422 and 485 Standards Overview and System Configurations:

Application Report
SLLA070D – June 2002 – Revised May 2010
RS-422 and RS-485 Standards Overview and System
Configurations
Manny Soltero, Jing Zhang, and Chris Cockril ..................................................... HPA - Industrial Interface
Updated by Kevin Zhang, Clark Kinnaird, and Thomas Kugelstadt .............................................................
ABSTRACT
ANSI TIA/EIA-422 and TIA/EIA-485 standards, commonly known as RS-422 and RS-485, respectively,
specify balanced data-transmission schemes for transmitting data over long distances in noisy
environments. These standards are compared, and their basic differences and similarities are discussed.
Techniques for impedance matching to minimize line reflections in several applications are presented, with
laboratory test results.
7
8
9
10
Contents
Introduction .................................................................................................................. 3
Overview of RS-422 and RS-485 Standards ............................................................................ 3
2.1
Selected RS-422 Electrical Specifications ...................................................................... 3
2.2
Selected RS-485 Electrical Specifications ...................................................................... 9
Failsafe Operation ......................................................................................................... 12
3.1
The Need for Failsafe Protection ............................................................................... 12
3.2
Internal Failsafe ................................................................................................... 13
3.3
External Failsafe .................................................................................................. 13
Suggested Termination and Grounding Techniques .................................................................. 13
4.1
No Termination ................................................................................................... 13
4.2
Parallel Termination .............................................................................................. 14
4.3
AC Termination ................................................................................................... 16
4.4
Multipoint Termination ........................................................................................... 17
4.5
Ground Connections ............................................................................................. 18
Typical System Configurations .......................................................................................... 21
5.1
Daisy-Chain Configuration ...................................................................................... 21
5.2
Bus and Stub Configuration ..................................................................................... 21
5.3
Point-to-Point Configuration ..................................................................................... 22
Summary Comparison of the Standards ............................................................................... 22
6.1
Common-Mode Range ........................................................................................... 22
6.2
Line Contention ................................................................................................... 23
6.3
Drive Current ...................................................................................................... 23
Conclusion .................................................................................................................. 23
Glossary .................................................................................................................... 24
Acknowledgment .......................................................................................................... 24
References ................................................................................................................. 24
1
RS-422 Balanced-Voltage Digital-Interface Circuit ..................................................................... 4
2
RS-422 Open-Circuit Test Circuit ......................................................................................... 4
3
RS-422 Output-Voltage Test Circuit ...................................................................................... 5
4
RS-422 Short-Circuit Output-Current Test Circuit ...................................................................... 5
5
RS-422 Power-Off Output-Current Test Circuit ......................................................................... 5
6
RS-422 Test Circuit and Output-Signal Waveform ..................................................................... 6
1
2
3
4
5
6
List of Figures
SLLA070D – June 2002 – Revised May 2010
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
1
www.ti.com
7
RS-422 Input Receiver Test Circuit and I/V Plot ........................................................................ 7
8
RS-422 Input-Sensitivity Test Circuit and Resultant Waveform ...................................................... 8
9
RS-485 Balanced-Voltage Digital-Interface Circuit ..................................................................... 9
10
RS-485 U.L. Test Circuit and I/V Relationship
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
........................................................................
RS-485 Open-Circuit Test Circuit .......................................................................................
RS-485 Output-Voltage Test Circuit ....................................................................................
RS-485 Output-Voltage Test Circuit With Common-Mode Loading ................................................
RS-485 Short-Circuit Output-Current Test Circuit .....................................................................
RS-485 Output-Signal Test Circuit ......................................................................................
Differential Unterminated Configuration ................................................................................
Differential Unterminated-Driver Output Waveforms ..................................................................
Differential Unterminated-Receiver Input Waveforms ................................................................
Differential Parallel-Terminated Configuration .........................................................................
Differential Parallel-Terminated Driver Output Waveforms ..........................................................
Differential Parallel-Terminated Receiver Input Waveforms .........................................................
Differential AC-Terminated Configuration ..............................................................................
Differential AC-Terminated Driver Output Waveforms ................................................................
Differential AC-Terminated Receiver Input Waveforms ..............................................................
Differential Multipoint-Terminated Configuration ......................................................................
Grounding Configuration with Isolated Local Ground and PE .......................................................
Grounding Configuration with Connected Local Ground and PE ...................................................
Isolated Configuration .....................................................................................................
Daisy-Chain Connection ..................................................................................................
Stub Cables Connected to the Main Backbone .......................................................................
Point-to-Point Connection ................................................................................................
10
10
11
11
11
12
13
14
14
15
15
16
16
17
17
18
19
19
20
21
22
22
List of Tables
2
1
Input Sensitivity and Resultant Voltages of 422-Compliant Devices ................................................. 8
2
Input Sensitivity and Resultant Voltages of 485-Compliant Devices
3
Summary of Termination Techniques................................................................................... 18
4
Summary Comparison of RS-485 and RS-422 Specifications ...................................................... 23
...............................................
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
12
SLLA070D – June 2002 – Revised May 2010
Introduction
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1
Introduction
The RS-422 and RS-485 standards, as they are known today, are balanced data-transmission schemes
that offer robust solutions for transmitting data over long distances and noisy environments. The official
titles for these two standards are ANSI TIA/EIA-422 and TIA/EIA-485, respectively, and are revised
periodically by the TR-30.2 DTE-DCE Interfaces and Protocols Subcommittee to the Telecommunications
Industry Association (TIA) TR-30 Data Transmission Systems and Equipment Committee. For
identification, RS-422 and RS-485 suffice.
