AB CompactLogix digital inputs; do they have internal pull up/down resistors?

Hi guys,

I was asked this morning how the CompactLogix Digital Inputs would behave if we lost power, and after doing some searching through the Technical Specs, I still am at a loss, so I thought I'd ask if you guys know.

Reason I ask is because the design I'm working on actually has two power supplies; one is on a UPS for critical systems, one is not. All the digital ins and outs are actually connected to the power supply that isn't backed up, but the PLC's power supply is; thus, we'd lose all but critical I/O (wastewater wet well level analog inputs, radio) in case of power failure.

So basically I'm wondering, if the digital ins lose power and are floating, what would their behavior be? Are there internal pull up/pull down resistors in the built-in I/O modules?

Lets say there is pull-downs: if you lost the 0v common connection from the power supply, they could float. Or more likely, another device would fill in as common and you would see erratic behavior. See where I'm going?

Normally I just dedicate a input that directly monitors the power supply but if your application is that critical than you need that and a method of detecting the loss of the common. There are devices out there for that very purpose though I'd have to do some searching as I haven't used them in quite some time. (back when ungrounded systems were common)

jstolaruk said:

Lets say there is pull-downs: if you lost the 0v common connection from the power supply, they could float. Or more likely, another device would fill in as common and you would see erratic behavior. See where I'm going?

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Yep, great point there. I keep forgetting that the common itself is not the same common as the PLC...frustrating!

Normally I just dedicate a input that directly monitors the power supply but if your application is that critical than you need that and a method of detecting the loss of the common. There are devices out there for that very purpose though I;d have to do some searching as I haven't used them in quite some time.

Click to expand...

I wasn't around during the design of this project so I'm not sure if the dual PS idea was a solution looking for a problem, or if it's really that critical; unfortunately it's what I've got to work with. If you happen to remember the name of the device you're talking about I'd definitely appreciate it if you could post again, and I'll do some digging too. Thanks for taking the time to respond, I appreciate it!