RS-422 and RS-485 Application Note

1 RS-422 /485 Application Note Cover Page Copyright B&B Electronics -- Revised 2506 B&B Electronics Mfg Co 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104 B&B Electronics Ltd Westlink Comm. Pk Oranmore, Galway, Ireland Ph 353-91-792444 Fax 353-91-792445 RS-422 and RS-485 Application Note International Headquarters B&B Electronics Mfg. Co. Inc. 707 Dayton Road -- Box 1040 -- Ottawa, IL 61350 USA Phone (815) 433-5100 -- General Fax (815) 433-5105 Home Page: Customer Service e-mail: -- Fax (815) 433-5109 Technical Support e-mail: -- Fax (815) 433-5104 European Headquarters B&B Electronics Ltd. Westlink Commercial Park, Oranmore, Co. Galway, Ireland Phone (353) 91 792444 -- Fax (353) 91 792445 Home Page: Sales e-mail: Technical Support e-mail: Copyright B&B Electronics -- Revised June 2006 RS-422 /485 Application Note Table of Contents i Copyright B&B Electronics -- Revised 2506 B&B Electronics Mfg Co 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104 B&B Electronics Ltd Westlink Comm.

2 Pk Oranmore, Galway, Ireland Ph 353-91-792444 Fax 353-91-792445 Table of Contents CHAPTER 1: 1 1 DATA TRANSMISSION 1 Unbalanced Line 1 Balanced Line Drivers .. 1 Balanced Line 3 EIA STANDARD RS-422 DATA 3 EIA STANDARD RS-485 DATA 6 TRISTATE CONTROL OF AN RS-485 DEVICE USING RTS .. 9 SEND DATA CONTROL OF AN RS-485 11 CHAPTER 2: SYSTEM CONFIGURATION .. 13 NETWORK 13 TWO WIRE OR FOUR WIRE 13 16 BIASING AN RS-485 17 EXTENDING THE 19 CHAPTER 3: SELECTING RS-422 AND RS-485 CABLING .. 20 NUMBER OF 20 20 CABLE 20 CHAPTER 4: TRANSIENT PROTECTION OF RS-422 AND RS-485 23 WHAT DOES A SURGE LOOK LIKE? .. 23 Surge 23 Common Mode vs. Differential Mode .. 25 GROUND 26 TRANSIENT PROTECTION USING 27 Isolation Theory.

3 27 Isolation 28 TRANSIENT PROTECTION USING 28 Shunting Theory .. 28 Connecting Signal 29 Shunting 29 COMBINING ISOLATION AND 29 SPECIAL CONSIDERATION FOR FAULT 31 CHOOSING THE RIGHT PROTECTION FOR YOUR 31 ii Table of Contents RS-422 /485 Application Note Copyright B&B Electronics -- Revised 2506 B&B Electronics Mfg Co 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104 B&B Electronics Ltd Westlink Comm. Pk Oranmore, Galway, Ireland Ph 353-91-792444 Fax 353-91-792445 CHAPTER 5: 33 33 RS-422 33 RS-485 DRIVER 33 RS-485 RECEIVER 34 MASTER-SLAVE 34 Four Wire Master-Slave 34 Two Wire Master-Slave Systems .. 34 MULTI-MASTER RS-485 35 SYSTEMS WITH PORT POWERED 35 CHAPTER 6: SELECTING RS-485 DEVICES.

4 36 CHAPTER 7: SOURCES OF FURTHER INFORMATION .. 37 APPENDIX A: EIA SPECIFICATION SUMMARY .. 38 APPENDIX B: EIA STANDARD RS-423 DATA TRANSMISSION .. 40 RS-422 /485 Application Note 1 Copyright B&B Electronics -- Revised 2506 B&B Electronics Mfg Co 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104 B&B Electronics Ltd Westlink Comm. Pk Oranmore, Galway, Ireland Ph 353-91-792444 Fax 353-91-792445 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.

5 Since both RS-422 and RS-485 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.

6 The RS-232 receiver typically operates within the voltage range of +3 to +12 and -3 to -12 volts as shown in Figure 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 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.

7 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. 2 RS-422 /485 Application Note Copyright B&B Electronics -- Revised 2506 B&B Electronics Mfg Co 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104 B&B Electronics Ltd Westlink Comm.

8 Pk Oranmore, Galway, Ireland Ph 353-91-792444 Fax 353-91-792445 Figure Figure RS-422 /485 Application Note 3 Copyright B&B Electronics -- Revised 2506 B&B Electronics Mfg Co 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104 B&B Electronics Ltd Westlink Comm. Pk Oranmore, Galway, Ireland Ph 353-91-792444 Fax 353-91-792445 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 is a schematic symbol for a balanced differential line receiver. Figure also shows the voltages that are important to the balanced line receiver.

9 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 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 is a typical RS-422 four-wire interface. Notice that five conductors are used.

10 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 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 RS-422 system and the - and + terminal markings used on many types of equipment. The A terminal is equivalent to the - designation, and the B terminal equivalent to the + designation.