1.4 Serial and Parallel Communications

The bits of a binary word may be transmitted either in parallel or serial form (see Figure 1.5). In the parallel form each bit is carried over a separate communications channel and the result is at least as many channels as there are bits in the word (there are normally additional lines for controlling the exchange of data on a parallel interface). Thus a 24-bit parallel interface would require at least 24 wires, an earth return, a clock line, and a number of address and handshaking lines. Such an approach is normally used for short distance communications ('buses') within a digital device, but is bulky and uneconomical for use over longer distances.

Figure 1.5: When a signal is carried in numerical form, either parallel or serial, the mechanisms of Figure 1.4 ensure that the only degradation is in the conversion process.

When data is carried serially it only requires a single channel (although electrically that channel may consist of more than one wire), and this makes it economical and simple to implement over large distances. On a serial interface the bits of a word are sent one after the other, and thus it tends to be slower than the parallel equivalent, but the two are so different that to say this is really oversimplifying the matter since there are some extremely fast serial interfaces around. Some serial interfaces carry clock and control information over the same channel as the data, whereas others accompany the data channel with a number of additional parallel lines and a clock signal to control the flow of data between devices. Depending on the standard protocol in use it is possible for serial data to be sent either MSB first or LSB first (see section 1.3), and knowing the convention is clearly important when interpreting received data. These matters are examined further in the next section.