Troubleshooting ONS Activation and Deactivation in PLC Program

11-01-2025
BeepBob
10 comments
412
57

Question:

I am having trouble activating or deactivating the ONS bit in this PLC program. The ONS bit should reset when there is a change from LOW to HIGH. Despite toggling N46:38/7 and adjusting the EQU to true and false, the ONS is not responding. It seems like the ONS is not being manipulated from any other part of the program. Can anyone identify what I am overlooking?

Top Replies

To troubleshoot, verify that the specific ladder file is being accessed. Then, navigate to the data file view and choose the Usage option for that specific bit. Check if the bit is utilized at the file or word level. Keep in mind that indirect addressing may not be visible in the Usage view. A helpful tip to locate indirection in a RSLogix 5/500 program is to use the Find All feature with a square bracket in the search field "[". This can help identify any instances of indirection in the program.

12-01-2025
OkiePC

Is the issue with the latched output on rung #27 not turning on? This result would be expected to occur. The OTE on the previous rung would only be energized for a single scan, so no action would be visible there. Additionally, I noticed that your counter is using C4 (data type counter, data file #4). Typically, counters are found in data file 5 (C5) and timers are in data file 4 (T4). It is possible to delete the default data files 3-8 and utilize them as needed, although this practice is uncommon. Have you encountered this before? While not a problem, it is certainly unusual.

12-01-2025
Operaghost

It is important to ensure that the ladder logic is properly called and not located within an MCR. If you are working with Logix5000, the ONS bit should be high when the ladder is activated as shown. Additionally, double-check that no other components are modifying the entire word. Review the cross-reference diagram to confirm if the word is being utilized elsewhere.

13-01-2025
robertmee

The ONS (One Shot) function operates by turning the output on for just one scan when the input preceding it becomes true. The bit assigned to the ONS is solely for status reference and should not be used elsewhere in the program to avoid potential issues. It is important not to assign the same bit to multiple ONS functions as it may lead to incorrect operation. Remember to reset the ONS appropriately to prevent unexpected behavior in the program.

13-01-2025
GaryS

GaryS mentioned that the ONS tag should only be used for status reference and should not be accessed or utilized elsewhere in the program. It reminds me of a situation I encountered at my previous job, where a PLC used the ONS tag in XIC/XIO instructions all over the routine. Surprisingly, it had been functioning properly for a decade before I came across it. Although the code worked, troubleshooting it was quite challenging. I advise caution against attempting to replicate this setup.

14-01-2025
joseph_e2

More Replies →

Joseph_e2 shared an interesting anecdote about encountering a PLC that utilized the ONS tag in XIC/XIO instructions consistently throughout the routine at their previous workplace. Despite running smoothly for a decade, troubleshooting it proved to be a challenging task initially. It is strongly advised against attempting to replicate this setup. GaryS rightly pointed out the risk of unpredictable operations when this technique is used in other scenarios. Understanding the intricacies of XIC and XIO instructions is crucial for successful implementation, but caution should be exercised to avoid unexpected results. In teaching experiences, XIO, OTU, TOF, and ONS/OSR were commonly misunderstood concepts. If troubleshooting ONS/OSR, ensure that the subroutine is being called correctly, especially if it is located in LAD 90. By checking the JSR instruction calling U:90, you can verify its accuracy. The OTL on the rung below should indicate the success of the operation, even if the right output does not show momentarily. Furthermore, include the complete word in the XREF report to facilitate easier navigation. To simplify the process, enable the embedded cross-reference by accessing Properties in the ladder window. By selecting the appropriate checkboxes under the Cross Reference section, you can easily identify where the address is used within the program. This method proves to be more efficient than manually scrolling through an XREF report.

