asynchronous link vs synchronous link

Hi Folks,

Can somebody please explain the basic differences between a synchronous/asynchronous link?

Thanks, syuga

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Serial connections use two "states" to transmit and receive traffic (1's and

0's), usually sent as two different voltage levels on the line (a high and low). The receiver samples the line during a "clock cycle" and reads the voltage level to determine if it received a high or a low voltage. The clock is simply the bit rate. So at 9600 baud, the line is sampled every 1/9600 of a second. The clock of the receiver must be synchonized with the transmitted signal in order to acurately determine if a high or low was sent. Why, because if the clock is off by half a cycle and a transition occurs between two consecutive states (a high to low) , the receiver can misinterpret the received signal as either high or low. The sample during the "clock" is not at the "high" level, and its not at the "low" level either.

On a synchonous serial link, both sides transmit basically transmit a signal all the time and one end of the link provides clock, and the other side recovers it from the received signal. On an asynchronous link, each side provides their own clock and each side only transmits when it's sending data. Because of this, asynchronous links must first send some bits before the actual data so that the receive side can "synch" up with the transmitted signal. On low speed serial connections it is only a single bit (start bit), followed by the data, followed by a "stop bit". On high speed asynchronous serial connections (like Ethernet), the "start of frame" is 9 bits (110101010). Synchonous serial connections require a much more sophisticated circuit to receive the data, generally use more sophisticated encoding schemes to transmit data, and are more expensive to build and implement. Asychronous circuits are simpler to build and cheaper. Synchronous lines make better use of the line because start and stop bits are not needed. The serial port on a PC is asynchronous and must send 10 bits on the line for each 8 bits of data, so on a 9600 bit/s asynchronous connection only 7,680 of those bits are data (only 80% efficient). On the other hand a 9600 bit/s synchonous connection is 100% efficient.

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Hi Syuga,

To some extent it depends on the context in which the terms are used. For example, BOTH terms are used within Serial Communications, where a serial data stream needs some method of synchronising the data stream between the sender and receiver, so a CLOCK signal is used.

Sync usually refers to a situation where the clocking signal is external to the Data signal but there is a strong relationship between them.

Async usually refers to a situation where the Clocking signal is embedded WITHIN the Data Signal, which effectively increases the size of the unit of data that is transferred slightly (typically where an 8 Bit Character stream sends 8 bits per character traffic when using SYNC transmission, is sent using 10 bits per character when sent as ASYNC traffic).

An ASYNC transmission method has a higher control component compared to a SYNC transmission method, but the later requires close synchronisation between the Clock and the data (IE closer tolerances are required for SYNC traffic). ASYNC is usually used for slower speed transmission, while SYNC is usually used for higher speed transmission.

I hope this

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