder Logic:**
“`
Input —[TP Timer1]— Output
Preset: 1s
“`
5. Accumulating Timer (ACC)
The Accumulating Timer adds up time periods over multiple cycles.
**Characteristics:**
– Accumulates time across multiple cycles
– Requires manual reset
– Useful for totalizing operations
– Common applications: Total production time, energy monitoring
**Example Ladder Logic:**
“`
Input —[ACC Timer1]— Output
Preset: 3600s
Reset —[RES Timer1]
“`
Timer Parameters
### Common Timer Parameters:
1. **Preset Value (PT)**
– Target time for timer operation
– Usually specified in milliseconds or seconds
– Range depends on PLC model
2. **Accumulated Value (ACC)**
– Current running time
– Can be monitored for process control
– Resets based on timer type
3. **Timer Status Bits**
– Done bit (DN)
– Timer timing bit (TT)
– Timer enabled bit (EN)
## Best Practices
1. **Timer Selection**
– Use TON for delayed starts
– Use TOF for delayed stops
– Use RTO for accumulating time
– Use TP for fixed-duration outputs
2. **Timer Programming**
– Always initialize timers
– Use an appropriate time base
– Consider scan time effects
– Implement proper reset logic
3. **Common Applications**
– Motor start/stop sequences
– Conveyor control
– Batch processing
– Safety interlocks
– Maintenance scheduling
## Example Applications
### 1. Motor Start Sequence
“`
Start —[TON T1]—[TON T2]— Motor1
Preset: 2s Preset: 3s
“`
### 2. Conveyor Control
“`
Sensor —[TOF T1]—[TON T2]— Conveyor
Preset: 5s Preset: 2s
“`
### 3. Batch Processing
“`
Start —[RTO T1]—[RTO T2]— Batch Complete
Preset: 30s Preset: 60s
“`
## Troubleshooting Tips
1. **Timer Not Starting**
– Check input conditions
– Verify the timer enable bit
– Confirm proper timer type selection
2. **Timer Not Completing**
– Verify preset value
– Check for reset conditions
– Monitor the accumulated value
3. **Unexpected Timer Behavior**
– Review scan time effects
– Check for conflicting logic
– Verify timer reset conditions
Conclusion
Understanding different PLC timer types and their applications is crucial for effective industrial automation programming. Each timer type serves specific purposes, and selecting the right timer for your application ensures reliable and efficient operation.
Further Reading
– [PLC Timer Programming Guide](https://www.automationdirect.com/static/specs/timercounter.pdf)
– [Industrial Automation Timer Applications](https://www.rockwellautomation.com/en-us/products/hardware/allen-bradley/plcs.html)
– [PLC Programming Best Practices](https://www.siemens.com/global/en/products/automation/industry-software/automation-software/tia-portal.html)
This content is based on standard PLC programming practices and common industrial automation knowledge.
