Synchronous and Asynchronous Transmission

 

 

 

 

 

Synchronous transmission

This is a form of data transmission where the transmitter and receiver are synchronised, ie, are running not only at the same clock rate, but the clocks are locked together.

This can be achieved in three ways -

  • The transmitter and receiver are part of one system which has one master clock

  • The interconnection between the transmitter and the receiver carries a seperate clock signal which is used to lock up the receiver to the transmitter

  • The serial data stream uses a coding system that enables the receiver to extract a clock signal from every bit cycle, even if the data signal is a continuous stream of logical 1`s or 0`s.

    This can be done, for example, by having a transition from high to low or low to high every cycle, with an additional transition during the cycle to represent one of the logical states, but no additional transition for the other logical state.

These methods tend to require that the transmitter and receiver are within a common system, in fairly close proximity to each other.

Also, because the receiver knows exactly when it is going to receive each bit, there is a high immunity to noise and distortion.

Synchronous transmission is therefore usually used for interconnection of computers within a system, where high data rates, and high data volume transfers can be achieved.

 

Asynchronous transmission

This is a form of data transmission where there is no synchronisation between the clocks of the transmitter and the receiver.

The transmitter fires off some data, and the receiver must be able to capture it, without knowing in advance that it is about to arrive.

A number of techniques are used to allow this to happen :

  • The receiver samples the incoming data line at a rate typically 8 to 10 time faster than the expected incoming data rate. It can therefore detect the existence of a new data burst within the first eighth or tenth of a bit cycle.

  • Each character within the data stream is wrapped in a packet, which typically has two start bits before the character data, and one stop bit after the character data.

  • The start bits are both a logical 1, and are represented by a voltage level opposite to that used when the line is idle. This means the receiver can "see" the new activity on the line.

    The stop bit is a logical 0, and is represented by a voltage level which is the same as when the line is idle.

    This means that at the end of the packet, the line is sitting at the idle level, without a further transition.

  • All the packets are the same length - ie, all the characters are described by the same number of bits, so the receiver knows when it should be receiving a stop bit, and hence the next start bit.

Most PC`s have a standard type of asynchronous transmission receiver built into them - this is known as a UART - which stands for Universal Asynchronous Receiver Transmitter. This is usually a single integrated circuit.

Asynchronous transmission links are typically used for peripherals such as keyboards and for modems.

Typical bit rates range from 300 bits/sec to 28.8 kbits/sec, but actual character data rate is lower, because of the start and stop bits.

 

© 1998 Ron Turner

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