AN-5023 LVDS Compatibility with RS422 and RS485 Interface Standards
AN-5023
Fairchild Semiconductor
Application Note
July 2002
Revised July 2002
LVDS Compatibility
with RS422 and RS485 Interface Standards
Abstract
Low Voltage Differential Signaling (LVDS) technology
offers one of the best serial data transmission profiles
available today for physical layer interfaces. With many
existing interface standards to choose from, this applica-
tions note provides guidelines on the inter-operation of
LVDS devices with other differential interface standards
such as RS-422/485. An example is included of an LVDS
receiver configured to be inter-operable with an RS-422
driver.
Interface Standards
With so many electrical interface standards having sepa-
rately evolved within particular industries, many serial data
interface standards now exist. To simplify designers’
choices, the following table summarizes some of the key
electrical specifications for the different serial data interface
standards listed.
TABLE 1. Interface Standards Specifications
Parameter
Differential Voltage Swing (typ)
Common Mode Voltage
Driver Output Rise Time
Receiver Common-Mode Voltage
Receiver Sensitivity
Signaling Rate
Transmission Distance (Note 1)
RS-422
3.0V
1.8V
10%tui
RS-485
3.0V
1.8V
30%tui
RS-644 (LVDS)
350mV
1.2V
30%tui
0V to
+
2.4V
±
7V
±
200mV
<
50Mbp/s
≤
1200m
−
7V/
+
12V
±
200mV
<
50Mbp/s
≤
1200m
±
100mV
>
400Mbp/s
≤
100m
Note 1:
The transmission distance is shorter with faster signaling rates.
The three standards in Table 1 are hardware specifications
that define the driver and receiver electrical characteristics.
Software protocols are not discussed in any of the stan-
dards, and it is up to the system designer to define a proto-
col suitable for their system. The RS-422 differential
standard was established to provide a balanced intercon-
nect system (in preference to the single-ended interface)
for use at higher signaling rates. The RS-422 standard is
suited for point-to-point and multi-drop interconnects and
not so well suited for multipoint systems. The RS-485 dif-
ferential standard was issued to provide the driver/receiver
electrical characteristics for balanced multipoint systems.
One of the principle differences between RS-422 and
RS-485 standards is that the RS-485 driver can be placed
into a high impedance tristate mode, allowing drivers to
transmit over the same pair of wires.
Compatibility with RS-422 and RS-485 Standards
The LVDS interface circuit is not intended for direct inter-
operation with other interface technologies, like RS-422,
RS-485, or even Positive Emitter Coupled Logic (PECL).
Under certain conditions, inter-operation of LVDS with
other interface circuits mentioned above may be possible
but may require modification in the interface or within the
equipment. Limitations on certain performance parameters,
like common mode range, may be required; satisfactory
operation is not assured and additional provisions may
need to be employed.
In determining whether direct inter-operation of LVDS TIA/
EIA-644 compliant devices is possible with other interface
standards, it is necessary to compare the generator (driver)
output and the receiver input electrical specifications. Spe-
cifically the driver’s differential output voltage (V
OD
) and
the driver offset voltage (V
OS
) must be within the bounds of
the LVDS RS-644 receiver’s input ranges. Correspond-
ingly, the receiver’s input thresholds and voltage range
must be able to accept the LVDS RS-644 driver’s output
levels.
© 2002 Fairchild Semiconductor Corporation
AN500526
Drivers compliant to the TIA/EIA-644 standard feature a
current source capable of delivering a loop current in the
range of 2.5 to 4.5mA. As illustrated in Figure 1, the result-
ing differential voltage (V
OD
) will be a minimum of 250 mV
up to a maximum of 450mV across the 100
Ω
termination
resistor. The driver offset voltage (V
OS
), also referred to as
the center point, is typically
+
1.2V referenced to circuit
common ground.
Any balanced receiver that guarantees the input voltage
range of 0V to
+
2.4V and input thresholds of 200mV or less
may be compatible and directly inter-operate with other
balanced drivers. Compatibility is possible provided the
balanced driver does not violate the maximum receiver
input voltage range and develops a differential voltage
(V
OD
) of at least 100mV and not greater than 600mV. Inter-
operation with drivers with larger output differential volt-
ages is possible by using an attenuation circuit with the
interface points of the components. Refer to Figure 2 for
the illustration.
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AN-5023
Compatibility with RS-422 and RS-485 Standards
(Continued)
FIGURE 1. Driver Output Levels
FIGURE 2. Application of RS-422 Data to an LVDS Receiver
The R1, R2, and R3 resistor divider network collectively is
a total differential load of 100
Ω
to match the characteristic
impedance of the transmission line and reduces the typical
RS-422 differential signal amplitude of 3.3V down to a
330mV level. Essentially, the resistor network is a 10:1
I
TOT
=
V
TOT
/R
TOT
attenuation circuit. Refer to the equation below to calculate
the voltage drops across the resistors.
The majority (90%) of the RS-422 voltage is dropped
across resistors R1 and R2, leaving the remaining voltage
to be dropped across R3 which is compatible to the LVDS
receiver.
(Current through the resistor network)
=
3.3V / (R1
+
R2
=
R3)
=
3.3V / 100
Ω
=
33 mA
(Voltage across R1)
V1
=
I
TOT
* R1
(Voltage across R2)
V2
=
I
TOT
* R2
(Voltage across R3)
V3
=
I
TOT
* R3
=
(33mA)(45
Ω
)
=
1.485V
=
(33mA)(45
Ω
)
=
1.485V
=
(33mA)(10
Ω
)
=
330mV
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2
AN-5023 LVDS Compatibility with RS422 and RS485 Interface Standards
Summary
Compatibility of TIA/EIA-644 LVDS drivers and receivers
with other interface standards can be achieved by employ-
ing additional attenuation circuitry within the interface
points of the system. When inter-operation of LVDS compli-
ant devices is desired, provisions should be made to imple-
ment the attenuation circuitry that adjusts the differential
amplitude voltages of the balanced driver to be within the
receiver voltage range. As always, application specific
requirements and needs may dictate what design tech-
niques must be implemented to make a reliable intercon-
nect system.
Fairchild does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and
Fairchild reserves the right at any time without notice to change said circuitry and specifications.
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