DCS connects to 3-wire instruments at the site
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The on-site 3-wire instrument terminals are 24V+, com, and signal+. The SITOP power supply provides 24V to 24V+ and com, while signals are sent from signal+ and com to the AI module in the DCS cabinet. Is it necessary to switch the power supply from SITOP to the 24V power supply provided by the DCS itself?
The 24V power supply of the DCS itself is no different from the SITOP power supply; it’s just a power supply of a different model. It’s fine to connect like this. On our end, it’s a three-wire system; I use three-core wires for connections – 24V+ is connected to 24V+, 24V- is connected to COM, and AI card+ is connected to signal+. Then, at the cabinet side, COM and AI card- are shorted together.
The field instruments have already been powered; for DCS wiring, passive AI wiring must be used.
There’s not much difference, it’s just for power supply. It doesn’t matter; just pick it up wherever is most convenient