Problems with active contacts
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The 24VDC active contact signal is fed into the DCS; the DCS cards have power distribution functions, so is it still necessary to provide an external 24VDC power supply? If it’s not needed, a 2-core cable should be sufficient, right?
Well, it depends on whether the original poster can understand this. If there are active contacts, then it’s definitely not a regular transmitter, such as a pressure transmitter; rather, it’s some kind of analyzer or similar device that requires external power supply. On the DCS side, you need to set up a power distribution cabinet. If there are other instruments at the site that also require power, such as 220V electric valves, or instruments located at greater distances, things get more complicated. A single power distribution cabinet cannot supply both 220V and 24V simultaneously; you’ll need to use additional conversion equipment. When purchasing, remember to keep the design consistent as much as possible.
If the instrument on site is equipped with its own 24VDC power supply, and the signal wires are connected to the DCS, then it is sufficient to connect the 2-wire cables to the card in a passive manner. If it is the card that provides the 24VDC power, then the connection to the card must be made in an active manner; in this case, the instrument on site does not require any external power supply
What is used are active contacts; the DCS requires a relay to convert them into dry contacts that can be fed into the DCS.
It is a 24VDC active contact output by a proximity switch; ordinary transmitters output 4–20mA signals, so they can be powered using a power supply card, but proximity switches that output contact signals cannot be powered in this way, right?
Yes, it’s not provided by DCS cards; it’s the distribution cabinet – see what the moderator said below
It’s better to follow the rules; don’t try to cut corners and end up causing other problems, especially safety issues, as that would be bad.