wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

2-4 Industrial Communication Systems
Diff. driver
Signal line
Data in V out +
Signal line
V out –
V CM
+
V Diff.
V CM
–
V GND
Diff. receiver
Data out
FIGURE 2.3
Differential transmission.
more than 1.m. These additional efforts significantly reduce the cost advantages. Disadvantage of SE
transmission is the poor noise immunity originated by the way of ground wiring.
For balanced or differential transmission, a pair of signal lines is necessary together with an additional
ground connection (see Figure 2.3). The signal is transmitted by a sender with differential outputs
in a way that the signal is placed on one line and the inverted signal on the second line. The receiver
detects the voltage difference between its inputs. Good noise performance comes from the use of a
twisted pair whereby noise is coupled into both signal lines in the same way. Due to the CM rejection
capability of a differential amplifier, this noise is without effect. Twisted-pair cables with correct termination
and differential signaling allow very high data rates up to 10.Gbps. Only cost is the primary
disadvantage of differential transmission. Furthermore, high data rates require a very well-defined cable
impedance and correct termination to avoid reflections.
2.2.4 Simplex and Duplex Communication
Media access for data transmission can be performed in different topologies in respect of directly realizable
communication links. These topologies are point-to-point, multidrop, and multipoint.
Point-to-point or simplex communication is characterized by one sender and one receiver per signal
line or line pair of a cable (Figure 2.4). Data transmission is possible only in one direction, i.e.,
unidirectional. Being the most elementary topology, it has the advantage of enabling well-controlled
cable impedances necessary for very high signaling rates. All signaling technologies may be used for
point-to-point links. However, very fast differential signaling technologies are primarily designed
for point-to-point signal transmission.
A topology with one sender and multiple receivers is called multidrop or distributed simplex (Figure
2.5). Only unidirectional transfer is possible. Terminating the cable on the far receiver side is advisable
only when the signal driver is on the opposite end of the cable from the terminated receiver. In
all other cases (e.g., driver connected to the middle of the bus), the bus needs to be terminated at both
ends of the bus.
The multipoint or multiplex topology is implemented with many transmitters and many receivers
per line (Figure 2.6). In practice, this solution is often realized with combined transmitter–receiver
pairs called transceivers. Any combination of receivers, transmitters, and transceivers is possible for
this topology. Transmission is possible in both direction, i.e., bidirectional, and all signal drivers and
receivers are sharing the same single interconnect. A frequently implemented variant of the multipoint
Single ended
Differential
D R D R
FIGURE 2.4
Point-to-point communication link.
© 2011 by Taylor and Francis Group, LLC