1756-if8

New system installed, I was not involved in programming or installation, but I'm being tasked with making it work. There are 3 inputs on card being powered off an external 24vdc power supply. (1) pressure transmitter (2) flow meter (3) tank level. Once Plc is powered up all all analog signals are correct, but after approximately 1 1/2 hours without any system changes all of the analog signals displayed on panel view are reading max values. During this time I've checked each instruments 4-20ma readings and they are correct. If I power down Plc for 20-30 minutes problem is gone, but returns in the next hour or so. Has anyone seen this before ????

Welcome to the forum!

I've not seen this before, but it definitely sounds like a faulty card if it works fine, and then after a period of time stops working with the current loop still correct.

To be sure, check whether the input data also goes to maximum scale (the input address which will look like I:1.0, where "1" is your card number and "0" is the input number). If so, you can be pretty sure the card is faulty. Two final things to check just in case - make sure that there is nothing overwriting the input data (check for the card as a whole, as well as the individual inputs) and check for loose wiring on a common, or incorrect wiring in general.

I suspect you're going to find that everything is fine and it's a faulty card, but it always pays to be sure before you spend the money on a new one

Thank you for response, unfortunately we are at the mercy of the contractor who designed project. We need to purchase RSLogix 5000 before we can access program. I thought maybe signals are getting saturated, maybe a voltage drop with the power supply, not sure. Will check all wiring again in the morning, thanks again.

No problem! Just one correction to make - I've been quoting an SLC system this morning, so I've got all the 1746 SLC part numbers on the brain and thought that's what you had. But I see now that this is a Control Logix platform with 1756 I/O. So, the input will not be I1:0, rather it will look more like Local:1:I.Ch0Data or something similar (again with the 1 being the slot number and the 0 being the input number). Not that that's a huge help to you until you get Logix 5000 sorted, but anyway!

ASF, today I removed the 1756-TBCH (terminal block) from the analog input card and then powered up the Plc. All three analog inputs went to max values without any signals from field devices. Still doesn't make me warm and fuzzy that it's the card because I still don't know what the 1756-IF8 is configured as. Isn't it possible that a signal loss could be programmed to a certain value?

I don't believe so - certainly, most analog transmitters can be programmed to send out 23mA or 3mA on error, but I don't believe there's any way to have the card itself report an off-scale-high value if the input is lost.

So, just to confirm - the input data itself went to maximum scale? i.e. the Local:X:I.Ch0Data tag? What was the displayed value, and what is the configured maximum for each input? Just because on those cards you can configure max/min values individually for each input, so I'm curious to know if it's gone to the maximum scaled value, or even higher.

Ashtyl said:

Isn't it possible that a signal loss could be programmed to a certain value?

Click to expand...

It's possible that there's logic to do that in the PLC code, but I think it's unlikely (because what would be the point?). But to be sure, I would run a cross reference on the whole input card. Open your controller tags, and right click on Local:X:I (where X is the slot number of the affected card) and click Cross Reference. You should come up with a list of everywhere that any information from that card is used. The big thing to check for is that every single entry in the list has an "N" for "No" in the "Destructive" column - not a "Y" for "Yes". If any of them have the Y, then have go looking there. If they're all N's, then you can be pretty certain the card itself is faulty. Perhaps just for your own peace of mind, go to each of the locations on the list and make sure you understand what each rung is doing, and that it's not contributing to what you see.

If you can't see the actual on-line logic, you cannot tell if the card is showing an error or not. They could have some sort of filter between the card and the display, which isn't properly working.

Hey guys, thanks for all your responses. I replaced the analog input card and problem is gone. Still waiting on my new RSLogix 5000 (studio) software but atleast one problem is gone, thanks again.

No worries, glad you got it working. Thanks for posting the follow up!