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

2-2 Industrial Communication Systems
Signaling rate (Mbps)
10,000
1,000
100
10
CML/ECL
LVDS
M-LVDS
485/422
CML/ECL
LVDS
M-LVDS
485/422
1
0.1
0.1 1 10 100 1,000 10,000
Distance (m)
FIGURE 2.1
Signaling rate versus cable length.
to the sender with an amplitude and sign depending on the mismatch’s impedance. Cable mismatch
mostly reduces the achievable bit rate and, therefore, should be avoided.
The form of a rectangular pulse remains equal as long as the pulse frequency components are transmitted
with the same attenuation and velocity. Frequency dependence of phase velocity is called dispersion.
Mostly, cable attenuation (in dB/m) increases with the frequency. This and increasing cable length
are limiting the maximum rate of transmitted bits considering a maximal bit error rate (BER). Defining
the distance as length of the signal transmission path between the sending and the receiving system and
the signaling rate as the bit rate at which data have to be passed to the receiving device, a characteristic
relation can be stated as defined by Figure 2.1.
Different transmission standards, such as TIA/EIA–485, low-voltage differential signaling (LVDS),
multiplex LVDS (M-LVDS), and current-mode logic (CML) provide solutions for various needs in terms
of bit rate and line length (see Table 2.1 for details of the standards). As shown in Figure 2.1, as the cable
length increases, the speed at which the information is transmitted must be lowered in order to keep
the BER down. More details with regard to physical properties, transmission types and standards, and
devices can be found in [TI04,NS08].
2.2.2 Cable Types and Operational Characteristics
Besides of cable data transmission characteristics, the main important aspect is cable protection against
electromagnetic influences of the environment, which gets growing importance with increasing cable
length and strong and high-frequent electromagnetic fields near the cable. Two parameters influence
protecting the cable against influences: cable type and construction.
Basically, a cable has two lines. This unshielded type can be additionally protected by shielding where
the two lines are placed within a conductive hose. Mostly connected with ground potential at one end,
this hose reduces the impact of electromagnetic noise.
Several cable constructions are well suited for supporting high bit rates: coaxial cable and one or
multiple twisted pairs. The coaxial cable is set up with a line in the center of a conductive hose that
delivers a shielding of the inner signal line by the hose at ground potential. Primarily used for unbalanced
signal transmission as described in the following, a coaxial cable can be additionally shielded
by an outer hose.
© 2011 by Taylor and Francis Group, LLC