Help request post!
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I’m asking kind people to patiently explain to me why a relay is usually used for isolation at DO points, while it’s generally not necessary for DI points
It cannot be assumed so simply that the need for isolation depends on the actual usage scenario. For electrical supply signals or control output signals, isolation is generally required to prevent voltage from leaking into the circuit and damaging the components. In the case of solenoid valves with high power requirements, if the DO cards may not be able to handle such loads, an isolation relay can also be used; its main function here is to supply power to the solenoid valve.