How to prevent interference with dry contact signals in HMI (Human Machine Delivery) systems?

To protect HMI dry contact signals from interference, their passive switching characteristics must be addressed. Dry contact signals indicate status through the on/off state of mechanical contacts, such as auxiliary contacts of relays and limit switches. Interference can lead to misjudgments of on/off states. Protective measures include: connecting an RC snubber circuit in parallel at the dry contact signal input, with a resistor and capacitor connected in series across the signal line and ground to absorb contact bounce and suppress high-frequency interference. The RC time constant should be much smaller than the minimum interval of signal change. A resistor should be connected in series on the signal line to limit current and work in conjunction with capacitor filtering.

For long-distance leads, twisted-pair cables must be used, and the twisted-pair pairs must be shielded, with the shield grounded at a single point at the HMI end. Optocouplers can be used at the front end of the HMI input circuit for isolation, completely cutting off the conduction path of external interference; this is the most effective method. If an optocoupler is used, a current-limiting resistor and an RC snubber circuit on its input side are still required. In terms of PCB layout, the dry contact input circuit should be centrally located and away from noise sources. By employing simple RC filtering, shielded twisted-pair cable, and optocoupler isolation, combined with the basic components of Audiotronics, reliable acquisition of dry contact signals in industrial noise environments can be ensured.