Is there an easy way to get an "Always On" tag in Logix5000?

defcon.klaxon · Dec 9, 2019

Hey guys,

I need to have a boolean in Logix5000 that's always on. In other PLCs I've programmed (like GE, for example) there is a system tag that is always on. I'm not seeing anything like that in Logix5000. I was thinking I'd just get a coil on a rail with no contact in front of it, is that an acceptable way of doing things or is there a reason why that isn't best practice? Thanks!

chavak · Dec 10, 2019

defcon.klaxon said:
Hey guys,

I was thinking I'd just get a coil on a rail with no contact in front of it,

Yes, that's the way in 5K

AustralIan · Dec 10, 2019

That's the old way, but I guess it isn't true between prescan and the first scan of the rung.
Better is a SET coil which isn't reset in prescan.
Best is a Boolean constant set to true.

defcon.klaxon · Dec 10, 2019

AustralIan said:
That's the old way, but I guess it isn't true between prescan and the first scan of the rung.
Better is a SET coil which isn't reset in prescan.
Best is a Boolean constant set to true.

Thanks for the input. This isn't a time critical process, so if it's not true for the first scan it doesn't matter. Basically I created an AOI that lets a pump run at a certain time of day, start and stop times based on hours and minutes. Recently the client asked if I could provide an enable/disable function for certain Times of Day, so I added an enable bit to the code. The problem is, only one set of pumps needs the enable/disable, the rest of the pumps that use the same code don't need the enable/disable. So as I change my TOD code, most of the pumps/aerators that use the AOI don't need the enable/disable bit but if I want to my AOIs to be identical across the board, I need to give an always on bit for the pumps/aerators that don't care. So I guess I could create a second Time of Day AOI for the one set of pumps that need the enable/disable, now that I think about it.

defcon.klaxon · Dec 10, 2019

chavak said:
Yes, that's the way in 5K

Thanks for the confirmation!

rdrast · Dec 10, 2019

AustralIan has the correct answer if you need an actual always on bit... Create a boolean constant set to one, and set external access to None or Read Only. You cannot write to it in logic, BUT you can still change it from the tag editor. If it is absolutely critical that it always be true, I'd probably create a routine to trigger a hard fault if it is ever zero and shutdown the CPU.

Using a plain tag latched on in the program is not guaranteed to be in any state, as you can potentially change its value anywhere in the scan.

Aabeck · Dec 10, 2019

For troubleshooting and future use I always add a OTL "True" to the program, and for SLC500's platforms it was OTL B3:0/1 "TRUE" along with OTU B3:0/0 for an "AFI" bit.

For other platforms (Mitsubishi, Omron, Idec, etc) I did similar too.

JZerb · Dec 10, 2019

Aabeck said:
For troubleshooting and future use I always add a OTL "True" to the program, and for SLC500's platforms it was OTL B3:0/1 "TRUE" along with OTU B3:0/0 for an "AFI" bit.

For other platforms (Mitsubishi, Omron, Idec, etc) I did similar too.

Same. In Studio 5000 first rung of the program Ill have an OTL and and OTU for an 'Always True' and 'Always False' bit for troubleshooting purposes.

saultgeorge · Dec 10, 2019

My old boss did exactly what you did, but he added what data type it was; that is if was a boolean, then the tag would be "balways_off", a DINT tag would be "dalways_off", etc. I still use it to this day. One job I had wanted place holders with a DINT data type and the tag name "ZZZ.xx" and then we could put a descriptor as to what exactly was needed if it was a little unclear.

jkerekes · Dec 10, 2019

Do you need more than one "always-on" bit? You can also use a NOP instruction.

James Mcquade · Dec 10, 2019

jkerekes,

I disagree with having more then 1 always on/off bits.
I had a 4 station machine with each station doing the same thing.
14 always on bits and 12 off bits were used, but the contacts were toggled, not in a rung. we don't know why the oem left it like this.

got a call at 1 am and had to go into the plant, one of the bits got turned off and it took most of the night and the rest of the day to figure out the program and eliminate these bits.

since that time, only 1 always on and 1 always off bit is allowed AND it must be the first rung of logic in the program.

james

jkerekes · Dec 10, 2019

James Mcquade said:
jkerekes,

I disagree with having more then 1 always on/off bits.
I had a 4 station machine with each station doing the same thing.
14 always on bits and 12 off bits were used, but the contacts were toggled, not in a rung. we don't know why the oem left it like this.

got a call at 1 am and had to go into the plant, one of the bits got turned off and it took most of the night and the rest of the day to figure out the program and eliminate these bits.

since that time, only 1 always on and 1 always off bit is allowed AND it must be the first rung of logic in the program.

james

I agree. I was just curious if he needed more than one.

Aabeck · Dec 10, 2019

As for other tags, I usually add a DINT called ZERO (=0), a REAL called Pi (=3.1415927), and depending on the project other set tags that commonly get used.

Some platforms don't have a FirstScan bit, so I create one

And if the platform doesn't support a NOP I label a bit NOP.

dmroeder · Dec 10, 2019

We typically have a rung like this:

BST OTE True NXB XIO True OTE False BND

JaxGTO · Dec 10, 2019

I just create a BOOL tag called AlwaysOn. Edit the Properties and make it Read only and a Constant. No logic needed.

Is there an easy way to get an "Always On" tag in Logix5000?

defcon.klaxon

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