Ethernet - Computer Busses - page 439

tal signal and transfer the digital signal to the Manchester decoder. The network interface
consists of three parts:

• BNC/RJ-45 connector.
• Reception hardware - the reception hardware translates the waveforms transmitted on
the cable to digital signals then copies them to the Manchester decoder.
• Isolator - the isolator is connected directly between the reception hardware and the rest
of the Manchester decoder; it guarantees that no noise from the network affects the com-
puter, and vice versa (as it isolates ground levels).

The reception hardware is called a receiver and is the main component in the network inter-
face. It acts as an earphone, listening and copying the traffic on the cable. Unfortunately, the
Ether and transceiver electronics are not perfect. The transmission line contains resistance
and capacitance which distort the shape of the bit stream transmitted onto the Ether. Distor-
tion in the system causes pulse spreading, which leads to intersymbol interference. There is
also a possibility of noise affecting the digital pulse as it propagates through the cable.
Therefore, the receiver also needs to recreate the digital signal and filter noise.
Figure 26.16 shows a block diagram of an Ethernet receiver. The received signal goes
through a buffer with high input impedance and low capacitance to reduce the effects of
loading on the coaxial cable. An equaliser passes high frequencies and attenuates low fre-
quencies from the network, flattening the network passband. A 4-pole Bessel low-pass filter
provides the average dc level from the received signal. The quench circuit activates the line
driver only when it detects a true signal. This prevents noise activating the receiver.

Line
driver

Receive
equaliser

RX+
RX-

GND

Squelch

Data
buffer

RXI

Low-pass
filter

GND is -9V (isolated)
RX is receive output
RXI is network signal receiver

Figure 26.16 Ethernet receiver block diagram

26.12.2 Manchester decoder
Manchester coding has the advantage of embedding timing (clock) information within the
transmitted bits. A positively edged pulse (low to high) represents a 1 and a negatively edged
pulse (high to low) a 0, as shown in Figure 26.17. Another advantage of this coding method
is that the average voltage is always zero when used with equal positive and negative voltage
levels.
Figure 26.18 is an example of transmitted bits using Manchester encoding. The receiver
passes the received Manchester-encoded bits through a low-pass filter. This extracts the low-
est frequency in the received bit stream, i.e. the clock frequency. With this clock the receiver