14-01-2025
Operaghost

The process may seem complex, but according to @Operaghost's observation, it should effectively toggle the value of N46:50/1 during each scan cycle when the PLC is running and Subroutine U:90 is activated. This occurs when the conditions are met: there are no active MCRs interfering with the subroutine evaluation, C4:0.ACC equals 1, N46:38/7 equals 1, and in the previous scan cycle, C4:0.ACC was 0 and/or N46:38/7 was 0. For alternative methods on toggling bits, check out the [Downloads] section above, then click on the [Allen Bradley] link and look for [ALTERNATOR METHODS.pdf] (not .rss). As an experiment, when C4:0.ACC and N46:38/7 both equal 1, you can manually change the value of N46:36/4 to 0 using the RSLogix (500?) IDE interface in online mode. This should trigger a toggle on N46:50/1. However, this is not recommended as a permanent solution, but rather a way to gain a better understanding of the process. If you alter the value of N46:36/4 to 0 under the stated conditions and it does not revert back to 1 immediately, it indicates that the rung is not being evaluated due to a JSR's input rung being false, an MCR, or the PLC not being in RUN mode.

15-01-2025
drbitboy

During my time at a previous job, I came across a unique situation with a PLC that utilized the ONS tag in XIC/XIO instructions throughout its programming. Despite running smoothly for a decade, troubleshooting it proved to be a bit tricky initially. I would strongly advise against attempting to replicate this setup. The ONS storage bit functions similarly to an OTE, turning on when the conditions before it are true and off when they are false. While this approach may have had some significance in the past when OTE was limited to the end of a rung, I still do not recommend using it in this manner. If memory constraints are a concern, it is advisable to upgrade to a larger processor instead.

16-01-2025
plvlce

According to plvlce, the storage aspect of an Output Enable (OTE) in a Programmable Logic Controller (PLC) operates similarly to an Output Energize (OTE) - turning on when the logic conditions are met, and turning off when they are not. While this approach may have been beneficial in the past when OTEs were only at the end of a ladder rung, it is generally not recommended now. It is suggested to invest in a more powerful processor instead of using this method, especially if memory constraints are a concern. It seems that this method was attempted in a CompactLogix system performing a straightforward task with ample resources available, perhaps in an attempt to be resourceful.

16-01-2025
joseph_e2

In a CompactLogix system with ample resources, a user observed potential unnecessary instructions in a task, speculating the programmer was trying to be clever. Putting quotes around the code snippet is a proper move. It appears that there are redundant rungs in the circuit, making it difficult for most to quickly identify it as a toggle-on-rising-edge. It is recommended that the code be rewritten for clarity and efficiency.

16-01-2025
drbitboy

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Frequently Asked Questions (FAQ)

FAQ: FAQs:

Answer: 1. How can I troubleshoot issues with activating or deactivating the ONS bit in a PLC program? - To troubleshoot ONS activation and deactivation issues, ensure that the conditions for resetting the ONS bit (e.g., change from LOW to HIGH) are correctly defined. Verify that the input toggling N46:38/7 and the EQU instruction are functioning as intended. Check for any potential conflicts or lack of manipulation of the ONS bit in other parts of the program.

FAQ: 2. What could be causing the ONS bit not to reset despite the expected conditions being met?

Answer: - The ONS bit may not be responding due to incorrect logic implementation, such as incorrect addressing, faulty conditions for resetting the ONS, or lack of manipulation from other parts of the program. Review the program logic carefully to identify any discrepancies or errors that may prevent the ONS bit from resetting as intended.

FAQ: 3. How can I ensure proper manipulation of the ONS bit in the PLC program?

Answer: - To ensure proper manipulation of the ONS bit, double-check the logic sequence involving the ONS bit and verify that all conditions for activation and deactivation are accurately defined. Confirm that the ONS bit is being correctly toggled and monitored within the program to trigger the desired reset behavior.

FAQ: 4. Are there any common pitfalls to watch out for when working with ONS activation and deactivation in PLC programming?

Answer: - Common pitfalls when working with ONS activation and deactivation include

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