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I just gave a class this morning on Unitronics/Visilogic and noticed that there is one common rookie mistake that seems to be made often with Visilogic.

His problem, as he perceived it, was that he couldn't get his V130 to accept a numeric input.

After fiddling around with his program a bit, and confirming that indeed the numeric input wasn't working, I started scanning through the ladder.

Sure enough, he had used regular contacts to invoke HMI screen calls. The numeric input wasn't working because the screen call was being initiated every scan.

This mistake seems to be common enough that it would be nice to maybe put some sort of warning at compile time if the logic sourcing the call does not contain a trasitional contact of some sort.

I do realize that there are other methods of generating a one shot outside of using transitional contacts for HMI calls, and that it would be difficult to create a rule that would exclude all possible erroneous warnings for this, but I can't think of any other simple tests you can perform with Visilogic to help find this error. I still think it is worth some kind of warning as I have seen this cost many man hours by first time users.

I also can't think of any good reason why you would ever want to call a screen in anything other than a one shot fashion. To this point, I think it should be considered to actually make the one-shot part of the call function itself. This way if someone invokes the HMI call it only invokes on the low to high transition of its enable input, and has to go from high to low again before you can re-invoke the call.

There are other functions, such as for example the TCP init bocks, that may also deserve consideration for the transition being built into the block.

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Hi Damian,

It seems like the capacity exists in Visilogic to look for these sorts of "gotchas". After all, if you try to use a CanOpen PDO or NMT, or a modbus command, the compiler checks to see if you put a buffer bit or busy bit ahead of the instruction.

I really think the simplest solution (for us users) would be something in the OS that checks what HMI is displaying an ignores additional calls to that screen. Then it wouldn't matter how you called the HMI, and the problem would go away altogether.

My two bits.

TM

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Hi Damian,

It seems like the capacity exists in Visilogic to look for these sorts of "gotchas". After all, if you try to use a CanOpen PDO or NMT, or a modbus command, the compiler checks to see if you put a buffer bit or busy bit ahead of the instruction.

I really think the simplest solution (for us users) would be something in the OS that checks what HMI is displaying an ignores additional calls to that screen. Then it wouldn't matter how you called the HMI, and the problem would go away altogether.

My two bits.

TM

Hi TM,

I agree. It would be nice to have the One-shot built in or the OS take care of it. Unless someone could come up with a rational reason how re-invoking the same screen on multiple consecutive scans (especially if it is already the current or loading screen) can be useful somehow. I've mulled it over and can't come up with one myself, but there are a lot of creative minds out there.

My fealing is, if any function block should only rationally be invoked in a one shot fashion, then build it in for us so we can't goof it up. Like the CanOpen PDO you mention...... If it forces you to have it there, then why not take care of it for us and eliminate the extra work or possible screw up.

D

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I remember getting caught by this "trap". Maybe it's Unitronics way of getting newbie's to understand positive transition contacts!

I like the idea Damian.

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