Serial and Parallel Transmission

Serial transmission

This is a form of data transmission in which all the data is piped one bit at a time through a single transmission channel. It therefore takes a minimum of eight bit cycles to transmit one byte.

A serial transmission path can take a number of forms -

• There may be a number of parallel channels, only one of which carries data in each direction. The other channels carry control signals used to open up and close down the transmission link.

  This form of serial transmission channel is commonly used to connect modems to PC`s, and the transmission is asynchronous.

• There may be only one channel, which is bi-directional.

  The data is wrapped up in packets, which provide the address for the data to go to, and the protocol used within the system handles the control of the transmission.

  This form of serial transmission is typical of computer networks such as Ethernet, where data rates up to 10 Mb/s are usual. Coaxial cable or unscreened twisted pairs are used as the hardware for these.

  It is also used in fibre optic networks such as FDDI, and in High Speed Ethernet, where data rates of 100 Mb/s are used.

  The actual data transfer is effectively slower than these figures, because of the packeting overhead.

Parallel transmission

This is a form of data transmission which utilises eight parallel channels to transmit a full eight bit byte in one bit cycle.

It is obviously much faster for transmitting data than a system such as serial transmission for the same basic bit rate.

However it does suffer from the problem that the parallel wires can induce crosstalk between channels, leading to false data transfers.

The faster the bit rate, the faster are the rise and fall times, and the more the crosstalk.

Cable lengths are therefore limited to 3 - 5 metres.

The most common use for parallel transmission is for connecting printers to PC`s via the computer parallel port ( usually a 25 pin D-type ) and the centronics connector on the printer.

© 1998 Ron Turner

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