This application report offers an overview of the RS-422 and RS-485 standards. While many specifications
are described in the official ANSI documents, only the most prevalent are discussed in this application
report. The purpose of this application report is to not duplicate the official documents, but to outline basic
differences and similarities between the RS-422 and RS-485 standards. Major specifications are described
in detail and the two standards are compared. Because impedance matching is an important aspect of
differential data transmission in minimizing line reflections due to transmission-line effects, techniques for
terminating different system applications are presented. Also, typical system configurations are taken into
consideration for optimal application performance and cost constraints.
2
Overview of RS-422 and RS-485 Standards
Officially, the RS-422 standard's title is Electrical Characteristics of Balanced Voltage Digital Interface
Circuits, and is published by the ANSI Telecommunication Industry Association/Electronic Industries
Association (TIA/EIA). In the industry, the term RS-422 is commonly used rather than the official name,
and this document does the same. RS-422 is specified as a simplex multidrop standard, which means
only one driver and up to ten receivers can be attached to a bus.
The RS-485 standard's title is Electrical Characteristics of Generators and Receivers for Use in Balanced
Digital Multipoint Systems. RS-485 is commonly used, rather than its official title. If more than one driver is
required, devices conforming to RS-485 are recommended. RS-485 specifications allow only one driver to
send data at a time, and up to 32 unit loads (U.L.) can be placed on the bus. The U.L. concept is
described in this application report in the Selected RS-485 Electrical Specifications section.
RS-422 and RS-485 initially might appear to be similar, but are distinct, and interchangeability is
determined by the bus architecture. The RS-485 standard is written to be electrically compatible with
RS-422. To illustrate their basic differences, a condensed description of each standard is presented in the
following subsections.
2.1
Selected RS-422 Electrical Specifications
The balanced-voltage digital interface is shown in Figure 1. The driver (or generator) is labeled D, the
receiver is labeled R, and the termination impedance is ZT. The termination impedance should be equal to
the characteristic impedance of the cable, Zo, and is used only once at the end of the cable. Because
matching termination impedance to Zo often is difficult to achieve and is application dependent, typically,
±20% is sufficient. Also, up to nine additional receivers can be placed along the cable from points A and B
to points A' and B', respectively. No restriction on maximum cable length is imposed by the RS-422
standard. Taking this into account, systems of up to 1 km are not uncommon, with signaling rates no
higher than about 100 kbps. Speed and cable lengths work against each other. In other words, the longer
the cable, the slower the signaling rate must be, while data can be transmitted faster on shorter cables. As
a rule of thumb, the data signaling rate (in bps) multiplied by the cable length (in meters) should not
exceed 108. For example, a system with a cable measuring 500 m should not transmit data at speeds
greater than 200 kbps (108/500).
SLLA070D – June 2002 – Revised May 2010
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
3
Overview of RS-422 and RS-485 Standards
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A
Logic
(1 or 0)
A’
ZT
D
B
R
B’
S0450-01
A
D = driver (or generator)
B
R = receiver
C
ZT = termination impedance
Figure 1. RS-422 Balanced-Voltage Digital-Interface Circuit
Although the input electrical characteristics of the RS-422-compliant receiver are identical to those of the
RS-423-compliant receiver (ANSI TIA/EIA-423 standard), the RS-423 specifies an unbalanced signaling
scheme, which is not within the scope of this application report.
Descriptions of selected specified parameters are presented in the following paragraphs.
2.1.1
Open-Circuit Output Voltage (VOD, VOA, and VOB Measured)
The output voltage shall not exceed ±6 V under unloaded conditions, and the differential voltage
[measured as the difference between an output voltage, VOA (VOB), and its complementary output voltage,
VOB (VOA)] is no greater than ±10 V. See Figure 2 for the test circuit.
A
Logic
(1 or 0)
+
–
D
VOD
B
VOB
+
–
+
–
VOA
S0451-01
A
|VOD| ≤ 10 V, |VOA| ≤ 6 V, and |VOB| ≤ 6 V
B
VOA = voltage on A output
C
VOB = voltage on B output
D
VOD = differential output voltage
Figure 2. RS-422 Open-Circuit Test Circuit
2.1.2
Differential and Offset Output Voltage (VOD and VOC Measured)
To ensure proper drive strength, a minimum of ±2-V VOD and a maximum of ±3-V VOS are measured (see
Figure 3). Furthermore, a check on driver output-voltage balance between the differential output voltages
is put in place to measure the change in these voltages (not to exceed 400 mV). The maximum limit of
400 mV most often is approached during transients when driver outputs are switching states.
4
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
SLLA070D – June 2002 – Revised May 2010
Overview of RS-422 and RS-485 Standards
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50 W
A
Logic
(1 or 0)
+
–
D
VOD
B
+
–
50 W
VOC
A. VOD| ³ 2 V, |VOS| £ 3 V
B. |DVOD| = ||VOD| - VOD|| £ 0.4 V
C. |DVOC| = ||VOC| - VOC|| £ 0.4 V
Figure 3. RS-422 Output-Voltage Test Circuit
2.1.3
Short-Circuit Output Current (IOS Measured)
With the driver shorted to ground, the magnitude of the output current shall not exceed 150 mA,
regardless of the state of the driver output (high or low) at the time of the short. This test ensures that the
device is not destroyed by excessive current flowing through the output stage. Figure 4 shows the test
circuit.
A
Logic
(1 or 0)
D
B
IOC
IOC
S0453-01
A
|IOC| to ground ≤ 150 mA
Figure 4. RS-422 Short-Circuit Output-Current Test Circuit
2.1.4
Power-Off Measurement (IOFF Measured; VO Applied)
As shown in Figure 5, with the driver powered down, the magnitude of the output leakage current shall not
exceed 100 µA for output voltages ranging from –0.25 V to 6 V. Currents higher than 100 µA can disrupt
the bus potential and lead to erroneous data at the receiver.
