Laader Logic Question

Hi my friend,
In the following network, can you tell me when the one-shot coil is on? The register 40005 is the "RTU second". as you see in first photo, both registers 40005 and 40065 are equal but the coil is on. in the second photo, both registers are still equal but the coil is not on. during one minute interval, sometimes the coil become on more than once. but sometimes only once and this will happen in various seconds not a specific second.
it is very confusing

Thanks in advance for your time.

It is very unlikely that you will be able to observe consistently the valid on or off states. Communications between your programming device and the PLC typically take place at the end of a scan. This may vary depending on the particular PLC. For verification of proper operation add a counter (or an add of 1 to a register) on a rung just after the move command. Trigger the counter or add instruction with your one-shot bit. See if the count rises as you would expect.

bernie_carlton said:

It is very unlikely that you will be able to observe consistently the valid on or off states. Communications between your programming device and the PLC typically take place at the end of a scan. This may vary depending on the particular PLC. For verification of proper operation add a counter (or an add of 1 to a register) on a rung just after the move command. Trigger the counter or add instruction with your one-shot bit. See if the count rises as you would expect.

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Thank you so much Bernie,
I did what you said and now it is clear that this logic is a 1 second pulse generator. the value of the counter increases every second.
It was very helpful. I really appreciate it.

bernie_carlton said:

It is very unlikely that you will be able to observe consistently the valid on or off states. Communications between your programming device and the PLC typically take place at the end of a scan. This may vary depending on the particular PLC. For verification of proper operation add a counter (or an add of 1 to a register) on a rung just after the move command. Trigger the counter or add instruction with your one-shot bit. See if the count rises as you would expect.

Click to expand...

This is precisely why I avoid using one shots.

Tom Jenkins said:

This is precisely why I avoid using one shots.

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Thanks Tom,

What do you suggest? What I can do to avoid one shot in this network?

What kind of logic is going on in these instructions? I have not see it before.

sparkie said:

What kind of logic is going on in these instructions? I have not see it before.

Click to expand...

The one shot is used to calculate the average of a well pressure each second. Every second that one shot is activated, it enables the average function.

sayahan2003 said:

The one shot is used to calculate the average of a well pressure each second. Every second that one shot is activated, it enables the average function.

Click to expand...

I mean the graphical representation of it.

sparkie said:

I mean the graphical representation of it.

Click to expand...

Not sure if I got your question but this ladder logic is in Telepace software for Scadapack.

In general while monitoring the system do not expect to see accurately information (like a one shot) which changes very quickly. If you are using data views as well as observing information directly on the ladder you may be observing data from different scans. You will need to think like the PLC and develop strategies, like the counter I suggested and which you used. Use a latch to verify rare quick signals.

By the way, does the one second count roll over? You may be losing a calculation of the average each rollover. You may want to OR the < and > outputs of the compare to your one shot coil.

To sparkie, after you have seen many ladder representations you tend to adapt quickly to other formats. I hadn't seen this either but it was similar enough to get the idea (a compare of current and former scan values yielding a '< = >' evaluation followed by a move of current to the previous scan register.)

bernie_carlton said:

In general while monitoring the system do not expect to see accurately information (like a one shot) which changes very quickly. If you are using data views as well as observing information directly on the ladder you may be observing data from different scans. You will need to think like the PLC and develop strategies, like the counter I suggested and which you used. Use a latch to verify rare quick signals.

To sparkie, after you have seen many ladder representations you tend to adapt quickly to other formats. I hadn't seen this either but it was similar enough to get the idea (a compare of current and former scan values yielding a '< = >' evaluation followed by a move of current to the previous scan register.)

Click to expand...

Thanks Bernie,
I like your idea of "think like the PLC".

I usually use a timer to set a coil with a holding contact ahead of the rungs being controlled. Then, after the operation, I use a timer done bit to release the holding coil. I think I have some sample logic - I'll dig it up and post it when I get back to the office.

Tom Jenkins said:

I usually use a timer to set a coil with a holding contact ahead of the rungs being controlled. Then, after the operation, I use a timer done bit to release the holding coil. I think I have some sample logic - I'll dig it up and post it when I get back to the office.

Click to expand...

Awesome! Thanks Tom,

Didn't find what I was looking for, but here is an example of using a timer for a periodic task without a one-shot.