The short answer is that if a device has a passive output, then you need an external power supply to support it. If the device is active, then you already have an output signal with the power supply, so you don’t need another one.
Devices with active vs passive output and two-wire vs four-wire transmitters
For instance, two-wire transmitters are passive because they need external power to close the loop before you can read the signal. But four-wire devices have different scenarios.
First off, you can see these devices providing a passive or active output. Second off, depending on the transmitter’s feature, you can select whether the device gets active or passive.
Four-wire transmitters have power supplies separate from the output. So if the output is active, then you have the signal and the power supply. But if the device is passive, then the control system should provide the power to close the loop and send the output.
One more thing – if the device has an active output, the input of the control system should be passive. But if the device is passive, the input should be active to provide power. Make sense? If not, drop me a comment and I’ll help out!
These links will tell you more about analog and HART communication:
What does a HART communicator do?
What is the difference between HART and FOUNDATION Fieldbus ?
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