CompactLogix Analog Input Enable/Disable

JHarbin · Feb 22, 2010

I want to be able to enable or disable 1769-IF8 inputs via my operator interface. I can toggle the Local:1:C.CH0En, but the channel doesn't disable. I suspect that I will need to inhibit and uninhibit the module to get it to work, but I wanted to verify this.

If so, is there a tag that I can access to do this?

blamb · Feb 22, 2010

What functionality are you looking to achieve by disabling the channels? Do you want to stop processing the disabled channel(s)? Are you looking to stop the scaling and set the input to a set value (0)? Perhaps this is something that could be done with logic in your scaling routine, rather than manipulating the Local config tags for the module? Inhibiting the module will affect all channels, I'm not sure if that would be the way to go.

JHarbin · Feb 22, 2010

blamb said:
What functionality are you looking to achieve by disabling the channels? Do you want to stop processing the disabled channel(s)? Are you looking to stop the scaling and set the input to a set value (0)? Perhaps this is something that could be done with logic in your scaling routine, rather than manipulating the Local config tags for the module? Inhibiting the module will affect all channels, I'm not sure if that would be the way to go.

I've been wondering if I need to bother doing this.

I'm trying to make this system startup tech friendly. I want him to be able to add an input without using the RSLogix5000 software. Since I am doing the scaling external to the AI, he will be able to set the Zero and Span very easily. I just wanted to add an enable/disable tag to the HMI.

The program won't be all that big, so scanning an unused input (or three) probably won't impact the system that much.

mordred · Feb 22, 2010

if you have spare regular inputs why not scan those analog inputs if a selector switch enables them? set it up so that if an optional analog input is added have an enabler switch used to activate the necessary rungs

JHarbin · Feb 22, 2010

mordred said:
if you have spare regular inputs why not scan those analog inputs if a selector switch enables them? set it up so that if an optional analog input is added have an enabler switch used to activate the necessary rungs

This is all being done in function block programming, BTW.

I'll probably just enable all the inputs and let the chips fall where they may.

mellis · Feb 22, 2010

My preference is to enable and configure all installed I/O.
Not sure what you think there is to be gained by disabling I/O. I can't think of anything.

JHarbin · Feb 22, 2010

mellis said:
My preference is to enable and configure all installed I/O.
Not sure what you think there is to be gained by disabling I/O. I can't think of anything.

I've always disabled unused Analog Inputs, not matter what controller/PLC I've used but in this case I may agree with you.

There has to be some reason that A-B puts an Enable/Disable check box in the I/O configuration. You wouldn't need it if you just always enabled it.

Dead horse officially beaten enough, methinks...

TConnolly · Feb 22, 2010

My guess is the enable is there for 1756 IO modules - which you can enable/disable with a message to the module. I'm not sure about it on the 1769 modules. Configure them all and then put a jumper across un-used inputs, no jumper on outputs.

daba · Feb 22, 2010

Enabling or Disabling individual channels in CompactLogix, ControlLogix, FlexLogix, etc., will have absolutely no impact on performance.

The module produces the Location:Slot:I tag which contains ALL of the data, channel values, alarms, status, etc. This tag is consumed by the controller.

Disabling channels will not change the size of the tag, and hence there is no improvement in performance.

JHarbin · Feb 22, 2010

Alaric said:
My guess is the enable is there for 1756 IO modules - which you can enable/disable with a message to the module. I'm not sure about it on the 1769 modules. Configure them all and then put a jumper across un-used inputs, no jumper on outputs.

It is the same with 1769 modules it just doesn't enable/disable until inhibited and uninhibited.

Thanks to all who offered advice.

JHarbin · Feb 22, 2010

daba said:
Enabling or Disabling individual channels in CompactLogix, ControlLogix, FlexLogix, etc., will have absolutely no impact on performance.

The module produces the Location:Slot:I tag which contains ALL of the data, channel values, alarms, status, etc. This tag is consumed by the controller.

Disabling channels will not change the size of the tag, and hence there is no improvement in performance.

I wasn't so worried about performance - I've just always thought it was bad form to leave unused analogs out there floating (even with a jumper). Personal preference, I guess, but it's been my MO since the PLC5 was a baby.

As I've said - thanks for the advice.

Mickey · Feb 22, 2010

I guess, but it's been my MO since the PLC5 was a baby.

How did you disable in the PLC5? You just shorted the unused channels.

JHarbin · Feb 23, 2010

Mickey said:
How did you disable in the PLC5? You just shorted the unused channels.

Sorry, my offhand comment was unclear. I was simply making a totally unnecessary comment about how long it had been my MO (using the life of the PLC5 as my reference). I didn't necessarily mean that I had been able to follow my MO in every case.

The only way to explain further would be to wax autobiographical and I see no need to bore y'all any more.

Thanks again to all for your help - signing off this thread.

CompactLogix Analog Input Enable/Disable

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