ADM3491
AP P LICATIO NS INFO RMATIO N
Cable and D ata Rate
D iffer ential D ata Tr ansm ission
T he transmission line of choice for RS-485 communications is a
twisted pair. T wisted pair cable tends to cancel common-mode
noise and also causes cancellation of the magnetic fields gener-
ated by the current flowing through each wire, thereby reducing
the effective inductance of the pair.
Differential data transmission is used to reliably transmit data at
high rates over long distances and through noisy environments.
Differential transmission nullifies the effects of ground shifts
and noise signals which appear as common-mode voltages on
the line.
T he ADM3491 is designed for bidirectional data communica-
tions on multipoint transmission lines. A typical application
showing a multipoint transmission network is illustrated in
Figure 23. Only one driver can transmit at a particular time, but
multiple receivers may be enabled simultaneously.
T wo main standards are approved by the Electronics Industries
Association (EIA) which specify the electrical characteristics of
transceivers used in differential data transmission. T he RS-422
standard specifies data rates up to 10 MBaud and line lengths
up to 4000 ft. A single driver can drive a transmission line with
up to 10 receivers.
As with any transmission line, it is important that reflections are
minimized. T his may be achieved by terminating the extreme
ends of the line using resistors equal to the characteristic im-
pedance of the line. Stub lengths of the main line should also be
kept as short as possible. A properly terminated transmission
line appears purely resistive to the driver.
T he RS-485 standard was defined to cater to true multipoint
communications. T his standard meets or exceeds all the re-
quirements of RS-422, but also allows multiple drivers and
receivers to be connected to a single bus. An extended common
mode range of –7 V to +12 V is defined.
Receiver O pen-Cir cuit Fail Safe
T he most significant difference between RS-422 and RS-485 is
the fact that the drivers may be disabled thereby allowing more
than one to be connected to a single line. Only one driver should
be enabled at a time, but the RS-485 standard contains addi-
tional specifications to guarantee device safety in the event of
line contention.
T he receiver input includes a fail-safe feature that guarantees a
logic high on the receiver when the inputs are open circuit or
floating.
Table III. Com parison of RS-422 and RS-485 Interface Standards
RS-422
Specification
RS-485
T ransmission T ype
Differential
4000 ft.
±2 V
Differential
4000 ft.
±1.5 V
Maximum Cable Length
Minimum Driver Output Voltage
Driver Load Impedance
Receiver Input Resistance
Receiver Input Sensitivity
Receiver Input Voltage Range
100 Ω
54 Ω
4 kΩ min
±200 mV
–7 V to +7 V
12 kΩ min
±200 mV
–7 V to +12 V
REV. 0
–11–
ADI [ ADI ]
