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Old December 24th, 2008, 06:20 AM   #1
michael childers
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Exclamation unlatch command in allen bradley

can you use a unlatch command in a program without using
the latch command?
looking at some programs at work it seems that it was
common with who ever created the programs to use a unlatch
command in the end of the rung with the ote.
----:1:-----------------------{}-----ote
{}unlatch1
trying to hook up a plc for experiments and I could not
see the data file change back to a 0.
so does that mean it doesnt work?
Thanks Mike
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Old December 24th, 2008, 06:48 AM   #2
LJBMatt
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The unlatch command will work with OTE's.
But remember that if the OTE isn't in a seal condition with the bit in the seal it will stay high during the next scan and you probably won't see it go low. Just keep that in mind during your ladder execution.
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Old December 24th, 2008, 07:04 AM   #3
michael childers
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example
lets say N:7/1 turns on 2:/1 and the unlatch command is on the same
rung is N:7/1 in parallel with the ote 2:/1.
just didnt know if this really unlatched when I saw this written in
a program like this.
I thought that was the purpose of latch and unlatch working together.
so its ok to use one without the other?
Thankksssss
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Old December 24th, 2008, 07:05 AM   #4
bernie_carlton
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I use an unlatch in the PLC program for bits from an HMI. Momentary buttons from an HMI depend on two communication events, a turn on and a turn off. I use just a 'turn on' type button. In the PLC program I perform the indicated task then unlatch the bit.
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Old December 24th, 2008, 07:13 AM   #5
michael childers
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still learning and could not see it change in the program.
Thanks
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Old December 24th, 2008, 08:02 AM   #6
milldrone
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Quote:
Originally Posted by michael childers View Post
still learning and could not see it change in the program.
Thanks
Ahh! Then perhaps some of this will help. http://www.plctalk.net/qanda/showthr...&highlight=ote Especially Ron's post #11.

If your up for a little reading over the holidays. Search advanced, ote in the search window and Ron Beaufort as the user name
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Old December 24th, 2008, 12:31 PM   #7
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To make a long story short: OTU writes a 0 to the address when the rung is true, so anytime you want to program IF CONDTION=TRUE THEN BIT := 0 then OTU is one way to do it.
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Old December 24th, 2008, 01:33 PM   #8
michael childers
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I dont always say what I want or explain things the way I should so I guess thats why I am not a very good teacher myself.Thanks for all the responses.I never did get a straight yes or no if you could use the unlatch command without ever using the latch command?
The intergal xio turns on a ote and the same intergal in the same rung is turned off with the ote command in parallel.
Mike
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Old December 24th, 2008, 01:36 PM   #9
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------[]------------===0te
n:7/1 branch-[u]n:7/1
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Old December 24th, 2008, 01:38 PM   #10
TConnolly
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Straight answer: Yes you can use OTU without OTL - they are not devices, they are computer instructions and a computer takes what it is instructed to do quite literally and carries it out.
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Old December 24th, 2008, 07:38 PM   #11
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Well put Alaric...
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Old December 25th, 2008, 07:42 AM   #12
michael childers
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Thank you.
And Merry Christmas.
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Old December 25th, 2008, 08:00 AM   #13
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Showing my Age Again

The use of an Unlatch in a program actually resembles a function you used to see on the TI PLC's.

The TI PLC's had an inverted output. What this ment was an Output was turned off whenrever the rung conditions became true. The unlatch comand in AB PLC's doses the same thing.

Most common use I remeber for it was to daisey chain drum timers together. This alllowed you to "jump" from one drum timer to the next and deactivate the previous drum timer while limiting the amount of code you used.

Another common use for the unlatch from my memeory was to act as a strobe for a parallel input text display. The strobing of the bit would allow you to setup your bits then send them to the screen.

Anyway that is the pratical uses I know of.
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Old December 25th, 2008, 08:33 AM   #14
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To expand on Alaric's reply:

OTE - IF condition is TRUE THEN output equals TRUE ELSE output equals FALSE

OTL - IF condition is TRUE THEN output equals TRUE


OTE - IF condition is TRUE THEN output equals FALSE
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Old December 25th, 2008, 09:32 AM   #15
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The OTL/OTU will fool you with an AB. When the bit is "on" you will see both the OTL and OTU bits "green". It looks like the OTU is on, but it isn't. You have to look at the rung logic to see what you have for an output. Usually "green" = output, when the bit is off, no green.
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