Electromagnetic interference is evaluated to apply metallic lines to digital transmission of ISDN. Especially, the interference on usernetwork interface is investigated in detail, because the unbalance about earth of terminal equipment increases the interference on bus wiring configuration. The interference by induced voltage from radio wave, lowvoltage supply line and analog telephone line is evaluated, and measures against the induced voltages are studied. Consequently, the interferences are eliminated by insertion of common-mode choke coil and improvement of balance of the equipment.
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