AN-1031 TIA/EIA-422-B Overview: Application Report
AN-1031 TIA/EIA-422-B Overview: Application Report
                                                                       ABSTRACT
        This application report covers topics associated with concerns for implementing a balanced interface
        circuit utilizing the TIA/EIA-422-B (formerly RS-422-A) electrical interface standard.
                                                                       Contents
        1        Introduction .................................................................................................................. 2
        2        Cable Length and Data Rate .............................................................................................. 2
        3        Termination .................................................................................................................. 3
        4        Failsafe ....................................................................................................................... 4
        5        Configuration ................................................................................................................ 5
        6        ESD Protection .............................................................................................................. 5
        7        Live Insertion ................................................................................................................ 7
        8        By-Pass Capacitors ......................................................................................................... 7
        9        Stub Length .................................................................................................................. 8
        10       Receiver Power Off Characteristics ...................................................................................... 8
        11       Typical Cable Media ........................................................................................................ 9
        12       Summary ................................................................................................................... 10
        13       References ................................................................................................................. 10
                                                                      List of Figures
        1        Terminated Configuration .................................................................................................. 2
        2        Receiver Operating Range      ................................................................................................         2
        3        Cable Length Versus Data Rate ..........................................................................................               3
        4        Multi-Drop Application With Parallel Termination .......................................................................               4
        5        Types of Failsafe ............................................................................................................         4
        6        Example of Multi-Point Configuration ....................................................................................              6
        7        A Point-to-Point RS-422 System With TransZorbs .....................................................................                   6
        8        Example of Protected Connectors ........................................................................................               7
        9        Stub Reflections .............................................................................................................         9
        10       Receiver ON/OFF Characteristics ........................................................................................               9
1      Introduction
       TIA/EIA-422-B (RS-422) is an industry standard specifying the electrical characteristics of a balanced
       interface circuit. Other prefixes are commonly used with the RS-422 standard. These include EIA, EIA/TIA,
       and RS; although previously correct, today TIA/EIA is the correct prefix. However, for simplicity, RS-422
       wil be used throughout the rest of this application report. Also, a suffix letter denotes the different revisions
       of the standard but various prefixes with the same suffix reference the same identical standard.
       RS-422 was introduced to solve the limitation problems of single-ended standards like TIA/EIA-232-E.
       Single-ended interfaces lack common-mode noise rejection capability; ideal for noisy environments. Also,
       data rates are usually limited to less than 0.5 Mbps. A RS-422 interface may be implemented to overcome
       these limitations.
       A RS-422 driver can drive up to ten unit loads (4 kΩ to circuit common is one unit load). The driver is
       capable of transmitting data across 4000 feet (recommended limit) of cable; but not at maximum data
       rates (see Figure 3 ). Standard RS-422 drivers are assured to source and sink a minimum 20 mA across a
       100 Ω load. This corresponds to a minimum differential output voltage, VOD, of 2 V across the load (see
       Figure 1).
       The complement RS-422 receiver must be equal to or less than one unit load. This is represented by the
       slope of the shaded region in Figure 2. The operating range of the receiver is defined between ±10 V and
       is represented by the shaded area in Figure 2.
       Also, RS-422 receivers have a ±200 mV threshold over the entire common mode range of ±7 V. A
       differential noise margin ≥1.8 V is assured between the driver's differential output swing and the receiver's
       threshold.
       RS-422 drivers and receivers are designed for point-to-point and multi-drop configurations but not multi-
       point. For multi-drop configurations, a daisy chain is the recommended interface configuration.
       Caution, at long distances or high data rates, termination is recommended to reduce reflections caused by
       a mismatch in the impedance of the cable and the impedance of the receiver's input. For further
       information, see Section 3.
       Significantly, the RS-485 differential interface standard is very similar to RS-422. However, there are
       differences that distinguish the two standards from one another; which include the output stage of the
       driver, the common mode range of the interface, the input resistance of the receiver, and the drive
       capability of the driver. For more details concerning the comparison of RS-422 and RS-485, see AN-759
       Comparing EIA-485 and EIA-422-A Line Drivers and Receivers in Multipoint Applications (SNLA023).
       A chart displaying the recommended operational region of a typical RS-422 standard interface is shown in
       Figure 3. Other electrical interface standards operating regions are also shown for comparison. The
       curves were obtained from empirical data using a 24 AWG, copper, 16 pF/ft, twisted-pair cable parallel
       terminated with a 100 Ω load.
3      Termination
       Termination is recommended for use when designing a RS-422 interface that is considered to be a
       transmission line. An interface is considered to be a transmission line if the propagation delay of the cable
       is greater than ⅛ the transition (rise or fall) time of the signal. This is a time-domain analysis. The same is
       also true in frequency-domain.
       For clarification, the transition time for the time-domain analysis is measured from zero to one hundred
       percent of the transition. The rising or falling edge may be used for analysis, whichever edge is the
       fastest.
