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ADM488ABRZ Datasheet PDF : 16 Pages
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APPLICATIONS INFORMATION
DIFFERENTIAL DATA TRANSMISSION
Differential data transmission reliably transmits 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 charac-
teristics of transceivers used in differential data transmission
are approved by the EIA.
The RS-422 standard specifies data rates up to 10 Mbps and line
lengths up to 4000 ft. A single driver can drive a transmission
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.
Table 8. Comparison of RS-422 and RS-485 Interface Standards
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
RT
D
ADM488A/ADM489A
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 causes cancellation of the magnetic fields generated
by the current flowing through each wire, thereby reducing the
effective inductance of the pair.
The ADM488A/ADM489A are designed for bidirectional data
communications on multipoint transmission lines. A typical
application with a multipoint transmission network is illustrated
in Figure 22. An RS-485 transmission line can have up to 32
transceivers on the bus. Only one driver can transmit at a par-
ticular time, but multiple receivers can be simultaneously enabled.
As with any transmission line, it is important to minimize reflec-
tions. This can be achieved by terminating the extreme ends of the
line using resistors equal to the characteristic impedance of the
line. Keep stub lengths of the main line as short as possible. A
properly terminated transmission line appears purely resistive
to the driver.
RS-422
Differential
10 Mbps
4000 ft.
±2 V
100 Ω
4 kΩ minimum
±200 mV
−7 V to +7 V
1/10
RT
RS-485
Differential
10 Mbps
4000 ft.
±1.5 V
54 Ω
12 kΩ minimum
±200 mV
−7 V to +12 V
32/32
D
R
R
R
D
R
D
Figure 22. Typical RS-485 Network
Rev. 0 | Page 13 of 16
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