The transport of data between digital devices may take place either in be parallel or in series. In the case of parallel transmission, one channel (one line) per bit is required, whereas in series transmission, only one channel is needed, with the individual bits transferred one after another as impulses.
As a rule, parallel data transmission takes place within devices. The exchange of information takes place over a data bus. Each bit is assigned a printed circuit trace. The individual bits of a data word are transferred in parallel, but the data words are transferred in series (bit parallel-word series). The longer the word, the greater the transfer speed.
The transfer of data in series (series transmission) takes place wherever multiple transmission channels are impossible or uneconomic (e.g. Field bus, LAN, telephone). A distinction is made between synchronous and asynchronous transmission.
The key characteristic of synchronous, series data transmission is the exact matching of the data with the clock signal on the bus. As soon as the baud rate of the series data transfer is present, the sender module must state a data bit for each clock signal. Before any series data transfer, the senders and receivers are synchronised by a special series data bit string (synchronisation pattern). Therefore, the receiver module knows exactly how it is to interpret the series data signal.
If series data is transferred asynchronously, the sender module only issues a symbol when it is ready to send such a symbol. Between the symbols, a continuous break signal, normally with a higher level, is issued:
Each data unit in an asynchronous data flow must carries its own synchronisation information.
An asynchronous data unit is therefore framed by an individual start bit and 1, 1½ or 2 stop bits:
In the case of series data transfers over a telephone line, a distinction is made between two operating modes:
If an individual telephone line is used to send data in both directions, this is called half-duplex operation
If the sender and receiver modules are connected via two lines and each line is intended for data transfer in just one direction, this is called full duplex (or simply duplex) traffic.
The advantage of a data connection with full duplex operation is that the data transfer can take place in both directions.