I have a question: how can one distinguish between active and passive inputs and outputs of instruments?
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1. In DCS, for a card, how can one distinguish between active and passive ones? For example, external thermal resistors or transmitters, external electric control valves, and the feedback signals from those electric control valves. 2. Does \"active\" and \"passive\" refer to the channel or to the input or output instruments?
For example, when connecting to a transmitter, the card needs to output 24V voltage, and it operates in a two-wire system; the signal transmission involves both 24V voltage and 4-20mA current. Is such a card active or passive? Is the channel active or passive? Is the transmitter active or passive?
Active means it is powered, while passive means it is not powered
For example: card holders, cables, transmitters. Does \"active/passive\" refer to primary instruments, secondary instruments, or signal channels?
For example, when a card is connected to a transmitter, the card provides 24V while also accepting 4-20mA signals; is such a card active? What about the transmitter? Where’s the channel?
Active and passive refer to whether the signal output by the field transmitter has 24V; for example, most mass flow meters are powered by 220V, and in such cases the signal line is considered passive
Most transmitters these days are powered by safety barriers; from my understanding, the safety barrier is the active component compared to the transmitter
Passive generally refers to dry-contact signals, such as the feedback signal from limit switches or the alarm signal from float switches. Active signals require power supply, such as two-wire transmitters and fork-type level switches. The obvious boundary is these instruments, which cannot function in a passive state. Of course, the power supply can also be connected using a four-wire system, with one wire for power and another for signals; this approach is generally used when two wires are not sufficient to provide enough power, such as in the case of mass flow meters and radar level gauges. The panel floor is connected to the 24V circuit, so no separate wiring is required. If the four-wire power supply is 24V, a separate wire must be drawn from the DC module. I’m not sure if I made myself clear; please correct me if I’m wrong.
For field instruments, a separate power supply is required. Instruments that operate on a four-wire system are active, meaning that the signal they transmit is an active signal. As for DCS systems, they do not need cards or safety barriers to provide power; in other words, such signals are passive； For field instruments with loop power supply, such as two-wire system instruments, the instrument itself is passive; therefore, the signal from such instruments is a passive signal. In the case of a DCS, it requires cards or safety barriers to provide power supply, which means it is an active system. So the specific understanding of it depends on each individual; everyone has their own way of understanding it. It cannot be said absolutely that active and passive are defined in terms of field instruments or control room cards.
Active and passive generally refer to DCS cards; active cards do not require an external power supply as they can directly supply power to 24V instruments on-site, with the power source built directly into the card. Passive card modules require an external 24V power supply to power the field instruments. Generally, in the DCS systems we use today, active card modules are chosen for 2-wire instruments.