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IO list - active or passive signals

IO list - active or passive signals

IO list - active or passive signals


I am getting crazy trying to understand if my IO signals are active or passive.

I have a new IO list template that require us to fill if our signals are active or passive.

To make it more confusing, we received and example of the list that I believe is incorrectly filled which is confusing myself even more.

We have a number of Siemens Sitrans P420 Pressure transmitters, 4-20 mA HART which should be 24VDC loop powered. Is the signal active or passive as per this snap:

To my own research, I believe these signals should be passive but here is the confusion because in the example of the list we received from our client, loop powered PT transmitters were listed as active...

We also have a Ultrasonic Flow transmitter, that has a 2-wire 4-20 mA HART signal cable as well as a 2-wire 24VDC external power supply cable. In this case I believe that signal is active since it is powered by the external power cable. Correct?

Looking forward to some feedback and clarification.

RE: IO list - active or passive signals

Hmm part of my confusion might have been resolved.

I am thinking that "Signal Type" refers to the DCS system AI port. Meaning that the system shall be ACTIVE toward the loop and therefore powers the loop.

Passive means that the system is passive toward the loop and therefore the loop has to be externally powered.

That means my passive 2-wire PT instruments shall be plugged into an ACTIVE AI to get powered by the PLC.

While my Flow transmitter that is externally powered shall be plugged into an PASSIVE AI since the loop is externally powered.

Meaning that the if the instrument is passive then the AI shall be active and vice versa.

Does this make sense? Have I understood correctly by myself? Otherwise, please correct me accordingly.

RE: IO list - active or passive signals

Your assessment is correct.

Here's a link to a Wika blog dealing with active/passive PLC inputs.

Here's the graphic from the blog:

The definition of active/passive is well defined industry-wide for field devices. This link confirms that a passive analog input does not provide loop (excitation) voltage, an active analog input provides loop (excitation) voltage.

The graphic shows the method for determining whether an analog input for an existing loop is active or passive

RE: IO list - active or passive signals

That is great, thank you!

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