IOFF
A
Logic
(1 or 0)
D
B
IOFF
vO
vO
S0454-01
A
|Ioff| ≤ 100 µA for –0.25 ≤ VO ≤ 6 V
Figure 5. RS-422 Power-Off Output-Current Test Circuit
SLLA070D – June 2002 – Revised May 2010
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Copyright © 2002–2010, Texas Instruments Incorporated
5
Overview of RS-422 and RS-485 Standards
2.1.5
www.ti.com
Output-Signal Waveform (VOD Measured)
Basically, this test ensures good signal quality on the bus. With a 100-Ω resistor across the differential
output, the voltage monotonically changes between 10% and 90% of VSS within a tenth of the unit interval,
tui, or 20 ns, whichever is greater. Figure 6 shows the test circuit and resultant waveform. In addition, the
resultant voltage shall not change more than 10% of VSS after a transition has occurred (limits overshoots
and undershoots).
A
Logic
(1 or 0)
VOD
D
+
–
100 W
B
tui
1.1 VSS
0.9 VSS
0.1 VSS
VOD
VSS
0-V Differential
VOD
0.1 VSS
0.9 VSS
1.1 VSS
tr
tf
S0455-01
A. tui = time duration of the unit interval
B. VSS = |VOD - VOD|
C. 2 V ? |VOD| ? 10 V
Figure 6. RS-422 Test Circuit and Output-Signal Waveform
2.1.6
Input I/V Characteristics (VIA, and VIB, Applied; IIA, and IIB, Measured)
A maximum limit on the input characteristic must be placed on the receiver to ensure a maximum load on
the bus when all ten receivers are placed on it. With the common-mode voltage VIA, (VIB) ranging from +10
V to –10 V while VIB, (VIA,) is held to 0 V, the resultant input current should remain within the shaded
region (see Figure 7) in both the power-on and power-off conditions. A device with input characteristics
within the shaded region reveals that the input impedance is no smaller than 4 kΩ, as defined by the
calculation. The inverse of the slope of the upper and lower bounds is exactly the minimum input
impedance allowed for the input.
6
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
SLLA070D – June 2002 – Revised May 2010
Overview of RS-422 and RS-485 Standards
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IIA’
A’
R
B’
vIB’
vIA’
IIB’
II
3.25 mA
–10V
VI
–3V
+3V
+10V
–3.25 mA
S0456-01
A
R = ΔV/ΔI = 13 V/3.25 mA = 4 kΩ
Figure 7. RS-422 Input Receiver Test Circuit and I/V Plot
2.1.7
Input Sensitivity (VCM, VIA, and VIB, Applied; VID Measured)
Figure 8 shows the test circuit used to determine a receiver's input sensitivity. To ensure functionality over
the full common-mode range, suggested test voltages for both inputs and the purpose of the
measurements are given in Table 1.
SLLA070D – June 2002 – Revised May 2010
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Copyright © 2002–2010, Texas Instruments Incorporated
7
Overview of RS-422 and RS-485 Standards
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IIA’
A’
R
B’
vIB’
vIA’
IIB’
+10
VID
Maximum
Operating
Range
+200 mV
–200 mV
VID
Transition
Region
–10
S0457-01
A
200 mV < |VID| < 10 V
Figure 8. RS-422 Input-Sensitivity Test Circuit and Resultant Waveform
For a common-mode voltage varying from –7 V to 7 V, VID need not be greater than ±200 mV to correctly
assume the intended state. As specified in the standard, the magnitude of the differential input voltage,
VID, varying from 200 mV to 10 V, is required to maintain correct operation over this range.
Table 1. Input Sensitivity and Resultant Voltages of 422-Compliant Devices
APPLIED VOLTAGE
VIA'
VIB'
(3)
RESULTING
VID (1)
RESULTING
VCM (2)
RECEIVER
OUTPUT
STATE
+10 V
–2 V
+12 V
+4 V
Q
–10 V
+2 V
–12 V
–4 V
Q
+10 V
+4 V
+6 V
+7 V
Q
–10 V
–4 V
–6 V
–7 V
Q
+100 mV
–100 mV
+200 mV
0V
Q
–100 mV
+100 mV
–200 mV
0V
Q
+7.1 V
+6.9 V
+200 V
+7 V
Q
–7.1 V
–6.9 V
–200 V
–7 V
Q
(1)
(2)
(3)
2.1.8
(3)
PURPOSE OF MEASUREMENTS
Ensures correct operation with maximum differential
voltage supply
Ensures correct operation with maximum common mode
voltage supply
200 mV threshold test across common mode voltage
supply
|VID| < 12 V (maximum input differential voltage without damaging device)
VCM is measured as the arithmetic average of VIA' and VIB', or (VIA' + VIB')/2.
|VIA'| < 10 V, |VIB'| < 10 V (maximum input voltages to ensure correct operation)
Cable Termination
Cable termination is required, unless the data rate of the application is less than 200 kbps or the signal
rise/fall time at the load end of the cable is greater than four times the one-way cable delay. The latter
rule-of-thumb typically is used to describe a system that does not behave like a transmission line. In most
other applications, cable termination is recommended. Cable termination for a RS-422-compliant system
8
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
SLLA070D – June 2002 – Revised May 2010
Overview of RS-422 and RS-485 Standards
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always is placed at the load end of the cable. Two options for cable termination are recommended in the
standard. The first option is to match the termination resistance to the characteristic impedance of the
cable, Zo, while the second option is to place an additional capacitor in series with the termination
resistance for designers that are concerned with power dissipation. These two options are discussed in
detail in the Suggested Termination and Grounding Techniques section.
