A communication device, an adapter device and a communication system (100) are provided. The system (100) includes: a master communication device (10) and a slave communication device (20), the master communication device (10) includes: a first external interface (130) including a first pin (131) and a second pin (132), and a signal generating module (110) electrically connected to the first pin (131); the signal generating module (110) is configured to generate X signals according to time interval corresponding to an N-bit string to be sent, in which the time interval corresponding to the N-bit string to be sent represents time interval between start times of each two adjacent signals, different time intervals corresponding to different bit strings, where N≥1, X≥1 and X is a natural number; said first external interface (130) is congured to send said X signals through the first pin (131).