Input Mapping Issue

[sutton]

Dear All,

I am currently facing a simple input mapping logic issue.
Basically, the input bit function well from the sensor and it trigger average at 300msec.

But sometimes we get some alarm due to timer watchdog.
Please find the attached screen shots.
Periodic task is used for this input mapping program.

I dont understand why i am getting pulse missing in the program tag while the input tag works well. i have tried with new tag and the same problem happen.

Hope anyone can advice.

Thanks a lot.

REgards,
Jason

 

[cjd1965]

Silly question.... but..Is the periodic task running when the sensor is triggering? That would be my guess

 

[dmroeder]

It appears that the routine that the logic is in is not being scanned.

 

[nehpets]

As a RSLogix500 programmer (just getting into RSLogix5000)I'm not sure if I am barking up the wrong tree, but the ONS appears to be an element of an array, is something writing into this array and effecting the operation of the ONS, please be my guest and shoot me down and correct me as this tag base system is proving a bit of a headache.

Steve

 

[sutton]

The routine is running with production. All the other I/O Points & program are running without any issue.

There are only two of these sensors that trigger much faster than any of other discrete IO points. It is running at 300msec average. The other one of this sensor works really fine.

I had tried to create a new tag OTE to this input sensor as well. The trend i received is the same.
I tried putting this rung to other task routine as well and still i am getting the same trending.

I really find this a bizzare case. hope anyone would have such experience please share your idea.
Thanks a lot.

 

[cjd1965]

Another idea... the ONS is relying on the input to go OFF before it will see the next ON and generate the one shot pulse. Does your periodic cycle time allow for that?

Have you tried making it continuous logic and see the result change?

 

[daba]

Ok, sutton, I can see a couple of things that could be producing the missed pulses. Answer the questions(?) I raise, and I can help you troubleshoot this.

1. You said the pulses were 300mS - and from the first trend I can see 9 pulses in just over 2 seconds - a quick bit of mental arithmetic - 270mS.

Now that is the time period between pulses. How long is each pulse ON for ? It is hard to tell from the trend, but I estimate 100mS

The logic has to see ALL of the ONs and OFFs of every pulse to work correctly, so what is the setting for the periodic task that looks at the input? It needs to be fast enough to "capture" the ONs of the pulses, so to guarantee that it sees it, it should be set to a faster period than the pulse ON time, whatever that is.

2. I can see the application has a lot of periodic tasks. Does the task that is capturing the pulses have a high enough priority? It could be being interrupted by another task with a higher priority.

3. What is the RPI value of the input module? The input tags have to be updated fast enough so that the periodic task can see the inputs ON and OFF.

4. If the input module supports digital filtering, is this enabled and set too high?


Seeing input pulses correctly relies on all of the above. A lot of the problems go away if you use an Event Task, instead of a Periodic task. The Event task can be triggered by a change of state of your inputs, both Off-to-On, and On-to-Off. The Input Data State Change overrides the modules RPI setting. It gets more complicated if the input module is in a remote chassis.

 

[daba]

I really find this a bizzare case. hope anyone would have such experience please share your idea.

I don't find it a bizarre case, I have such experience, and I share my idea - too bad sutton hasn't responded...

 

[sutton]

Hi Daba,

Thank you so much for your sharing.
I dont feel that bizarre after all with your guidance.

Here are the finding for your troubleshooting question:
1. the Periodic task is set at 50msec.
2. The input tag is program in the task of priority no.2. There are 2/7 task is set as no.2, 1/7 is set as no.1, 1/7 is set as no.4, and 4 others being set as no.10. I dont understand how priority works here. I will read some info from techconnect later.
3. The RPI value is 10msec and this is a local module.
4. The digital filtering is set as 1msec. Again i dont understand how this works. Another homeworks to do for this.


Hmm.. i dont think i can change to event task with production runnning 24/7. I can only wait for shutdown to perform a download.

I just noticed something different. The other production line with similar program & machine setup doesnt has this issue. i just checked it doesnt use "enable change state". All the points were being unchecked. Does it tell the whole story. I am not sure if i can unchecked this one at the moment. Probably i will do that with shutdown time.

The other short term solution i can think of is creating new tag with latch and unlatch to the input tag. Not sure if this can temporary solve the issue while waiting for the shutdown. I will try this on wednesday when i get to the plant.

Thanks a lot for your valuable sharing.

Best regards,
sutton

 

[sutton]

Hi Daba,

Here are more finding for troubleshooting works i have done.

1. I have put in the local input tag to map to a temporary tag in the period task with priority 1. The result is same still with miss pulse.

2. I have tried to latch and unlatch with another output tag to this input. I even tried with a timer to unlatch the bit. Again, miss pulse remain. So, we are clear this is miss scanning issue.

Looking at your suggestion, is it the only solution to use event task?
Or is there another method to solve this, ie. change the digital filter time.
/ disable state change?

Many thanks for your help.
Regards,
sutton.

 

[jdbrandt]

I guess I don't understand why the OP would NOT use an EVENT task, triggered by the input, and do all the processing in the associated routine for the Task's Main Program... seems straight forward... It's 1 1756- processor, and the I/O is local.
Then, just play with the RPI of the module until it works.

 

[sutton]

Hi Daba,

I have solve the miss pulses issue thanks so much to your advices.
Basically i just set the input filter longer for that module and it works perfectly.

Many big thank you for all your sharing.
I really learnt a big lesson here.
Thank you.

Best regards,
sutton