One-Shot Latch equivalent in FBD (RSLOGIX 5000)

I would like to be able to set a bit on with logic, and then be able to turn it off somewhere else in the code. This is really easy in ladder:

|---|XIC|---|ONS|------------(OTL)---|

     TAG1                     TAG2

TAG1 can be used to set TAG2, however, tag two can be unlatched elsewhere in the code.

I'm struggling to achieve the same in a Function Block Diagram.

In FBD, the block attached to the OREF for TAG2 will constantly stay keep the tag turned on.

I'm not experienced with FBD, so I'm hoping I'm just missing something simple.

Please show what you have attempted.

Not sure about Rockwell but you need to use a "Positive Transition" or "Trigger".
if input true ---> P_Trigger
P_Trigger.q -----> SET (whatever your latch is).
This can also be combined in a single network (rung).

The answers are already posted on this site.
It is a mater of searching.

Not just on this site, the exact instructions are listed right in the RSLogix Help. Help | Instruction Help. Under Bit Instructions if you can't find it from the Help file.

I have tried searching. I have looked through manuals etc.

If the answer is using something like a rising edge trigger into a set/reset block, it doesn't work as I need it to. I need to be able to ddirectly turn on/off TAG2 on other parts of the program. The output of the set/reset block holds the value and writes to the OREF for TAG2...

... I'll post some code tomorrow...

snoochems said:

I have tried searching. I have looked through manuals etc.

If the answer is using something like a rising edge trigger into a set/reset block, it doesn't work as I need it to. I need to be able to ddirectly turn on/off TAG2 on other parts of the program. The output of the set/reset block holds the value and writes to the OREF for TAG2...

... I'll post some code tomorrow...

Click to expand...

And why doesn't Set/Reset work?
You can still turn them on/off from elsewhere.
Why must you use FBD for this anyway? If you have perfectly valid LAD logic, why not use that?

this might not be exactly what you're looking for - but maybe the basic ideas will help ...

I don't mind telling you - I don't think that this is necessarily a "good" idea ... keep in mind that the "controlled" bit (RETENTIVE_BIT in this example) will be "retentive" ... that means that whatever you're controlling can come back ON AGAIN whenever the processor starts up (after a power failure for example) ... this could fall into the "bad" column of life's experiences ...

depending on what you're trying to accomplish, you might want to consider adding some "one-shots" (OSRI, etc.) into your code also ...

I used the Input and Output Wire Connector elements to give you the "turn it off from other places" operation that you asked for ...

also read up on the SETD (Set Dominant) instruction ... basically if the "START" button and a "STOP" button are BOTH pushed at the same time, which one of those do you want to "win" and be in control of the output? ... I chose "STOP" for my example ...

also ... PLEASE consider what this type of code is going to look like to Gomer and Goober when they can't get the pump to run on nightshift ... if it takes this much effort to "write" – how much effort (and TIME $$$) is it going to take to "figure it out" at 3:00 o'clock in the morning when the pump won't run? ... this looks like "bad ju-ju" – but I think that it works for what you were asking for ...

frankly – I'd just use Ladder Logic and get on with my life ...

good luck with your project ...

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