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Active Loop and Passive Loop

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Hi all,

    I put together a project with a v570 controller and a E3XB snapin module.  I have two anaolg inputs (AI1 & AI2).  AI1 has a loop powered signal for a pressure transducer hooked up and AI2 has a self powered signal coming from a pH meter hooked up.  We noticed that the two inputs seemed to not play well with each other because when they were both hooked up the pH meter reading would come in at less than 3.4mA.  It seemed like they were fighting with control of the refernce ground so we installed a signal isolator that takes a 0-10V (all we had at the time).  The customer is complaining that the value on the meter doesn't match up to the value displayed on the PLC and this has to do with converting the 4-20mA (only option for retransmission).  If we can, we would like to eliminate the signal isolator.  Any input would be greatly appreciated, thank you

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  • MVP 2014

In general terms you can use a loop-powered device with Unitronics if you include a loop power supply in series with the loop.  It is simple if there is only one analogue device connected.  However in this case you may still get problems with the reference ground, depending on the overall circuit for your second analogue signal.  


If it's not a simple case of switching sensors as Alex suggested, I would put a signal isolator on the loop-powered sensor.  Either purchase an isolator with support for loop-powered sensors, or add a loop supply to the sensor circuit.  Adding the loop power supply in this case should not affect the reference ground as the isolator is between the PLC and the loop supply.

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