1756-IF16 Question

mjp123gp

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Join Date
Mar 2012
Location
Savannah, GA
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94
While troubleshooting an issue with an analog input card the other day, I removed the terminal block from the input card. Looking at the values in the program, everything went high to a value just over the high scaled value. Since we have the input range set to 0-20mA and all of our scaling is 4-20mA I would expect the input value to go low or negative. I've checked the other settings for the card but can't figure out why it did this. I thought it may have been an issue with the module so I swapped it with a new one and it did the same thing. You can see in the attached pic that channel 2 and 3 are scaled 4-20mA / 0-750 but the actual value show 777. In this pic, all wiring has been removed from the card so there should be 0mA to each input.

Can someone explain please why it does this?

Input Card.JPG
 
This is a guess but…
It sounds like the Analog inputs are actually current differential circuits which would mean that they produce a current (probably on the positive connection) that is returned through the 4-20mA device’s negative connection. When you remove the connector, you break the circuit and the module sees the full current at the input’s positive connection because there is no where for it to go. It does sound like normal behavior which is why you want to disable channels that aren’t being used (so you don’t show an open circuit fault).
 
Differential vs. Single-Ended Analog Channels

Your IF16 only showing 8 input channels is correct, as you stated it is in "Float Data - Differential Mode". You can achieve 16 channels in single-ended mode, but you would need to delete the module and then add it back in, selecting the "Float Data - Single-Ended Mode" comm format when you are creating the module.

I found a nice explanation in another post:
http://www.plctalk.net/qanda/showthread.php?t=80631
 
Your IF16 only showing 8 input channels is correct, as you stated it is in "Float Data - Differential Mode". You can achieve 16 channels in single-ended mode, but you would need to delete the module and then add it back in, selecting the "Float Data - Single-Ended Mode" comm format when you are creating the module.

I found a nice explanation in another post:
http://www.plctalk.net/qanda/showthread.php?t=80631

Thanks! The explanation is kinda funny because that's exactly what started our issue. We're doing new construction on an existing system and the electrical contractor wired up a new flow meter without telling anyone and it started affecting other channels on the input card. Luckily someone had noticed him in the cabinet and told me so we had a place to start looking.

I've always considered myself pretty knowledgeable with analog signals but after this I'm realizing I don't know as much as I thought.

Thanks again for the help!
 
On a related note, while the analog cards are rock solid in general, you should really tie any unused input to common. Failing to do so, and leaving them to float, can introduce a lot of wierdness and noise on the channels actually being used.
 

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