       Transmission lines may restrict the use of a multi-drop configuration and limit the maximum data rate of
       the RS-422 interface.
       Transmission line theory will not be discussed in this application report but is useful knowledge. For more
       details on transmission line theory, see AN-806 Data Transmission Lines and Their Characteristics
       (SNLA026), AN-807 Reflections: Computations and Waveforms (SNLA027), and AN-808 Long
       Transmission Lines and Data Signal Quality (SNLA028).
4      Failsafe
       For typical RS-422 interfaces, open, terminated, and shorted input are the three types of failsafe to
       consider as shown in Figure 5. A receiver with full failsafe protection assures a known receiver output for
       all three types failsafe conditions.
           Rt = ZO
           where:
           Rt = termination resistor
           ZO = characteristic impedance of cable
                              Figure 4. Multi-Drop Application With Parallel Termination
5      Configuration
       For RS-422 interface devices, usually three types of configurations are commonly used.
5.3    Multi-Point
       The last type of configuration is multi-point, which uses two or more drivers connected to one or more
       receivers (see Figure 6 ). RS-422 drivers are normally not designed into this type of configuration.
       However, a multi-point system can be accomplished if certain issues are addressed. The three issues are
       ground potential differences between drivers, contention between drivers, and the drive capability of the
       drivers. Therefore, RS-485 devices are recommended for multi-point applications.
       For further details and illustrations concerning these issues, see AN-759 Comparing EIA-485 and EIA-
       422-A Line Drivers and Receivers in Multipoint Applications (SNLA023).
6      ESD Protection
       Electrostatic discharge (ESD) is normally an event of very high potential for a short period of time. This
       may be damaging to some integrated circuits (IC). ESD is not limited to a one time occurence in the life of
       an IC, unless it is fatal the very first time. It may be an ongoing transpiration that can wear down an IC
       until it eventually fails fatally.
       The RS-422 standard does not specify requirements for ESD protection. However, the industry has
       developed a defacto minimum standard of 2,000 V ESD protection under human body model (HBM)
       conditions. However, many systems today require much higher levels of ESD protection. Possibly as high
       as 10 kV or 15 kV.
       ESD protection may be enhanced in different ways. IC protection circuitry, TransZorbs®, and protected
       connectors are three possible solutions for increasing an IC's ESD protection. The first method is built-in
       IC protection circuitry that requires no external components. This type of protection is the result of the
       manufacturer's IC design. In the early years of IC manufacturing, ESD was not as well publicized or
       standardized as it is today. Today, IC designers strive to achieve five, ten, even fifteen thousand Volts of
       ESD protection.
      The later two solutions, TransZorbs and protected connectors, are both external to the IC and usually
      implemented, by the system manufacturer, to provide additional protection, if needed, beyond that which is
      supplied by the IC itself.
6.2   TransZorb
      TransZorbs are like back-to-back diodes. They are connected between the interface line that needs
      protection and ground. They act like voltage clamps, clamping voltages that are above the TransZorb's
      specified reverse stand-off voltage (Vrs). Take care when selecting a TransZorb, if the Vrs is too high, the
      IC may become damaged before the TransZorb ever turns on. One TransZorb should be used per
      interface line. This solution could become expensive as the number of interface lines requiring protection
      increases. Figure 7 shows an example of a RS-422 point-to-point system with TransZorbs.
       In the later two solutions, the system manufacturer provides additional ESD protection. Ideally, system
       manufacturers would like the IC manufacturers to provide all the ESD protection required for their
       systems, internal to the device. Texas Instruments interface group recognizes the importance of ESD
       protection and has released the DS36276 and the DS36277 with ESD protection up to 7,500 V. Future
       Interface products will trend toward higher ESD protection.
7      Live Insertion
       RS-422 does not specify how to insert an IC (driver or receiver) into a live interface. The same is true for
       removing the IC. PC board connectors, cable connectors, and sockets are likely interface points. A device
       may be inserted live via one of these interfaces. Live insertion is a larger concern, since removing an IC,
       from a live interface, normally is less damaging to an IC. For live insertion, the device may not be powered
       up when being inserted. Thus, the RS-422 device may receive bus potentials that exceed the power
       supply voltage (VCC). First, this may cause biasing of diodes and bus clamping, or result in large current
       faults that damage devices. Second, the potential difference between ICs connections and the interface
       connector may be large enough to cause ESD strikes which may be harmful to the IC. In addition, the IC
       may transmit signals when the supply voltage is below minimum operating level which may result in data
       transfer errors. On the other hand, when removing an IC from an active interface, the VCC should be within
       normal operating levels with no device pins higher than the supply pin. Also, the potential difference at the
       instance of the disconnect is minimized since the potential is the equivalent just prior.
