Synchronous vs. asynchronous

synchronous vs. asynchronous signalling

Whenever an electronic device transmits digital (and sometimes analog) data to another electronic device, there must be a certain rhythm established between the two devices, i.e., the receiving device must have some way of knowing, within the context of the fluxuating signal that it's receiving, where each unit of data begins and where it ends.

For example, a television transmitter produces a continuous stream of data in which each horizontal line of image must be distinguishable from the preceding and suceeding lines, so that a TV will be able to distinguish between them upon reception.

Or, a serial data signal between two PCs must have individual bits and bytes that the receiving PC can distinguish. If it doesn't, then the receiving PC can't tell where one byte ends and the next one begins. Or where one bit ends and begins.

So the signal must be synchronized in a way that the receiver can distinguish the bits and bytes as the transmitter intends them to be distinguished.
There are two ways of synchronize the two ends of the communication.

In synchronous transmission, the stream of data to be transferred is encoded as fluxuating voltages on one wire, and a periodic pulse of voltage is put on another wire that tells the receiver "here's where one bit/byte ends and the next one begins".

In asynchronous transmission, there is only one wire/signal carrying the transmission. the transmitter sends a stream of data and periodically inserts a certain signal element into the stream which can be "seen" and distinguished by the receiver as a synch signal.

Obviously, the term "asynchronous" is misleading in its literal interpretation and must be understood as a term which is dictated by conventional usage.