2.2
Selected RS-485 Electrical Specifications
By comparing Figure 1 and Figure 9, it is evident that RS-422 and RS-485 system topologies are different.
The RS-485 can operate in balanced digital multipoint systems, whereas the RS-422 can support only one
driver per bus line (multidrop). Parameter values specified in 485 are similar to those specified in RS-422.
Furthermore, RS-485-compliant receiver and driver electrical characteristics are specified such that they
cover requirements of RS-422. This allows RS-485-compliant drivers and receivers to be used in most
RS-422-compliant applications.
A
A’
A/A’
ZT
ZT
B
B’
D1
B/B’
R2
R3
D3
S0458-01
A
D1 = driver
B
D3/R3 = transceiver
C
R2 = receiver
D
ZT = termination impedance
E
Up to 32 U.L.s [receiver, driver (off state), transceiver]
Figure 9. RS-485 Balanced-Voltage Digital-Interface Circuit
Although RS-485 specifies that only one driver can talk at any given time (half-duplex operation), fault
conditions might occur (caused by inadvertent shorts on output drivers or line contention). Therefore,
RS-485-compliant devices must provide for this. For example, consider the case when driver D1 in
Figure 9 is intended to send a signal to receiver R2, but driver D3 still is enabled. If the designer did not
disable driver D3 before initiating the transmission, a fault condition occurs and erroneous data might be
transmitted to receiver R2. This condition also is known as line contention (see Summary Comparison of
the Standards section).
The maximum recommended cable length is about 1200 m. Usually, the amount of noise a designer is
willing to tolerate is the deciding factor in choosing the cable length. The same relationship of speed
versus cable length applies to RS-485-compliant systems, as well as to RS-422-compliant systems.
2.2.1
U.L. Concept (VIA and VIB Applied; IIA and IIB Measured)
As with the RS-422, a maximum limit on the I/V characteristic must be placed on the receiver, driver (off
state), and transceiver to ensure a maximum load on the bus when all 32 U.L.s are used. With the voltage
VIA (VIB) ranging from –7 V to 12 V, while VIB (VIA) is grounded, the resulting input current IIA (IIB) should
remain within the shaded region in both power-on and power-off conditions (see Figure 10). A device with
input characteristics that fall within the shaded region conforms to having a 1-U.L. characteristic. The
SLLA070D – June 2002 – Revised May 2010
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
9
Overview of RS-422 and RS-485 Standards
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RS-485 standard specifies the capability to sustain up to 32 U.L.s. The RS-485 often is thought of as a
12-kΩ load standard. Because the output current of the driver is dependent on loading, a design that
requires a large number of stations (drivers, receivers, or transceivers) attached to the bus needs a larger
load resistance to allow more connections. For example, a 0.5-U.L. transceiver can be placed up to 64
times on a bus, because this configuration complies with the maximum 32-U.L. specification.
IIA
A/A’
Load
B/B’
IIB
vIB
vIA
II
1 mA
–7V
VI
–3V
+5V
+12V
0.8 mA
S0459-01
Figure 10. RS-485 U.L. Test Circuit and I/V Relationship
2.2.2
Open-Circuit Output Voltage (VOD, VOA, and VOB Measured)
The output voltage shall not exceed ±6 V under unloaded conditions, and the differential voltage
generated will be no smaller than ±1.5 V and no greater than ±6 V. Figure 11 shows the test circuit used.
A
Logic
(1 or 0)
+
–
D
VOD
B
VOB
+
–
+
–
VOA
S0451-01
A
1.5 V ≤ |VOD| ≤ 6 V
B
|VOA| ≤ 6 V; |VOB| ≤ 6 V
Figure 11. RS-485 Open-Circuit Test Circuit
10
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
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2.2.3
Differential and Offset Output Voltage (VOD and VOC Measured)
Similar to the RS-422, RS-485 also specifies a minimum output voltage to ensure proper drive strength,
but also places a maximum limit. Figure 12 shows the limitations that have been placed on the differential
and offset voltages. It also shows the magnitude in the change in these voltages shall not exceed 200 mV.
27 W
A
Logic
(1 or 0)
+
–
D
VOD
B
+
–
27 W
VOC
A. 1.5 V £ |VOD| £ 5 V, and |DVOD| = ||VOD - |VOD|| £ 0.2 V
- |V || £ 0.2 V
B. -1 V £ |V | £ 3 V, and |DV | = ||V
OC
OC
OC
OC
Figure 12. RS-485 Output-Voltage Test Circuit
2.2.4
Differential Output Voltage With Common-Mode Loading (VOD Measured; VCM Applied)
With the test circuit shown in Figure 13, the magnitude of the differential output voltage falls within 1.5 V
and 5 V.
375 W
A
Logic
(1 or 0)
D
60 W
+
–
VOD
–7 V £ |VCM| £ 12 V
B
375 W
S0460-01
A
1.5 V ≤ |VOD| ≤ 5 V
Figure 13. RS-485 Output-Voltage Test Circuit With Common-Mode Loading
2.2.5
Short-Circuit Output Current (IOS Measured; VO Applied)
With the driver shorted to a voltage source that is varied from –7 V to 12 V, current shall not exceed 250
mA, and the driver shall not be damaged (see Figure 14). Texas Instruments incorporates into all its
RS-485-compliant devices a circuit that meets this requirement.
