I/O module digital long-distance transmission EMC

Ensuring robust EMC performance for long-distance transmission of digital signals from I/O modules requires a robust design. First, a physical layer standard with strong anti-interference capabilities should be selected, such as RS-485 differential transmission, which has a high common-mode rejection ratio and is suitable for kilometer-level transmission. Matching resistors, typically 120Ω, should be installed at both the driver and receiver ends to eliminate reflections. Common-mode chokes, such as the CML series, should be installed at both ends of the line or at regular intervals to suppress induced common-mode noise.

Shielded twisted-pair cable should be used as the transmission medium, with the shield grounded at both ends or only at one end, depending on the grounding system, to avoid ground loops. For switching signals, current loop transmission, such as a 20mA current loop, can be used, as it is not sensitive to noise. Surge protection devices must be installed at the module interface, such as a multi-level protection circuit composed of P-series gas discharge tubes and SMBJ-series TVS diodes. For high-speed pulse signals, the signal rise time needs to be controlled to avoid excessive high-frequency radiation. Signal isolation is recommended before transmission. In actual deployment, signal cables should be laid separately from power cables, with a minimum distance of 30cm.