Hidden Output

lombac55

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Nov 2020
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Australia
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Hi, I'm a bit new to this and this is my first post, so I hope I'm doing this right.
I have an Allen Bradley Family 5 PLC in our plant and I am trying to fault find an issue. I've attached an image of what I need help with. I am looking for the highlighted bit B27:0/3 as an output, however whenever I do a search on this bit, and multiple others in this program it only comes up with input instructions, no outputs. Can someone tell me where I can find what controls these bits as this is the first time I have come across this.
Grateful for any guidance on this issue.

Program.PNG
 
Thanks for the quick reply.
I was searching for the actual bit.
There is an HMI panel but there is another instruction bit that is labelled as coming from the HMI when the machine is in manual mode. The bit I'm looking for is used in the Auto mode and I assume would bypass the HMI. Am I wrong to assume this or could it still be coming out of the HMI?
 
The reason I asked about searching for the entire B27:0 word is because a line in the program writing to the entire word might not show up in a search for just the individual bit. I say "might not" instead of "would not" due to my lack of familiarity with the details of RSLogix's search function.
The name of the bit "HEF_ST3" suggests it might mean Hoist End Fence Step 3 which may or may not be coincident with bit 3 of word B28:0.
The bit I'm looking for is used in the Auto mode and I assume would bypass the HMI. Am I wrong to assume this or could it still be coming out of the HMI?
Click to expand...
It's not a bad assumption, especially if the original programmer chose to label the bits coming from the HMI. But it's always better to make sure rather than to assume.
 
Just in case you need it
B27 is the file name
0 is the word
and 3 in the bit in that word
without having all the information ? program
i would thig it's used as input command / Bottom on a HMI
the operator activates a button on the HMI the the B27:0. is linked to that HMI button
it looks like to Lower the Fence
it's common to use a separate file for the HMI interface it helps to identify then in program
 
Be sure to do a Cross Reference of B27:0/3 instead of Find All. As you can see in the 2 attached images, the find all just finds the XIC but the cross reference points to a move instruction that can also act upon B27:0/3. If you don't find anything like that in cross reference surely it is coming from HMI.

0105211144.jpg
0105211144a.jpg
 
You can also look for a Bit Shift Left(or Right) that may be energizing that bit. As Bob said, search on the entire Word. Worst case scenario, someone is using a Message Write from another PLC to set that bit, in which case, unless you know which one would write to this one, you may never find it.
 
In RSLogix, do the following:
  1. Right-click on a blank part of the ladder window and select Properties
  2. Click on the Address Display tab
  3. Check the box for Display Output Cross Reference for Inputs on Rung and click OK.

Any instruction affecting that bit (or the entire word) will be identified. Underneath the instruction you will see a yellow box with the file and rung number where that bit is affected. Click on that number and it will jump to the location.

If a programmer is really trying, they can hide this through indirect addressing. If it does not show up here, then look at the HMI or other network devices writing to that bit.

OG
 
Steve Bailey said:
The reason I asked about searching for the entire B27:0 word is because a line in the program writing to the entire word might not show up in a search for just the individual bit. I say "might not" instead of "would not" due to my lack of familiarity with the details of RSLogix's search function.
Click to expand...

Good instincts. You are 100% correct Steve. Searching the bit only finds that specific bit. Search for the word and it will find each bit within that word.

OG
 
Operaghost said:
Good instincts. You are 100% correct Steve. Searching the bit only finds that specific bit. Search for the word and it will find each bit within that word.

OG
Click to expand...

I always had a problem with that in RSL 5 & 500,

Click on XIC T4:0/DN and it only finds the usage of the DN bit.

I like Studio500 there it shows the timer instruction as well as all the other bits used, as the others should be.
 
I_Automation said:
I always had a problem with that in RSL 5 & 500,

Click on XIC T4:0/DN and it only finds the usage of the DN bit.

I like Studio500 there it shows the timer instruction as well as all the other bits used, as the others should be.
Click to expand...

I am not referring to the Cross Reference report. That does what you said. The embedded cross reference I described works differently.

If you have a timer done bit, the file:rung displayed below would show the location of the timer. If you also had a RESET, then it would show two locations. One for the timer and one for the reset.

It is a great feature that I used extensively back in the day. It is similar to the Studio 5000 but embedded into the display.

You can also select the option to turn on the Output Cross Reference. That will show underneath an output all the locations where it is used as an input. For example for a timer it would show everywhere the status bits (or the PRE or ACC) are used as inputs. This particular option takes a lot of screen space so I don't typically turn this on. Example below is from an SLC but this works the same for PLC-5.

OG

PLC Xref.PNG
 
Last edited:

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