2.12 Communication

2.12 Serial and Parallel Communication

Data can be transmitted between a sender and a receiver in two main ways: serial and parallel.

Serial communication is the method of transferring one bit at a time through a medium.

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Parallel communication is the method of transferring blocks, eg: BYTEs, of data at the same time.

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As you can appreciate parallel communication is faster than serial. For this reason, the internal connections in a computer, ie: the busses, are linked together to allow parallel communication. However, the use of parallel communication for longer distance data communication is unfeasible for economic and practical reasons, eg: amount of extra cable required and synchronisation difficulties. Therefore, all long distance data communications takes place over serial connections.

Comparison of Serial and Parallel Transmission

Parallel transmission requires a separate channel for each bit to be transmitted. Therefore, to transfer a byte, eight channels will be required between the sender and receiver. Added to these eight are additional channels that are needed for control information and if full duplex communication is required then even more channels would be required. Parallel transmission is rare, other than for very short distances, eg: within a computer, eg: data bus, or between a computer and a printer, eg: Centronics printer interface.

Serial transmission is much more common, particularly over longer distances. It is generally much cheaper as only a single channel between sender and receiver is required, eg: The seven bits (plus one parity check bit) making up an American Standard Code for Information Interchange (ASCII) character are transmitted serially in sequence by the sender and are reassembled into the character by the receiver. A common example of a serial interface standard is Recommended Standard 232 (RS232).

Serial Communications Equipment

Three things should be considered when discussing serial communications and the equipment to carry this out:

  • Electrical standards associated with the interface
  • Mechanical standards associated with the interface
  • Standards organisations involved

It is the organisations that govern the first two items on the list.

Next: 2.13 Transmission Problems