How to ensure EMC of dry contact signals in electrical force measurement switches (PMS)?

The dry contact signals (passive contact input/output) of a PMS appear simple, but in industrial environments, they are susceptible to induced overvoltage, contact bounce, and spatial interference. EMC assurance requires a combination of external protection and internal processing. For input dry contacts, an RC snubber network (e.g., 0.1μF + 100Ω) is connected in parallel across the contact, followed by a current-limiting resistor (e.g., 10kΩ) in series before entering the optocoupler. The optocoupler provides isolation and level conversion. For output dry contacts (relay output), an RC snubber (e.g., 0.1μF + 100Ω) or an MOV (e.g., 7D390K) is connected in parallel across the relay contact, and a freewheeling diode is connected in reverse parallel across the relay coil. All dry contact signal lines should use shielded twisted-pair cable, with the shielding layer connected to the chassis ground at a single point at the PMS terminal. In terms of PCB layout, dry contact-related circuitry should be centrally located and isolated from the ground plane of digital circuitry. In terms of software, input debouncing (e.g., 10ms delay) should be applied, and feedback confirmation can be added to the output. Through design, the dry contact interface meets the requirements of IEC 61000-4-4 EFT Level 4 and IEC 61000-4-5 Surge Level 3, and can operate reliably in harsh industrial environments.