I read the posts about active and passive signals and I’m confused; please help me understand.
Thread Content
Is the output of a two-wire transmitter an active signal or a passive signal? I browsed Baidu, and some people said it’s an active signal while others said it’s a passive signal; that doesn’t make any sense. I think it’s active, but the professional answer is that it’s passive. :dizzy:
The last edit to this post was made by Rosemary’s Only on 2015-12-15 at 21:28. The output should be an active signal; when there is no power, a signal generator should be used to provide an active signal! Under normal conditions, a passive signal is sent. I don’t know if it’s right?
This post was last edited by lotos1987 on 2015-12-15 at 21:45. Active and passive refer to DCS systems, and the distinction between them depends on the power supply method. Two-wire field instruments require a DCS card to supply them with 24V power; the instrument sends passive signals to the DCS ； Four-wire instruments come equipped with a 24V power supply of their own, so no DCS card is required to provide power; for the DCS system, these instruments transmit active signals.
Regarding active and passive types in the field context, field instruments with a separate power supply are active, while those without a separate power supply are passive
A signal naturally has a transmitter and a receiver. A passive signal is one that does not contain its own power source and requires external power supply; this ‘external’ source can be the signal receiving end, or it can be a third party other than both the transmitting and receiving ends ； An active signal is one that has a power source built into it, and therefore does not require external power supply ； For example, in the case of 4–20 mADC signals, two-wire transmitters transmit passive signals that require power at the receiving end; this power can be supplied through a safety barrier, a power distributor, or another third-party device. If it is mistakenly assumed that these are active signals and no power is provided, then there will be no power in the circuit, and consequently no 4–20 mADC signal will be generated ； For example, there is a 4–20 mADC signal, which is an active signal; if it is mistakenly considered to be a passive signal and a power supply is added to the circuit, then two power supplies will be present in the circuit, causing the signal to become chaotic and the circuit to fail to function.
This question shouldn’t be asked in this way; if it has to be asked, then it can only be described as passive. Why? Because if you disconnect the wires and then use a multimeter to check whether there is a 4-20mA output from the transmitter terminals, there certainly will be no output. Therefore, two-wire transmitters and other instruments are passive devices.