Answer / carl_ellis

It depends on how much voltage the field transmitter needs to operate and the total loop resistance of the loop circuit .

Every 2 wire loop powered transmitter has a specification for the voltage required to drive a given loop resistance. Sometimes that is a formula, sometimes a load line graph, sometimes a statement like "minimum liftoff voltage is 17.7V", or rarely, a vague statement of maximum loop resistance (common with active analog outputs, rare with loop powered transmitters)

Excessive wire resistance in a 2 wire loop powered transmitter circuit can affect the operation by
- preventing the transmitter from turning on and operating
- allowing partial operation at lower current outputs, but failing to operate at higher current outputs. That failure can be either an output that just never exceeds a certain value or a transmitter that cycles on-off-on-off an never initializes.

If there is sufficient voltage to run drive the total loop resistance, then the transmitter could function normally.

You need to calculate the wire resistance over its total wire run length (out and back) and know what the receiver's analog input resistance is. Then look up the specification for how much resistance the transmitter's can drive and see whether your loop resistance is lower than the maximum.

Carl Ellis
Measure First

In RTU Panel we are having several Analogue & Digital Inputs /Outputs. We are recently facing some problems over Analog inputs , which are not updating properly (But Digital Input are updating normally) Can any one tell me from where the problem is creating , we have checked PLC CPU card, Analog Input card & we reconfigured ,But still the issue is there. Issue : 1. Sometimes Analog values are updating & sometimes not. 2. Analog values are not updating normally , showing some wrong Values 3. Total dead (0 value) Analog values are Level & flow measurement from the field instruments. (But in field the instruments are working normally & Displaying the values normally in local display unit) This problem we are facing in our Water Plant. If any one experienced this same problem or having any ideas related to our issue PLS send your valuable Answers/Replies...

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