AN-960 查看數據表（PDF） - Analog Devices
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AN-960 Datasheet PDF : 12 Pages
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APPLICATION NOTE
RS-485 OR RS-422?
RS-422 is specified as a simplex multidrop standard, which
means that only one driver and up to ten receivers can be
connected to the same bus. If more than one driver needs to
be connected on the same bus, then RS-485 is recommended.
RS-485 is specified as a multipoint standard, which means up
to 32 transceivers can be connected on the same bus.
Figure 2 shows a typical RS-422 interface circuit. Although
an RS-485 circuit may appear similar, the main difference is
in the bus architecture. Figure 3 shows a typical RS-485 appli-
cation circuit.
RS-422
The RS-422 standard specifies data rates up to 10 Mbps and
line lengths of up to 4000 feet. A single driver can drive a
transmission line with up to 10 receivers. The common-mode
voltage (VCM) is defined as the mean voltage of A and B
terminals with respect to signal ground (VCM = (VIA + VIB)/2).
The RS-422 receivers can withstand a common-mode
voltage (VCM) of ±7 V. When all ten receivers are placed on the
bus, a maximum load condition occurs. The input impedance
of the RS-422 receiver must be larger than or equal to 4 kΩ.
RS-485 AND THE UNIT LOAD CONCEPT
The input impedance of the RS-485 receiver is specified as
larger than or equal to 12 kΩ. This impedance is defined as
having one unit load (UL). The RS-485 specification specifies
the capability to sustain up to 32 ULs.
Some RS-485 receivers are specified as having ¼ UL or
⅛ UL. A receiver specified to have ¼ UL means that the
receiver only loads the bus by ¼ of the standard UL and,
therefore, 4 times as many of these receivers can be connected
to the bus (4 × 32 = 128 nodes).
RO
RO
RO
AN-960
Similarly, if an RS-485 receiver is specified to have ⅛ UL,
the receiver only loads the bus by ⅛ of the standard UL and,
therefore, 8 times as many of these receivers can be connected
to the bus (8 × 32 = 256 nodes). See Table 1 for UL and receiver
input impedance details.
Many of the RS-485 transceivers characteristics are the same as
for RS-422. The common-mode voltage range for RS-485 is
expanded to −7 V to +12 V. The RS-485 transceivers must
withstand this common-mode voltage range while tristated
(disconnected from the bus).
An RS-485 system must have a driver that can be disconnected
from the transmission line when a particular node is not
transmitting. The DE (RTS) pin on the RS-485 transceiver
enables the driver when a logic high is set to DE (DE = 1).
Setting the DE pin to low (DE = 0) puts the driver in a tristate
condition. This effectively disconnects the driver from the
bus and allows other nodes to transmit over the same twisted
pair cable.
RS-485 transceivers also have an RE pin that enables/disables
the receiver. The DE and RE pins combined allow some RS-485
transceivers to be put into a low power shutdown mode. This is
important for battery-powered applications.
Table 1. UL Receiver Input Impedance
Unit Load No. of Nodes Min. Receiver Input Impedance
1
32
12 kΩ
½
64
24 kΩ
¼
128
48 kΩ
⅛
256
96 kΩ
RO
RO
R1
R2
R3
R4
R5
DI
D
RT R10
RO
R6
R7
R8
R9
RO
RO
RO
RO
Figure 2. Typical RS-422 Interface Circuit
Rev. 0 | Page 3 of 12
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