RS-422 AND RS-485 APPLICATION NOTE

RS-422 and RS-485 Application Note B&B Electronics Mfg. Co. Inc. P.O. Box 1040 -- Ottawa, IL 61350 PH (815) 433-5100 -- FAX (815) 434-7094 Internet Addresses: B&B Home Page: http://www.bb-elec.com Customer Service: sales@bb-elec.com Technical Support: techsupt@bb-elec.com © B&B Electronics - Revised October 1997 RS-422/485 Application Note B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350 PH (815) 433-5100 -- FAX (815) 434-7094 Cover Page Table of Contents CHAPTER 1: OVERVIEW ........................................................................... 1 INTRODUCTION ............................................................................................. 1 DATA TRANSMISSION SIGNALS ...................................................................... 1 Unbalanced Line Drivers ......................................................................... 1 Balanced Line Drivers ............................................................................. 1 Balanced Line Receivers.......................................................................... 3 EIA STANDARD RS-422 DATA TRANSMISSION .............................................. 3 EIA STANDARD RS-485 DATA TRANSMISSION .............................................. 6 TRISTATE CONTROL OF AN RS-485 DEVICE USING RTS.................................. 9 SEND DATA CONTROL OF AN RS-485 DEVICE .............................................. 11 CHAPTER 2: SYSTEM CONFIGURATION ............................................ 13 NETWORK TOPOLOGIES............................................................................... 13 TWO WIRE OR FOUR WIRE SYSTEMS ............................................................ 13 TERMINATION............................................................................................. 16 BIASING AN RS-485 NETWORK.................................................................... 17 EXTENDING THE SPECIFICATION .................................................................. 19 CHAPTER 3: SELECTING RS-422 AND RS-485 CABLING .................. 20 NUMBER OF CONDUCTORS .......................................................................... 20 SHIELDING ................................................................................................. 20 CABLE CHARACTERISTICS ........................................................................... 20 CHAPTER 4: TRANSIENT PROTECTION OF RS-422 AND RS-485 SYSTEMS.................................................................................................... 23 WHAT DOES A SURGE LOOK LIKE? ................................................................ 23 Surge Specifications............................................................................... 23 Common Mode vs. Differential Mode..................................................... 25 GROUND ≠ GROUND.................................................................................... 26 TRANSIENT PROTECTION USING ISOLATION .................................................. 27 Isolation Theory..................................................................................... 27 Isolation Devices ................................................................................... 28 TRANSIENT PROTECTION USING SHUNTING .................................................. 28 Shunting Theory..................................................................................... 28 Connecting Signal Grounds ................................................................... 29 Shunting Devices ................................................................................... 29 COMBINING ISOLATION AND SHUNTING ....................................................... 29 SPECIAL CONSIDERATION FOR FAULT CONDITIONS ....................................... 31 CHOOSING THE RIGHT PROTECTION FOR YOUR SYSTEM ................................. 31 RS-422/485 Application Note Table of Contents i B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350 PH (815) 433-5100 -- FAX (815) 434-7094 CHAPTER 5: SOFTWARE ....................................................................... 33 INTRODUCTION ........................................................................................... 33 RS-422 SYSTEMS ....................................................................................... 33 RS-485 DRIVER CONTROL .......................................................................... 33 RS-485 RECEIVER CONTROL ....................................................................... 34 MASTER-SLAVE SYSTEMS ........................................................................... 34 Four Wire Master-Slave Systems............................................................ 34 Two Wire Master-Slave Systems............................................................. 34 MULTI-MASTER RS-485 SYSTEMS .............................................................. 