IOS
A
Logic
(1 or 0)
D
B
IOS
vO
vO
S0461-01
Figure 14. RS-485 Short-Circuit Output-Current Test Circuit
Although the internal circuitry limits the amount of current flowing from the output driver, complying with
the full range from –7 V to 12 V for an indefinite time is a very stringent test. For customers seeking
SLLA070D – June 2002 – Revised May 2010
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
11
Failsafe Operation
www.ti.com
devices with various levels of robustness, TI offers products that comply fully (indeterminate time period
and full voltage range) and partially with this specification. Partially compliant devices may limit the current
over the full range, but cannot sustain the current over a long period of time. Another example of partial
compliance is the capability to keep the current within specification for long periods, but at a smaller
voltage range.
2.2.6
Output-Signal Waveform (VOD Measured)
To ensure signal quality, RS-485 also places a constraint on the output-signal waveform. The
output-signal waveform is identical to the one described for the RS-422, but a different test circuit is
necessary (see Figure 15). The output voltage should change monotonically between 0.1 VSS and 0.9 VSS
within 0.3 tui. Thereafter, the output voltage shall not change more than 10% of VSS, and |VOD| ≤ 5 V.
A
Logic
(1 or 0)
D
50 pF
+
–
54 W
VOD
B
S0462-01
Figure 15. RS-485 Output-Signal Test Circuit
2.2.7
Input Sensitivity (VCM, VIA, and VIB, Applied; VID Measured)
Using the same test circuit as in RS-422, Table 2 shows the operating voltage extremes of the receiver
and the purpose of each measurement.
Table 2. Input Sensitivity and Resultant Voltages of 485-Compliant Devices
APPLIED VOLTAGE
3
VIA'
VIB'
RESULTING
VID
RESULTING
VCM
RECEIVER
OUTPUT STATE
–7 V
–6.8 V
–200 V
–6.9 V
Q
Minimum VI at extreme – VCM
+12 V
+11.8 V
+200 V
+11.9 V
Q
Minimum VI at extreme + VCM
–7 V
–2 V
–5 V
–4.5 V
Q
Maximum VI at extreme – VCM
+12 V
+7 V
+5 V
+9.5 V
Q
Maximum VI at extreme + VCM
PURPOSE OF MEASUREMENTS
Failsafe Operation
The feature of receiver failsafe is a benefit in many RS-422 and RS-485 applications; however, its
usefulness needs to be considered and understood at an application level.
3.1
The Need for Failsafe Protection
In any party-line interface system with multiple driver/receivers, long periods of time may occur when all
the driving devices are inactive. This state is known as line idle or bus idle and occurs when all drivers
place their outputs into a high-impedance state. During bus idle, the differential bus voltage is left floating
(i.e., indeterminate: neither logic-high nor logic-low state) if there is no termination resistors, and the
differential bus voltage is close to zero in the case where termination resistors are used. In both cases, as
a result, the receiver can be falsely triggered into either a logic-high or logic-low state, depending on the
presence of noise and the last polarity of the floating lines. Obviously, this is undesirable, as the circuitry
following the receiver could interpret this as valid information. It is best to detect such a situation and place
the receiver outputs into a known and predetermined state. The name given to methods that ensure this
condition is receiver failsafe. An additional, desirable feature that a failsafe provides is to protect the
receiver from shorted line conditions, which can again cause erroneous processing of data.
12
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Suggested Termination and Grounding Techniques
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3.2
Internal Failsafe
TI and other manufacturers have devices with failsafe design by including some form of open-circuit,
short-circuit, and idle-bus failsafe circuitry within the integrated circuits, which simplify the system design.
Further discussion of internal failsafe is found in references SLYT080 and SLYT064.
3.3
External Failsafe
External failsafe uses external circuitry together with termination to provide a defined voltage across the
receiver's input, regardless of whether the signal pair is shorted together or is left open circuited. It is
recommended to use when there is no internal failsafe circuit, or to reinforce the effect of internal failsafe
circuits in applications where extremely high levels of noise are possible. Further discussion of internal
failsafe is found in reference SLYT324.
4
Suggested Termination and Grounding Techniques
When designing a system that uses drivers, receivers, and transceivers that comply with RS-422 or
RS-485, proper cable termination is essential for highly reliable applications with reduced reflections in the
transmission line. Because RS-422 allows only one driver on the bus, if termination is used, it is placed
only at the end of the cable near the last receiver. In general, RS-485 requires termination at both ends of
the cable.
Factors to consider when determining the type of termination usually are performance requirements of the
application and the ever-present factor, cost. The different types of termination techniques discussed are
unterminated lines, parallel termination, ac termination, and multipoint termination. Laboratory waveforms
for each termination technique (except multipoint termination) illustrate the usefulness and robustness of
RS-422 (and, indirectly, RS-485). Similar results can be obtained if 485-compliant devices and termination
techniques are used. For laboratory experiments, 100 feet of 100-Ω, 24-AWG, twisted-pair cable (Bertek)
was used. A single driver and receiver, TI AM26C31C and AM26C32C, respectively, were tested at room
temperature with a 5-V supply voltage. Two plots per termination technique are shown. In each plot, the
top waveform is the driver input and the bottom waveform is the receiver output. To show voltage
waveforms related to transmission-line reflections, the first plot shows output waveforms from the driver at
the start of the cable; the second plot shows input waveforms to the receiver at the far end of the cable.
Resistor and capacitor (if used) termination values are shown for each laboratory experiment, but vary
from system to system. For example, the termination resistor, ZT, must be within 20% of the characteristic
impedance, Zo, of the cable and can vary from about 90 Ω to 120 Ω.
