Reset Instruction (RES) and Resets a retentive timer (RTO)

Understanding the Retentive Timer (RTO)

A Retentive Timer, or RTO, is a type of timer instruction commonly found in PLC programming. Its primary characteristic is its ability to "remember" the elapsed time even when the input signal is interrupted or power is lost. This contrasts with a standard timer-on delay (TON) or timer-off delay (TOF), which resets their accumulated value (ACC) under the same circumstances. The RTO timer continues counting from where it left off once the input signal is re-established, making it ideal for applications where cumulative timing is essential.

The RTO instruction typically has three key parameters:

• Timer Name: A unique identifier for the timer instance.
• Preset Value (PRE): The target time value the timer must reach to trigger the Done (DN) bit.
• Accumulated Value (ACC): The current elapsed time, which is retained even when the input is off.

The Role of the Reset Instruction (RES) in PLC Timers

The Reset Instruction (RES) in PLC programming is specifically designed to clear the Accumulated Value (ACC) of a retentive timer (RTO). When the RES instruction is executed, it forces the ACC of the specified RTO timer to zero, regardless of its current value. This effectively resets the timer, allowing it to start counting from zero again when the input signal is active. Crucially, the RES instruction also resets the timer's status bits, such as the Enable (EN), Timer Timing (TT), and Done (DN) bits, back to their initial inactive states.

The RES instruction is essential because, without it, an RTO timer would continue to accumulate time indefinitely, potentially leading to incorrect or unexpected behavior in the controlled process. By providing a means to explicitly reset the timer, the RES instruction ensures that the RTO can be reused for subsequent timing cycles without carrying over previous accumulated values.

Example Scenario: Using RES with an RTO

Let's illustrate the use of the RES instruction with a practical example. Imagine a machine requires a total run time of 60 minutes to complete a specific process. We can use an RTO to track the cumulative runtime, even if the machine is stopped and started multiple times. A PLC Reset Instruction is then used to start the timer from the beginning when a new batch is ready.

Here's how the RES instruction and RTO timer would work together:

1. An input signal energizes the RTO timer.
2. The timer starts counting, accumulating time in the ACC.
3. If the input signal is interrupted (e.g., the machine is stopped), the timer holds the ACC value.
4. When the input signal is re-established (e.g., the machine is restarted), the timer continues counting from the held ACC value.
5. Once the ACC reaches the PRE (60 minutes in this case), the DN bit is set, indicating the process is complete.
6. To start a new process, a RES instruction is triggered, resetting the ACC to zero and deactivating the DN bit, preparing the timer for the next cycle.

The following SVG diagram illustrates the timing behavior of an RTO timer with and without the RES instruction:

In this diagram, the blue line represents an RTO timer accumulating value until the preset is reached. The green line shows the effect of the PLC Reset Instruction resetting the timer back to zero after intervals.

Difference Between RTO and RES in PLC

It's crucial to understand the distinct roles of the RTO timer and the RES instruction. The RTO timer is responsible for accumulating time, retaining the accumulated value even when the input signal is interrupted. The RES instruction, on the other hand, is solely responsible for resetting the ACC of the RTO timer to zero. They are two separate but complementary instructions that work together to achieve controlled timing behavior.

Here's a table summarizing the key differences:

Best Practices for Using RES Instructions

To ensure the correct operation and maintainability of your PLC programs, consider these best practices when using RES instructions:

• Clearly Label RES Instructions: Use descriptive labels that indicate which RTO timer is being reset.
• Use RES Sparingly: Only reset the RTO timer when necessary to avoid unintended resets.
• Consider Interlocks: Implement interlocks to prevent the RES instruction from being triggered accidentally.
• Document Your Logic: Clearly document the purpose of each RES instruction in your PLC program.

Advanced Applications of RTO and RES Instructions

Beyond simple timing applications, the combination of RTO and RES instructions can be used in more complex scenarios. For example, you can implement a maintenance schedule based on cumulative machine runtime. The RTO tracks the total operating hours, and when a predefined threshold is reached, a maintenance flag is set. The RES instruction is then triggered after the maintenance is completed, resetting the timer for the next interval.

Another advanced application is in batch processing, where the RTO timer tracks the total processing time for a batch, and the RES instruction is used to reset the timer at the start of each new batch. This allows for accurate tracking of processing times and ensures consistent product quality.

The following SVG diagram illustrates the state transitions of an RTO timer based on input and RES instructions:

This diagram shows the state transition between idle and timing modes based on input and reset (RES) conditions.