35 SYSTEMS WITH PORT POWERED CONVERTERS .............................................. 35 CHAPTER 6: SELECTING RS-485 DEVICES ........................................ 36 CHAPTER 7: SOURCES OF FURTHER INFORMATION .................... 37 APPENDIX A: EIA SPECIFICATION SUMMARY ................................ 38 APPENDIX B: EIA STANDARD RS-423 DATA TRANSMISSION........ 40 ii Table of Contents RS-422/485 Application Note B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350 PH (815) 433-5100 -- FAX (815) 434-7094 Chapter 1: Overview Introduction The purpose of this application note is to describe the main elements of an RS-422 and RS-485 system. This application note attempts to cover enough technical details so that the system designer will have considered all the important aspects in his data system design. Since both RS-422 and RS485 are data transmission systems that use balanced differential signals, it is appropriate to discuss both systems in the same application note. Throughout this application note the generic terms of RS-422 and RS-485 will be used to represent the EIA/TIA-422 and EIA/TIA-485 Standards. Data Transmission Signals Unbalanced Line Drivers Each signal that transmits in an RS-232 unbalanced data transmission system appears on the interface connector as a voltage with reference to a signal ground. For example, the transmitted data (TD) from a DTE device appears on pin 2 with respect to pin 7 (signal ground) on a DB-25 connector. This voltage will be negative if the line is idle and alternate between that negative level and a positive level when data is sent with a magnitude of ±5 to ±15 volts. The RS-232 receiver typically operates within the voltage range of +3 to +12 and -3 to -12 volts as shown in Figure 1.1. Balanced Line Drivers In a balanced differential system the voltage produced by the driver appears across a pair of signal lines that transmit only one signal. Figure 1.2 shows a schematic symbol for a balanced line driver and the voltages that exist. A balanced line driver will produce a voltage from 2 to 6 volts across its A and B output terminals and will have a signal ground (C) connection. Although proper connection to the signal ground is important, it isn't used by a balanced line receiver in determining the logic state of the data line. A balanced line driver can also have an input signal called an “Enable” signal. The purpose of this signal is to connect the driver to its output terminals, A and B. If the “Enable” signal is OFF, one can consider the driver as disconnected from the transmission line. An RS-485 driver must have the “Enable” control signal. An RS-422 driver may have this signal, but it is not always required. The disconnected or "disabled" condition of the line driver usually is referred to as the “tristate1” condition of the driver. 1 The term “tristate” comes from the fact that there is a third output state of an RS-485 driver, in addition to the output states of “1” and “0.” 1 B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350 PH (815) 433-5100 -- FAX (815) 434-7094 RS-422/485 Application Note Figure 1.1 Figure 1.2 2 RS-422/485 Application Note B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350 PH (815) 433-5100 -- FAX (815) 434-7094 Balanced Line Receivers A balanced differential line receiver senses the voltage state of the transmission line across two signal input lines, A and B. It will also have a signal ground (C) that is necessary in making the proper interface connection. Figure 1.3 is a schematic symbol for a balanced differential line receiver. Figure 1.3 also shows the voltages that are important to the balanced line receiver. If the differential input voltage Vab is greater than +200 mV the receiver will have a specific logic state on its output terminal. If the input voltage is reversed to less than -200 mV the receiver will create the opposite logic state on its output terminal. The input voltages that a balanced line receiver must sense are shown in Figure 1.3. The 200 mV to 6 V range is required to allow for attenuation on the transmission line. EIA Standard RS-422 Data Transmission The EIA Standard RS-422-A entitled “Electrical Characteristics of Balanced Voltage Digital Interface Circuits” defines the characteristics of RS-422 interface circuits. Figure 1.4 is a typical RS-422 four-wire interface. Notice that five conductors are used. Each generator or driver can drive up to ten (10) receivers. The two signaling states of the line are defined as follows: a. When the “A” terminal of the driver is negative with respect to the “B” terminal, the line is in a binary 1 (MARK or OFF) state. b. When the “A” terminal of the driver is positive with respect to the “B” terminal, the line is in a binary 0 (SPACE or ON) state. Figure 1.5 shows the condition of the voltage of the balanced line for an RS-232 to RS-422 converter when the line is in the “idle” condition or OFF state. It also shows the relationship of the “A” and “B” terminals of an RS422 system