ADM488 Analog Devices, ADM488 Datasheet - Page 11

• Manufacturer Part Number: ADM488
• Description: Full-Duplex/ Low Power/ Slew Rate Limited/ EIA RS-485 Transceivers
• Manufacturer: Analog Devices

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APPLICATIONS INFORMATION
Differential Data Transmission
Differential data transmission is used to reliably transmit data
at high rates over long distances and through noisy environ-
ments. Differential transmission nullifies the effects of ground
shifts and noise signals, which appear as common-mode volt-
ages on the line. Two main standards are approved by the
Electronics Industries Association (EIA), which specify the
electrical characteristics of transceivers used in differential
data transmission.
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.
In order to cater to true multipoint communications, the RS-
485 standard was defined. This standard meets or exceeds all
the requirements of RS-422 and also allows for 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 RS-422 and RS-485 is the fact
that the drivers may be disabled thereby allowing more than
one (32, in fact) 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.
Specification
Transmission Type
Maximum Data Rate
Maximum Cable Length
Minimum Driver Output Voltage
Driver Load Impedance
Receiver Input Resistance
Receiver Input Sensitivity
Receiver Input Voltage Range
Number of Drivers/Receivers Per Line
Table V. Comparison of RS-422 and RS-485 Interface Standards
D
R
RT
Figure 25. Typical RS-485 Network
D
R
–11–
RS-422
Differential
10 MB/s
4000 ft.
100
4 k min
–7 V to +7 V
1/10
2 V
200 mV
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 is designed for bidirectional data com-
munications on multipoint transmission lines. A typical applica-
tion showing a multipoint transmission network is illustrated in
Figure 25. An RS-485 transmission line can have as many as
32 transceivers on the bus. Only one driver can transmit at
a particular time but multiple receivers may simultaneously
be enabled.
As with any transmission line, it is important that reflections are
minimized. This may be achieved by terminating the extreme
ends of the line using resistors equal to the characteristic imped-
ance 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.
D
R
RT
RS-485
Differential
10 MB/s
4000 ft.
54
12 k min
–7 V to +12 V
32/32
R
1.5 V
200 mV
D
ADM488/ADM489