Active and passive issues?

Active/passive signals generally refer to current signals, such as 4–20 mA. Active/passive is determined by whether the device that provides a 4–20mA signal has its own separate power supply cable; if it does (220VAC or 24VDC), then the signal it outputs is an active signal, otherwise it is a passive signal. For 4-20mA signals, \"active\" means that the signal source has its own power supply; it’s similar to what you mentioned as 4-WIRES, with two signal wires and two power wires. \"Passive\" means that the signal source does not have its own power supply and must draw power from the signal wires, which is similar to 2-WIRES. As for signal sources such as transmitters, both active and passive types require external power supply; the only difference lies in the source of that power. As for signals from thermal resistors, they can also be considered passive signals; moreover, thermal resistors do not require a power supply to function. The DCS supplies power to the thermal resistors so that it can detect their resistance values ； A thermocouple can be considered an active signal, and it is an active signal that does not require external power supply

There are many similar questions about LZ; it’s more convenient to search first, as there’s a lot of information available.

Active and passive signals are not limited to analog current outputs; they also exist for other types of signals, such as digital signals or contact signals, which can likewise be classified as active or passive, also known as dry contacts or wet contacts.

For AI signals, those that require power from a card are passive signals, while those that do not require power from a card and are powered separately on-site are active signals. For DI signals, contacts coming from the field or MCC that carry voltage are active signals; otherwise, they are passive signals. For DO signals, signals sent by the control system that carry voltage are active signals; otherwise, they are passive signals； In short, for the signal receiving side, a signal in which voltage is present in the signal circuit is an active signal, while the opposite is a passive signal

That’s how I understand it: in a two-wire system, the same wire serves both as a power line and a signal line; therefore, the signal is an active signal, while the device is a passive device, as it needs to be powered by DCS. Therefore, in a four-wire system, the signal lines transmit passive signals and do not require power supply, while the device in question is an active device. Ask an expert to evaluate: lol

For field instruments, active means 4-wire system, while passive means 2-wire system. In the active type, the signal is originally a 4–20 mA electrical signal; therefore, the instrument has two power wires and two signal wires, for a total of 4 wires (in some cases, two pairs of wires share a common wire, resulting in a 3-wire system). A passive signal can be considered a resistive signal; it adjusts its own resistance in response to the voltage applied to it, so that the output current remains within the range of 4–20 mA. An external 24V supply is required, which is why it is called passive.

Active and passive: Active and passive apply to AI. If a field instrument requires a DCS card to supply it with 24V power, then such an instrument is considered a passive signal for the DCS. Conversely, if the field instrument already has 24V power supplied to it from another source and does not need power from a DCS card, then it is considered an active signal. Active and passive signals have different wiring requirements. Active and passive signals: Active and passive signals generally refer to current signals, such as 4–20 mA. Active or passive is determined by whether the device that provides a 4–20mA signal has its own separate power supply cable; if it does (220V AC or 24V DC), then the signal it outputs is an active signal, otherwise it is a passive signal. The acquisition of active signals is simple; simply connect the positive and negative terminals of the input interface of the acquisition device to the corresponding positive and negative terminals of the 4–20mA signal source device. When it comes to the acquisition of passive signals, it is important to note that the device that provides these passive signals (let’s call it Device A) does not have its own independent power supply; therefore, when it needs to output a 4–20mA signal, its operating voltage must be supplied by an external device. Therefore, the device used for signal acquisition (assuming it is Device B) usually needs to have a power distribution function, such as a 24VDC output, so that it can supply 24VDC to Device A while acquiring 4–20mA signals. If Device B does not have a power distribution function, when it collects signals from Device A, it is possible to connect a 24VDC power supply, but the wiring must be done in accordance with the wiring diagram provided by the manufacturer. Two-wire, three-wire, and four-wire instrument signals: Two-wire instruments do not have wires for power supply; in other words, they lack an independent power source, and power must be supplied from outside. Their two wires are used to transmit both power and signals. The signals they transmit are called passive signals. A three-wire instrument has a positive power line and a positive signal line, while the negative power terminal and the negative signal terminal share one common line; in total, there are three wires. Their output signals are called active signals. A four-wire instrument, in which two wires are connected to the positive and negative terminals of the power supply, while the other two wires represent the positive and negative terminals of the output signal. Their output signals are called active signals.