Understanding Retentive Timers (TONR)

A retentive timer, unlike a standard timer, "remembers" the elapsed time even if the input condition that initiated the timing sequence is interrupted. The timer continues to accumulate time until it reaches its preset value or is explicitly reset. This functionality is crucial in scenarios where intermittent signals or events need to be tracked cumulatively.

The primary components of a retentive timer typically include:

• Input (IN): The signal that initiates the timing process.
• Preset Value (PT): The target time value that the timer needs to reach.
• Accumulated Value (ACC): The current elapsed time.
• Done Bit (DN): A status bit that becomes true when the accumulated value equals or exceeds the preset value.
• Reset (RST): An input that resets the accumulated value to zero.

Benefits of Using Retentive Timers

Using retentive timers in PLC programming offers several advantages over traditional timers, especially in specific applications:

• Persistence: The primary benefit is the ability to retain the accumulated time value across multiple cycles or events.
• Accuracy: By accumulating time accurately, retentive timers provide a reliable way to track total elapsed time.
• Flexibility: They can be used in various applications, from tracking machine uptime to monitoring process durations.
• Reduced Complexity: In some cases, using a retentive timer can simplify the logic required to track accumulated time compared to using standard timers and manual accumulation methods.

Here's a comparison table highlighting the key differences between Retentive and Non-Retentive Timers:

Applications of Retentive Timers in PLCs

Retentive timers find applications in various industries and processes. Here are a few examples:

• Machine Uptime Tracking: Monitoring the total operating time of a machine for maintenance scheduling.
• Process Monitoring: Tracking the cumulative duration of a chemical reaction or a manufacturing process.
• Equipment Usage: Measuring the total usage time of equipment for billing or performance analysis.
• Maintenance Scheduling: Tracking the time since the last maintenance service to trigger reminders.
• Counting Events: Accumulating time based on the occurrence of specific events or conditions.

Example: Machine Uptime Tracking

Consider a scenario where you need to track the total uptime of a machine. The machine might be turned on and off multiple times during the day. A retentive timer can accumulate the total on-time, even when the machine is switched off, providing an accurate record for maintenance scheduling.

// Ladder Logic Example (Simplified)
                    
                    // Input: Machine_Running (boolean)
                    // Output: Machine_Uptime_Timer.EN (Enable bit)
                    // Output: Machine_Uptime_Timer.ACC (Accumulated Value)
                    // Output: Machine_Uptime_Timer.DN (Done Bit)
                    // Input: Reset_Machine_Uptime (boolean)
                    
                    IF Machine_Running THEN
                        Machine_Uptime_Timer.EN := TRUE;
                    ELSE
                        Machine_Uptime_Timer.EN := FALSE;
                    END_IF;
                    
                    IF Reset_Machine_Uptime THEN
                        Machine_Uptime_Timer.ACC := 0;
                    END_IF;
                    
                    // The PLC system handles the actual timer incrementing.

Implementing Retentive Timers in Ladder Logic

Implementing a retentive timer in ladder logic is straightforward. Most PLC programming environments provide a dedicated instruction for retentive timers (e.g., TONR in Allen-Bradley PLCs). Here's a general approach:

1. Define the Input: Identify the input signal that triggers the timing sequence.
2. Set the Preset Value: Determine the target time value that the timer needs to reach.
3. Insert the TONR Instruction: Add the retentive timer instruction to your ladder logic program.
4. Connect the Input: Connect the input signal to the timer's input terminal.
5. Set the Preset Value: Configure the timer's preset value.
6. Implement the Reset Logic: Add logic to reset the timer's accumulated value when necessary.
7. Use the Done Bit: Utilize the timer's done bit to trigger subsequent actions or events.

Visual Representation of Timer Functionality

Troubleshooting Retentive Timers

While retentive timers are powerful, they can also present challenges if not implemented correctly. Here are some common issues and troubleshooting tips:

• Timer Not Accumulating: Verify that the input signal is active and that the timer is enabled.
• Timer Not Resetting: Ensure that the reset signal is being asserted when needed. Check the reset logic for errors.
• Incorrect Preset Value: Double-check the preset value to ensure it matches the desired timing duration.
• Unexpected Behavior: Review the entire ladder logic program to identify any conflicting logic or unintended side effects.
• PLC Scan Time: Consider the PLC's scan time, especially for very short timing intervals. The timer's accuracy might be affected.

Example of Ladder Logic for a Retentive Timer with Reset