Work summary: selection between passive and active DI and DO signals; everyone is welcome to provide corrections and additions
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This post deals only with the DI signals received by the digital control system in digital form: 1. On-site buttons, dry contacts (i.e., passive contacts). 2. Pneumatic valve position signals, dry contacts. 3. Electric valve position and alarm signals, dry contacts (it must be specified in the technical requirements for the electric actuator that the valve position signal should be in dry contact form). 4. The status of pumps and motors comes from the MCC (ac contactors); relays are used for isolation to obtain dry contacts. 5. Status and alarm signals from the on-site PLC control panels, dry contacts. DO signals output by the DCS: 1. On-site alarm lights, with a 24V output. 2. Control of pneumatic valve solenoids, with a 24V output. 3. Control of electric valves, either via dry contacts or 24V; these days it is required that dry contacts be used for output (this must be specified in the technical requirements for the electric actuator). 4. Control for starting and stopping pumps and motors comes from the MCC (ac contactors), with a 24V output; relays are used for isolation. 5. The start/stop control for on-site pump and motor equipment is handled by the built-in PLC control panel, with passive contacts being used for output (in other words, the PLC’s DI inputs require dry contacts). My understanding of active and passive connections is that an active connection means that the other party provides power, and there is polarity between the two contacts； In the passive case, I provide the power supply; there is no polarity between the two contacts ； Therefore, DI connections must be of the direct-contact type. To protect the DCS system, it is required that signal transmission between 220V and other components be isolated using relays. In simple terms, if there is an error in the electrical wiring and the terminals are connected incorrectly, will it damage the relay or a DCS card? Furthermore, isolating alternating current outside the control room cabinets helps to maintain signal stability
I cannot agree with the statement that in a passive setup I provide the power supply and there is no polarity between the two contacts. In pneumatic control valves, too, the 24V power supply supplied to the actuator has a specific wiring polarity.
Regarding active and passive, my understanding is that it is a relative concept. It’s like saying that a person is fat or short – one must compare them to another person who is thin or tall in order to make such a statement. Similarly, if you are charged, I am passive to you ； You are not charged, so to you I am active. It is inaccurate to simply state whether a signal is active or passive. I think that’s how I understand it.
Friend on the 3rd floor, the original poster is discussing DI DO; what you’re talking about goes beyond that scope :lol The comment on the 4th floor makes sense, but what’s the truth? Let’s wait for the friend downstairs to analyze it
I think the output of DCS doesn’t necessarily have to be active; it can also output dry contacts. If it is active, the voltage isn’t necessarily 24V – it could also be 220V. This is just my personal opinion, for discussion.