4.1
No Termination
Figure 16 shows a configuration with no termination. Figure 17 and Figure 18 show that, although
reflections are present at the receiver inputs at a data signaling rate of 200 kbps with no termination, the
RS-422-compliant receiver reads only the input differential voltage and produces a clean signal at the
output.
DY
DIN
RB
D
R
DZ
ROUT
RA
S0463-01
Figure 16. Differential Unterminated Configuration
•
•
Advantages
– Driver is only required to source a minimal amount of current to produce a signal at the receiver.
– Minimizes the driver's on-chip power dissipation
– Ensures that the receiver's output is in a known state if the receiver internally features open-line fail
safe
Disadvantage
– Signal reflections due to mismatched line impedance at high data signaling rates (should be used
SLLA070D – June 2002 – Revised May 2010
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
13
Suggested Termination and Grounding Techniques
www.ti.com
only in applications with data rates ≤200 kbps and short distances) (If tr > 4tdelay, the cable is not
considered a transmission line.)
DIN
DZ
DY
ROUT
G001
Figure 17. Differential Unterminated-Driver Output Waveforms
DIN
RA
RB
ROUT
G002
Figure 18. Differential Unterminated-Receiver Input Waveforms
4.2
Parallel Termination
Figure 19 shows a typical configuration using an impedance termination across the differential inputs at
the far-end receiver. As shown in Figure 20 and Figure 21, parallel termination ensures that there are no
impedance mismatches and eliminates reflections. A termination resistance of 100 Ω was applied, and
only one receiver was used in this experiment, while transmitting at a data signaling rate of 1 Mbps.
14
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
SLLA070D – June 2002 – Revised May 2010
Suggested Termination and Grounding Techniques
www.ti.com
RB
DY
DIN
ZT
D
DZ
LSTUB
ROUT
R
RA
R
S0464-01
A
ZT = Zo
Figure 19. Differential Parallel-Terminated Configuration
•
•
Advantages
– Elimination of reflections: higher data rates and longer cables
– Multidrop applications are also supported.
Disadvantages
– Long stub lengths (LSTUB) reintroduce reflections.
– Increase in driver's power dissipation (when compared to the unterminated case)
– Receiver's differential input is zero when all drivers are idle.
DIN
DZ
DY
ROUT
G003
Figure 20. Differential Parallel-Terminated Driver Output Waveforms
SLLA070D – June 2002 – Revised May 2010
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
15
Suggested Termination and Grounding Techniques
www.ti.com
DIN
RA
RB
ROUT
G004
Figure 21. Differential Parallel-Terminated Receiver Input Waveforms
4.3
AC Termination
Because the differential outputs are complementary when the driver is activated, one output is in the high
state, while the other is in the low state. Current flows from the high-state output to the low-state output
through the termination resistor, as in the parallel configuration. A capacitor and resistor are used for ac
termination to eliminate the dc current path from one differential output to the other. Figure 22 shows the
connectivity. However, this added RC time-constant delay significantly reduces transmission speeds.
Figure 23 and Figure 24 illustrate acceptable receiver input and output waveforms using an ac termination
of ZT = 100 Ω and a 1000-pF capacitor. The data rate was limited to 200 kbps in laboratory
measurements. If a RS-485-system with multiple drivers is used, place another ZT and CT across the
balanced line at the other end of the cable.
DY
RB
ZT
DIN
D
ROUT
R
CT
DZ
S0465-01
RA
Figure 22. Differential AC-Terminated Configuration
•
•
16
Advantages
– Driver power dissipation is decreased compared to parallel termination, but not as much as in the
unterminated case.
– Line reflections are reduced.
– Ensures that the receiver's output goes to a known state if the receiver internally features open-line
fail-safe
Disadvantage
– Limitation on maximum data signaling rate and cable distance due to RC time constant (typically
used on low-speed control lines)
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
SLLA070D – June 2002 – Revised May 2010
Suggested Termination and Grounding Techniques
www.ti.com
DIN
DZ
DY
ROUT
G005
Figure 23. Differential AC-Terminated Driver Output Waveforms
DIN
RA
RB
ROUT
G006
Figure 24. Differential AC-Terminated Receiver Input Waveforms
4.4
Multipoint Termination
Multipoint configurations are supported by RS-485, but not RS-422. Figure 25 shows a typical
RS-485-compliant configuration, with a transceiver at both ends of the cable and drivers/receivers placed
along the length of the cable. RS-485 requires termination at both ends of the cable (see Figure 25). No
waveforms are provided for this configuration, but performance similar to that of the parallel-termination
case can be expected.
SLLA070D – June 2002 – Revised May 2010
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
17
Suggested Termination and Grounding Techniques
DIN
www.ti.com
ZT
D
R
ZT
R
R
D
D
S0466-01
A
ZT = Zo
Figure 25. Differential Multipoint-Terminated Configuration
•
•
Advantages
– Same as parallel termination
– Optimum signal quality
Disadvantage
– Same as parallel termination, but two termination resistors are required, one at each end of the
cable, which adds more loading to the drivers.
Table 3. Summary of Termination Techniques
TECHNIQUE
POWER
DISSIPATION
OPEN-LINE
FAIL-SAFE
SHORTED-LINE
FAIL-SAFE
SPEED
SIGNAL INTEGRITY
Low
Supported if available
on receiver
Supported if available
on receiver
Low
Good at low signaling
rates, Poor at high
speeds
Medium
Supported if available
on receiver
Supported if available
on receiver
High
Excellent
Low
Supported if available
on receiver
Supported if available
on receiver
Medium
Good
Medium
Supported if available
on receiver
Supported if available
on receiver
High
Excellent
No termination
Parallel termination
AC termination
Multipoint termination
4.5
Ground Connections
Commonly RS-422 and RS-485 system configurations are presented without a separate ground wire.
