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GordoP

PLC Analogue Input 101

8 posts in this topic

I'm sure this is a very basic question, rest assured I'm not in the PLC business, just wondering about applying one. The PLC manual says common terminals are common and the drawing enclosed shows them not used. There is also IN + and IN - 1) Are the “In-” terminals effectively isolated from one another so I can connect channel 1, 2, and 3 to different parts of the same circuit that are at different potentials (i.e. would the enclosed picture work?) (I understand maximum input voltages, etc, the drawing is just for concept.) 2) Curious, what are Common terminals used for then? Thanks for your assistance! Gord Edited by GordoP

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Can you tell the forum what PLC you have and what analog card you have?

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It could be different ones depending on the location. One I do know of is an AB SLC500 based system and one of their standard I/O cards, though I don't know the exact model. The SLC 500 Analogue Input Module manual I was reading appears to suggest the whole series have the same interface. Will different manufacturers not always have IN+, IN-, COM? And if so, do some have single-ended inputs only? Thanks again! Gord

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Some have signal-ended some have differential ( some have both) some have isolated some a non-isolated inputs. And not all AB input cards are the same. http://www.ab.com/en/epub/catalogs/12762/2181376/2416247/1239758/2554063/Analog-I-O-Modules.html Edited by Mickey

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Ok, so let's say we have a AB SLC 500 with a 1746-NI4 Analog Input Module and no other input or output modules installed. The manual shows the input module input terminals are as attached. Can I connect channels 1, and 2 to different parts of the same circuit that are at different potentials (i.e. would the picture enclosed with the original post work?) (I understand maximum input voltages, etc, the drawing is just for concept.) Thank you, Gord

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Yes, as long as the dip switches are set up for voltage and the voltage inputs are within the specified range. I see no reason why not. Edited by Mickey

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The OP's drawing shows a power supply at 100Vdc. The "thumbnail" warning posted ( I assume it applies to the card involved) states a limit of +/- 20Vdc (common mode). Making the voltage measurements shown on the original drawing is NOT doable, because IN 1 is at 100V above ground, which sees 5x greater common mode than the allowed + 20Vdc. Exceeding the common mode limitation will first peg the readings off scale, then fry the input. The restriction limit of 20V common mode includes the IR drop across the output load up to 20V above ground. If the output load dropped 10V and IN2 dropped 8V, then it would be possible to measure IN2 because the common mode would only be 18V, less than the limit of 20V. Any combination of output and IN2 drops up to 20V means those two voltages can be measured because the combination is within the 20V common mode limit. Any combination greater than 20V means the measurement cannot be made. IN 1 is not measurable with the 20V limitation. Edited by DanW

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There is also an arrow through the power supply symbol indicating an adjustable power supply ( hopefully).

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