Answer / carl_ellis

What is passive connection of instrument in different type of I/O module?

What is passive connection of instrument in different type of I/O module?

The output of a field instrument or analyzer that makes a measurement and provides a conditioned, high level signal like 4-20mA is characterized as either active or passive.

An active output is powered internally by the instrument. These are 3 or 4 wire instruments with a separate power wire or wires.

A passive output requires a DC power supply in series with its output in order to excite or power the output. This done to
- limit field wiring to 2 wires
- reduce the likelihood of a ground loop due to difference in ground potential between the field instrument and the receiver.

So, a connection of a passive (2 wire, loop powered) device has to include connections to a DC power supply, observing correct polarity at the field device and the receiver's analog input.

Carl Ellis
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