A communication circuit is provided in which reads from a device are controlled by sensing a transition by a host communicating with a device. The device then accepts a one which holds the line high for a predetermined time period or accepts a zero when the line is held high for a different time period....http://www.google.com.tw/patents/US5864872?utm_source=gb-gplus-share專利 US5864872 - Single wire communication system
