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7.3.2 Synchronous transmission

In synchronous transmission there are two circuits between the transmitter
and receiver: the data line and the clock line.

The data line is used for the zeros and ones of the data as shown in the
previous section for asynchronous transmission. However, the data has
no structure in synchronous transmission—it is a continual stream of
bits. This means that if there is no real data ready for transmission, then
some form of padding data must be sent to keep the line busy. There can
be no gaps or idle states in this form of transmission.

The clock line carries the ticks of a clock from one component to the
other, so that the transmitter can set the state of the line, and the receiver
can detect the state, using the same clock. This guarantees keeping the
two ends in synchronization.

The procedure for sending 0110 is shown in Fig. 7.4.

Fig. 7.4 Procedure for synchronous transmission of 0110

The receiver will normally detect the state of the line in the middle of
the bit time to ensure that the transmitter has had a chance to set the
state. It does not matter which of the transmitter or receiver contain the
clock. It can be in the receiver, in which case the clock line transmits in a
different direction to the data.

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