Synchronous data transmission requires a clock signal to control the bit detection. This enables the transmission of several characters in sequence or of large binary data such as images or floating-point numbers. The clock signal can be exchanged via an individual connection of the interface or it can be retrieved by the receiver from the data stream (clock signal retrieval). This retrieval requires the RZ code or the Manchester code, as only these guarantee clock signal retrieval.
In the case of RZ coding, the level state returns to zero after half the clock rate. This ensures that the receiver can retrieve the clock signal due to the level change.
Signal level of RZ code:
Due to the return to zero, the data signal is not free of DC voltage, and electromagnetic isolators for electric isolation can therefore not be used.
The signal processing of the Manchester code according to G. E. Thomas is a result of the clock signal being connected iwth the XOR data signal. A logic 1 is represented by a downwards arrow ↓, and a logic 0 by an upwards arrow ↑. For the Manchester coding as defined for Ethernet (IEEE 802.3) the signals are practically negated. A logic 1 is represented by an upwards arrow, and a logic 0 by a downwards arrow.
Signal level of Manchester code:
Further information in Wikipedia: