ADM488/ADM489
Rev. D | Page 14 of 16
APPLICATION INFORMATION
DIFFERENTIAL DATA TRANSMISSION
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. Two main standards that specify the electrical
characteristics of transceivers used in differential data
transmission are approved by the EIA.
The RS-422 standard specifies data rates up to 10 MBaud and
line lengths up to 4000 ft. A single driver can drive a transmis-
sion line with up to 10 receivers.
To cater to true multipoint communications, the RS-485 stan-
dard was defined to meet or exceed the requirements of RS-422.
It also allows up to 32 drivers and 32 receivers to be connected
to a single bus. An extended common-mode range of −7 V to
+12 V is defined. The most significant difference between the
RS-422 and RS-485 is that the RS-485 drivers can be disabled,
thereby allowing up to 32 receivers to be connected to a single
line. Only one driver should be enabled at a time, but the RS-
485 standard contains additional specifications to guarantee
device safety in the event of line contention.
CABLE AND DATA RATE
The transmission line of choice for RS-485 communications is a
twisted pair. Twisted-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.
The ADM488/ADM489 are designed for bidirectional data
communications on multipoint transmission lines. A typical
application showing a multipoint transmission network is
illustrated in
Figure 30. An RS-485 transmission line can have
up to 32 transceivers on the bus. Only one driver can transmit
at a particular time, but multiple receivers can be simultane-
ously enabled.
As with any transmission line, it is important that reflections be
minimized. This can 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.
Table 10. Comparison of RS-422 and RS-485 Interface Standards
Specification RS-422 RS-485
Transmission Type Differential Differential
Maximum Data Rate 10 MB/s 10 MB/s
Maximum Cable Length 4000 ft. 4000 ft.
Minimum Driver Output Voltage ±2 V ±1.5 V
Driver Load Impedance 100 Ω 54 Ω
Receiver Input Resistance 4 kΩ minimum 12 kΩ minimum
Receiver Input Sensitivity ±200 mV ±200 mV
Receiver Input Voltage Range −7 V to +7 V −7 V to +12 V
Number of Drivers/Receivers per Line 1/10 32/32
D
R
D
R
D
R
RT RT
D
R
00079-030
Figure 30. Typical RS-485 Network
