TIA-485-A designates the two lines in a differential pair as A and B. At a typical
RS-485 driver, a TTL logic-high input brings line A more positive than line B,
while a TTL logic-low input brings line B more positive than line A. At the
RS-485 receiver, if input A is more positive than input B, the TTL output is
logic high, and if input B is more positive than input A, the TTL output is logic
low.
Referenced to the receiver??™s ground, each input must be within the range -7V to
+12V. This range allows for differences in ground potential between the driver
and receiver. The maximum allowed differential voltage, or the difference
between the voltages on line A and line B, is ?±6V.
3$ & # 4
Balanced lines are quiet because the two signal wires carry nearly equal but
opposite currents. The currents reduce received noise because most noise is
Inside RS-485
81
Figure 6-1: An unbalanced line uses one wire per signal while a balanced line uses
two wires per signal. Both types of line must have a common ground reference.
Table 6-1: TIA-485 and TIA-422 are balanced interfaces. TIA-422 allows just one
transmitter per line.
+"'-))'
%#'-))(
+"'-5*'
%#'-5*(
Cable length @90 kbps, max. (ft), approximate 4000 4000
Cable length @10 Mbps.,max. (ft), approximate 50 50
Data rate, max. (bps) 10M 10M
Differential output (minimum, V) ?± 2 ?± 1.
Pages:
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113