Laws of physics, however, still require a solid ground connection to ensure error-free communication
between drivers and receivers.
4.5.1
Local Ground and Protective Earth Is Not Connected
If all the power supplies for all the transceivers and related processing circuitry are isolated and have
disconnected local ground and PE (Protective Earth), no ground loop exists. Local ground of each
transceiver can be connected directly (Figure 26).
18
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
SLLA070D – June 2002 – Revised May 2010
Suggested Termination and Grounding Techniques
www.ti.com
VCC1
L
VCC2
D
N
Isolated
Power
Supply 1
L
R
R
ZT
ZT
Isolated
Power
Supply 2
D
E
N
E
No Connection
No Connection
No Ground Loop
Local
Ground 1
PE1
(Protective
Earth 1)
Local
Ground 2
PE2
(Protective
Earth 2)
Grounding Network
in Factory, Office,
House, etc
S0467-01
Figure 26. Grounding Configuration with Isolated Local Ground and PE
4.5.2
Local Ground and Protective Earth Is Connected
In many cases, local ground and PE are connected by wire, chassis, or leakage for many reasons, such
as lowest cost or simplest power supply design. Thus, if a high-voltage potential difference exists between
remote grounds, especially during transients, then current flows through the remote ground because a
ground loop does exist. If the ground loop has no resistance , then the ground current is large and creates
some problems. For this reason, the RS-485 standard recommends adding some resistance between
logic and chassis ground to avoid excess ground-loop currents (Figure 27).
VCC1
L
VCC2
D
N
L
R
Power
Supply 2
Power
Supply 1
R
ZT
ZT
D
100W
E
PE1
(Protective
Earth 1)
Connected
by Wire,
Chassis,
Leakage, etc.
100W
Ground Loop Exists
Local
Ground 1
N
Local
Ground 2
Connected
by Wire,
Chassis,
Leakage, etc.
Grounding Network
in Factory, Office,
House, etc
E
PE2
(Protective
Earth 2)
S0468-01
Figure 27. Grounding Configuration with Connected Local Ground and PE
This approach reduces loop current, but the existence of a large ground loop keeps the data link sensitive
to noise generated somewhere else along the loop.
SLLA070D – June 2002 – Revised May 2010
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
19
Suggested Termination and Grounding Techniques
www.ti.com
S0469-01
PE2
(Protective
Earth 2)
E
VCC(n-1)
R
GND(n – 1) D
VCC2
R
D
GND2
(b) Isolation of Multiple Remote Transceiver Stations
GNDn
GND1
Non-Isolated
Transceiver
R
D
Local
Ground 1
R
E
PE1
(Protective
Earth 1)
Connected
by Wire,
Chassis,
Leakage, etc.
D
N
Power
Supply 1
VCC1
L
D
R
Isolated
Transceiver
VCCn
VCC1
Grounding Network
in Factory, Office,
House, etc
No Ground Loop
(a) Isolation of Two Remote Transceiver Stations with Single-Ground Reference
Digital
Isolator
ZT
R
D
The Isolated
DC/DC Converter
and
Local
Digital Isolator
Ground 2
Break the
Ground Loop
Connected
by Wire,
Chassis,
Leakage, etc.
Power
Supply 2
VCC2
Isolated
DC/DC
Converter
ZT
Isolated
Transceiver
VCC2_ISO
Isolated
Transceiver
L
N
The approach to tolerate ground potential differences up to several kilovolts across a robust RS-485 data
link and over long distance is the galvanic isolation of the signal and supply lines of a bus transceiver from
its local signal and supply sources (Figure 28).
Figure 28. Isolated Configuration
In this case, supply isolators, such as isolated DC/DC converters, and signal isolators, such as digital
capacitive isolators, prevent current flow between remote system grounds and avoid the creation of
current loops. The non-isolated transceiver on the left provides the single-ground reference for the entire
bus.
20
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
SLLA070D – June 2002 – Revised May 2010
Typical System Configurations
www.ti.com
5
Typical System Configurations
This section presents a general idea about connecting balanced differential drivers, receivers, and
transceivers in different situations. Properly connecting the devices greatly reduces reflections.
The following discussions on system configurations apply only to RS-485-compliant system designs.
Conversion to a RS-422-compliant system is straightforward, knowing that the termination impedance, ZT,
is placed only once in close proximity to the last receiver at the end of the cable farthest from the driver.
Also, only one driver is required, with up to nine additional receivers allowed in a RS-422-compliant
system design.
5.1
Daisy-Chain Configuration
R
R
D
D
One widely used connectivity scheme is called daisy-chaining. In this topology, each station is attached
successively as close to the input/output as possible (see Figure 29). The idea is to make the main bus
seem like only one transmission line.
ZT
D
D
R
R
ZT
S0470-01
Figure 29. Daisy-Chain Connection
5.2
Bus and Stub Configuration
Another popular connection scheme (shown in Figure 30) is connecting stations directly to the main bus
(referred to as a backbone). To reduce line reflections, it is essential to keep the stubs (cable distance
from main bus line) as short as possible. Again, the intent is for the driver to see only one transmission
line.
SLLA070D – June 2002 – Revised May 2010
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
21
Summary Comparison of the Standards
R
R
D
D
www.ti.com
ZT
D
D
R
R
ZT
S0471-01
Figure 30. Stub Cables Connected to the Main Backbone
5.3
Point-to-Point Configuration
Point-to-point connectivity, in its simplest form of data transmission, is shown in Figure 31.