       Recommendations for implementing a live interface are, one, use an IC that supports the feature. Look for
       this feature in the device's datasheet. However, it will not always appear there so be sure to ask the chip
       manufacturer's technical support group, to be certain. These ICs usually have specially designed input
       and output structures that prevent damage to the device even if bus voltages exceed VCC. Two, since
       removal is less harmful than inserting, try to create similar conditions for inserting the chip that would exist
       when removing the chip. In other words, it is ideal to have the device powered and referenced to the same
       ground potential as the interface before the inputs and outputs make physical contact.
       For a PC board, a staggered connection where the ground trace is the longest, so that it makes contact
       first, and the VCC trace is the second longest followed by the input traces then the output traces last would
       suffice. Live insertion via cables or sockets may require special design but should make contact in the
       same pattern.
8      By-Pass Capacitors
       By-pass capacitors help reduce transients on output signals. Therefore, by-pass capacitors are
       recommended for better signal quality. One 0.1 μF capacitor is recommended for each powered IC in the
       system. If a device has more than one power supply (VCC and VEE) then use one capacitor for each supply.
       The capacitor should be placed between the power supply pin and ground. Place the capacitor as close to
       the VCC pin of the device as possible. Additionally, a 10 μF capacitor may be used near the main path
       where VCC is delivered to the system for bulk charge storage. If a system is large, additional bulk
       capacitance may be distributed across the system.
9     Stub Length
      Since RS-422 is a multi-drop standard, receivers may be connected to the bus via a stub. The length of
      the stub is important because of the affect it may have on the signal. As the stub length is increased, its
      characteristics begin to act as a transmission line. When is a stub considered to be a transmission line? A
      typical guideline for stubs and transmission lines is stated here:
      Time Domain:
      If the propagation delay of the stub (one way trip) is greater than ⅛ the transition time, measured from 0%
      to 100% of the signal transition, then the stub may be considered a transmission line.
      The boundary conditions at which a stub begins to act as a transmission line are not precise. Therefore,
      designers may use slightly different ratios.
      Furthermore, the maximum length of the stub depends on the transition time measured at the point of the
      stub interconnect. This is very important to remember because if the total length of the cable is 1,000 feet,
      a longer stub can be hung off the cable at 750 feet away from the driver than at 75 feet away from the
      driver. This is because the cable capacitance slows the transition time of the driver's output as it
      propagates down the cable and the transition time is longer.
      The length of the stub may be increased by slowing down the transition edge at the stub interconnect.
      This can be done using a bulk capacitance load or a RS-422 driver with output wave shape control like
      TI's DS3691, DS3692, or DS36C280.
      To implement a RS-422 interface with stubs of equal length. Apply the stub guideline rule to the stub(s)
      closest to the driver's output and use this length for the maximum length for all stubs on the interface bus.
      If a stub is too long and causes a noticeable reflection, it will be measured positive on the stub because
      the voltage reflection coefficient is positive since the load impedance ZL is larger than the stub impedance
      Zo. The formula for the reflection coefficient at the load is shown in Equation 1.
                                                                                                                                   (1)
      Note, the load impedance is equivalent to the input impedance of the receiver, which is equal to or greater
      than 4 kΩ. The stub impedance is typically about 100 Ω–130 Ω. The reflection created at the stub will
      propagate in both directions away from the stub (see Figure 9). Keep in mind that reflections are time
      dependent events.
12   Summary
     RS-422 is a standardized differential electrical interface capable of transmitting data in point-to-point and
     multi-drop applications. When implementing a RS-422 interface, make sure design-in concerns are
     addressed at the early stages of design to eliminate problems later in the application, which may be a lot
     more costly. Understanding RS-422 means understanding the issue associated with the interface.
     RS-422 is a well-defined standard and has many applications. However, RS-422 has been superseded by
     RS-485, which builds onto the existing foundation and creates an even more rugged standard. For more
     information on RS-485, see AN-979 The Practical Limits of RS-485 (SNLA042).
13   References
     •    TIA/EIA-422-B, Electrical Characteristics of Balanced Digital Interface Circuits, Electronic Industries
          Association Engineering Department. Washington D.C. 1994.
     •    INTERFACE: Data Transmission Databook
     •    AN-759 Comparing EIA-485 and EIA-422-A Line Drivers and Receivers in Multipoint Applications
          (SNLA023)
     •    AN-806 Data Transmission Lines and Their Characteristics (SNLA026)
     •    AN-807 Reflections: Computations and Waveforms (SNLA027)
     •    AN-808 Long Transmission Lines and Data Signal Quality (SNLA028)
     •    AN-847 FAILSAFE Biasing of Differential Buses (SNLA031)
     •    AN-903 A Comparison of Differential Termination Techniques (SNLA034)
     •    AN-916 A Practical Guide To Cable Selection (SNLA219)
     •    AN-979 The Practical Limits of RS-485 (SNLA042)
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