Retained (latched) bit in an unscanned rung ??

timbo_uk · Apr 27, 2009

Hi,

I am not sure here about a simple problem on an ML1100 I am working on.

I am calling a program file only during a certain part. This ladder has some fault detection that latches a bit should it detect flow failure of a pump. This program file will then not be scanned again.

If a bit is "latched" with an XIC of itself, if that part of the ladder is not scanned; then does the bit stay latched?

Hope that make sense!

OkiePC · Apr 27, 2009

When the subroutine is not scanned, the output addresses are not changed by the subroutine, even if they are OTE instructions.

I think that is what you are asking...

Paul

widelto · Apr 27, 2009

timbo_uk said:
If a bit is "latched" with an XIC of itself, if that part of the ladder is not scanned; then does the bit stay latched?

Yes, until you unlatch it. It will remain latched even if you disconnected plc from power and start it up again.

timbo_uk · Apr 27, 2009

LOL, I knew I would word it wrong!!!

Main program calls another ladder (Lad 8)
Ladder 8 runs and turns on B3:10/2
B3:10/2 is retained by XIC B3:10/2
Program returns to main ladder
Ladder 8 is not scanned again

If I check B3:10/2 in the main program, will it still be made?

The rung is below:

BST XIC B3:12/2 XIC T4:22/DN NXB XIC B3:10/2 BND OTE B3:10/2

timbo_uk · Apr 27, 2009

widelto said:
Yes, until you unlatch it. It will remain latched even if you disconnected plc from power and start it up again.

What, even if I am using an OTE not an OTL ?

gmferg · Apr 27, 2009

timbo_uk said:
What, even if I am using an OTE not an OTL ?

No, if an OTE is used, power cycle will clear the bit.

widelto · Apr 27, 2009

gmferg said:
No, if an OTE is used, power cycle will clear the bit.

Yes, I only mention OTL (latched) not OTE.

Ron Beaufort · Apr 27, 2009

Greetings timbo_uk ...

I think you've already realized that some of the confusion is the result of your terminology ...

the rung that you described is often called a "seal-in" type rung ... in contrast, the term "latched" is often reserved for a rung which makes use of an OTL instruction ...

a LOT of people (and some books) sort of blur the distinctions between the two ... there's a little bit of discussion in the following video that might be helpful ...

http://www.youtube.com/watch?v=KraWAA_i9GM

based on the way asked your question, I'd bet that the part about "prescan" is likely to be very interesting to you ...

PeterW · Apr 27, 2009

In the UK the term should be 'retained' not 'latched'. As others have said, latched means using a latch instruction.

Ron Beaufort · Apr 27, 2009

In the UK the term should be “retained” not “latched”.

well, I certainly won't argue - but some people get confused with THAT terminology too ... consider that Allen-Bradley officially describes the OTL (LATCH) instruction as being “retentive” – as in it can “RETAIN” its ON status even after a power cycle ...

let’s face it – there are plenty of “mistakes and misconceptions” about this stuff – no matter how hard we try to nail down the specifics ... the best possible policy is to understand WHAT happens – and WHY it happens – and then do the best that we can to describe the facts in non-ambiguous ways ...

six eye-witnesses to a robbery ... six totally different descriptions of what the robber looked like ... that’s just human nature ...

timbo_uk · Apr 28, 2009

Nice discusion building here

I cannot get onto utube here at work, not being cheeky, but does the video answer my original question? Or could somebody?

(PeterW, I just noticed I used "latched" in my first post and "retained" in my second one!)

Cheers

OkiePC · Apr 28, 2009

timbo_uk said:
Or could somebody?...
...answer my original question?

The rung you posted, if true during the last scan of the subroutine, would remain true when the subroutine is not being scanned.

Paul

timbo_uk · Apr 28, 2009

OkiePC said:
The rung you posted, if true during the last scan of the subroutine, would remain true when the subroutine is not being scanned.

Paul

Cheers Paul.

(You answered while I lashed up my mobile phone via a bluetooth dongle to an old PC in the workshop to get a look at Rons EXCELLENT u-tube video)

Ron Beaufort · Apr 28, 2009

quite a few people have reported that they can't access YouTube at work ... try going through the Sample Lessons page on my website ... I've embedded all eleven videos there - which seems to make them more accessable to people with YouTube restrictions ... please let me know if you're still having problems ...

timbo_uk · Apr 28, 2009

Ron Beaufort said:
quite a few people have reported that they can't access YouTube at work ... try going through the Sample Lessons page on my website ... I've embedded all eleven videos there - which seems to make them more accessable to people with YouTube restrictions ... please let me know if you're still having problems ...

Thanks Ron, but the actual videos are blocked. Our company uses a (actually very good from a company point of view) security proxy server from Websense

this strips the access to the file.

How big are the actual files? What about getting people to email you if they cannot access them, then give them a link to a directory on your server that has a "robots.txt" file to stop it being searched? Not sure how this could affect your server bandwidth charges.

I did manage to see the OTE vs OTL video (via my lash-up) and was very impressed with the clarity and content; very proffesional!

Retained (latched) bit in an unscanned rung ??

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