Rslogix 500 Integers

[p0zburn55]

I'm looking at program for a air compressor (SLC 04). I'm trying to figure out why I can't change a setpoint that's located in a N: address. The program allows me to change it for split second but something is rewriting it back to a maximum setpoint. I've tried using cross reference but the results are all over the program didn't make much sense. Is there a another tool I could use to find what's writing to it?
Thanks

 

[plvlce]

As you have noticed, something in the program is writing a value to the address.


There is no magic bullet that will somehow give you the answer, but the cross-reference and access to the online program will allow you to get there yourself.


Start by eliminating anything that does not have your address as the destination of an output instruction (eg MOV, COP)


If there are multiple such instructions, see which of them is on a currently true rung. Additionally, if it is a simple MOV or COP, the correct instruction should have the value you see overwriting your value in the source, either in a different integer address or as a constant.

 

[muusic_man]

Does this compressor have any kind of operator interface. If so, it could be writing the value to your integer.

 

[Operaghost]

An easier way to use cross-reference

1. Open the project
2. Right-click in a blank section on the ladder display
3. Select Properties
4. Click on the Address Display tab along the top of that window
5. Check the box for "Display Cross Reference for Outputs on Rung" and click OK

This is a computer setting so it will continue to use this for all projects until you reset it.

Now go to the location where you were trying to change the value. Underneath the instruction you should see in yellow the address, the type of instruction, and the file:rung number where it is being used (ie..N7:0 - MOV - 2:5). If you click on the file:rung it will jump to that location. And from that location you can jump back to where you were.

At that location check to see if that address is being used as the "destination". If so, then that instruction is writing to your address. If it isn't try the other locations if there are more.

If there isn't more, then it is likely that a device is writing to that address over a network. Like the HMI muusic_man suggested.

 

[p0zburn55]

Does this compressor have any kind of operator interface. If so, it could be writing the value to your integer.

It has a panelview 600 but i have to unplug that port for my rs485 adapter to talk to the plc

 

[p0zburn55]

An easier way to use cross-reference

1. Open the project
2. Right-click in a blank section on the ladder display
3. Select Properties
4. Click on the Address Display tab along the top of that window
5. Check the box for "Display Cross Reference for Outputs on Rung" and click OK

This is a computer setting so it will continue to use this for all projects until you reset it.

Now go to the location where you were trying to change the value. Underneath the instruction you should see in yellow the address, the type of instruction, and the file:rung number where it is being used (ie..N7:0 - MOV - 2:5). If you click on the file:rung it will jump to that location. And from that location you can jump back to where you were.

At that location check to see if that address is being used as the "destination". If so, then that instruction is writing to your address. If it isn't try the other locations if there are more.

If there isn't more, then it is likely that a device is writing to that address over a network. Like the HMI muusic_man suggested.

Wow thanks for the info I will definitely try that.

 

[Operaghost]

It has a panelview 600 but i have to unplug that port for my rs485 adapter to talk to the plc

That's good. Since you see this issue when the PV is unplugged then we've eliminated an external device as the problem. That means we can find it within the logic. And cross reference is the key.

However...there is one more possibility that cross-reference won't be able to help us with. It is possible that your logic includes something called "indirect addressing". This is where a portion of the address can be varied. If you look through your logic and notice any addresses that look something like this:

N7:[C5:0.ACC]

The square brackets with an address inside them are the indicator for indirect addressing. The address inside the brackets is often referred to as a pointer. Whatever the current value of the pointer gets plugged inside the brackets. If my counter accumulator is 5 then that address becomes N7:5. When the counter goes to 6 then it becomes N7:6. This allows me to, for example, write a value to an address and then the next time write to the next address and then the next, all with just one instruction. Cross-reference in RSLogix 500 cannot account for this variable addressing (the newer RSLogix/Studio 5000 for the Logix 5000 controllers can identify this).

If you still cannot locate the source of the mysterious overwrite issue, post your logic here and we can help locate it. Or if you don't want to post it openly, you can send it via a direct message. I'd be happy to take a look.

OG

 

[Nova5]

Besides the HMI, are there physical operator controls that could be sending it in via analog and a MOV instruction or similar moving entire card's values to set series of dints? I've not tried it with analog -> dints, works with discrete -> Binarys.

 

[James Mcquade]

as already mentioned, something is writing to that register.
move, copy, file fill, indirect addressing, messaging, comparison instructions, and block transfer from another plc can be the culprit.
hope this helps,
james

 

[bernie_carlton]

The SLC5/04 could have a DH+ connection. Check for any other cables plugged in to the front of the processor.

 

[roxusa]

This is a good example of where good descriptors are helpful in a program. Knowing someone else will eventually be trying to figure out the code written. I also try to only use Read messages between devices so I can see where an integer is coming from.