D
R
R
ZT
ZT
D
S0472-01
Figure 31. Point-to-Point Connection
6
Summary Comparison of the Standards
As discussed previously, RS-422 and RS-485 have similar requirements. RS-485-compliant drivers and
receivers generally are interchangeable with those compliant to RS-422, but the reverse is not necessarily
true. RS-422-compliant drivers used in multipoint applications have at least three major problems:
common-mode range, line contention, and output drive current.
6.1
Common-Mode Range
RS-485-compliant drivers and receivers are specified for operation with a common-mode range of -7 V to
12 V. RS-422-compliant drivers are specified over the range of only 0.25 V to 6 V. RS-422-compliant
receivers are specified regarding the 200-mV threshold levels over the common-mode range of -7 V to +7
V, but are specified regarding bus pin leakage over the common-mode range -10 V to +10 V.
22
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
SLLA070D – June 2002 – Revised May 2010
Conclusion
www.ti.com
6.2
Line Contention
Line contention, as discussed previously, is caused when two or more drivers are turned on at the same
time. For example, if driver 1 is driving VA > VB while driver 2 also is turned on and driving VB > VA, a
line-contention situation arises. Both RS-422 and RS-485 require driver output current limits that protect
against high currents in these situations. However, the data on the bus lines is unreliable during line
contention.
Thermal shutdown disables the output driver when it senses a high temperature and turns it back on after
the device cools. If line contention still is present after the driver is re-enabled, thermal shutdown disables
the driver again as soon as the temperature reaches a certain point. The output cycles in and out of
thermal shutdown indefinitely until line contention no longer is present.
The problem with line contention is exacerbated when the ground potential between the remote grounds is
stretched to its maximum limit of ±7 V. If driver 2 is connected to the ground that is 7 V lower than the
ground for driver 1, a potential close to +12 V can exist on the output of driver 2 (assuming VCC = 5 V). If
driver 2 is connected to the ground that is 7 V higher than the ground for driver 1, a potential close to –7 V
can exist on the output of driver 2. The RS-485 standard, as noted, limits the current out of the output
when a voltage ranging from –7 V to 12 V is applied. Therefore, the standard does require protection
against the effects of line contention by limiting the output current.
6.3
Drive Current
Because RS-485 requires two termination resistors, but RS-422 requires only one termination resistor,
RS-485 outputs typically are stronger. Furthermore, RS-422 allows driving up to 10 4-kΩ loads (equivalent
to 400 Ω), whereas RS-485 allows driving up to 32 12-kΩ loads (equivalent 375 Ω). Therefore, with the
same output drive strength, a RS-485-compliant driver can handle triple the number of loads.
Table 4 summarizes the main differences between RS-422 and RS-485; Table 3 summarizes attributes for
each termination technique discussed.
Table 4. Summary Comparison of RS-485 and RS-422 Specifications
PARAMETER
Number of drivers and receivers
Maximum theoretical cable length
RS-485
32. U.L.s
1200
1200
m
10
Maximum common-mode voltage
±7
–7 to +12
2 ≤ |VOD| ≤ 10
1.5 ≤ |VOD| ≤ 5
V
≥100
≥60
Ω
Driver load
Driver output short-circuit current limit
> 10
(1)
UNIT
Maximum data rate
Driver differential output level
Mbps
V
150 to GND
250 to –7 V or 12 V
mA
High-impedance state, power off
60
12
kΩ
Receiver input resistance
4
12
kΩ
±200
±200
mV
Receiver sensitivity
(1)
7
RS-422
1 driver / 10 receivers
TI devices operating at up to 50 Mbps.
Conclusion
Although similarities exist between RS-422 and RS-485, board designers must consider distinct
differences between devices specified for one standard or the other. This application report has outlined
the major differences and provided suggested connection schemes. RS-485-compliant devices can be
used in RS-422-compliant systems, but the opposite is not true.
TI offers a variety of devices that meet or exceed requirements of RS-422 and RS-485. TI's main
competitive advantage is its extensive selection of bipolar and BiCMOS differential devices that are
available at competitive prices, allowing the board designer to reduce system costs. For a complete listing
of available devices from each standard, data sheets are available on the Internet at http://www.ti.com
under interface products.
SLLA070D – June 2002 – Revised May 2010
RS-422 and RS-485 Standards Overview and System Configurations
Copyright © 2002–2010, Texas Instruments Incorporated
23
Glossary
8
Glossary
BiCMOS
bps
Line contention
Multidrop
Multipoint
U.L.
9
www.ti.com
Bipolar and complementary metal-oxide-semiconductor process
Bits per second
At least two drivers on the same bus inadvertently enabled simultaneously
Multiple receivers driven by a single driver per bus line
Multiple transceivers, drivers, or receivers per bus line
Unit load
Acknowledgment
Technical assistance was provided by Kevin Gingerich and Frank Dehmelt, both of the TI Advanced
Analog Products Group.
10
References
1. Electrical Characteristics of Balanced Voltage Digital Interface Circuits, ANSI/TIA/ EIA-422-B-1994,
Telecommunications Industry Association, 1994
2. Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems,
ANSI/TIA/EIA-485-A-1998, Telecommunications Industry Association, 1998
3. Application Guidelines for TIA/EIA-485-A, TIA/EIA Telecommunications Systems Bulletin,
Telecommunications Industry Association, 1998
4. A Comparison of Differential Termination Techniques, Joe Vo, National Semiconductor, Application
Note AN-903
5. Comparing EIA-485 and EIA-422-A Line Drivers and Receivers in Multipoint Applications, John Goldie,
National Semiconductor, Application Note AN-759
24
RS-422 and RS-485 Standards Overview and System Configurations
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SLLA070D – June 2002 – Revised May 2010
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