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1 AN-960. APPLICATION NOTE. One Technology Way Box 9106 Norwood, MA 02062-9106, Tel: Fax: RS-485/RS-422 Circuit Implementation Guide by Hein Marais INTRODUCTION WHY USE DIFFERENTIAL DATA TRANSMISSION? Industrial and instrumentation applications (I&I) require The main reason why RS-485 can communicate over long transmission of data between multiple systems often over distances is the use of differential or balanced lines. A com- very long distances. The RS-485 bus standard is one of the munication channel requires a dedicated pair of signal lines most widely used physical layer bus designs in I&I applica- to exchange information. The voltage on one line equals the tions. The key features of RS-485 that make it ideal for use inverse of the voltage on the other line. in I&I communications applications are TIA/EIA-485-A designates the two lines in this differential pair Long distance links up to 4000 feet. as A and B.

2 Line A is more positive than Line B (VOA > VOB) on the driver output if a logic high is received on the input of the Bidirectional communications possible over a single pair of transmitter (DI = 1). If a logic low is received on the input of the twisted cables. transmitter (DI = 0), the transmitter causes Line B to be more Differential transmission increases noise immunity and positive than Line A (VOB > VOA). See Figure 1. decreases noise emissions. A. DI RO. Multiple drivers and receivers can be connected on the VOD. same bus. VOA VOB B VIA VIB RE. DE. 07395-001. Wide common-mode range allows for differences in ground potential between the driver and receiver. Figure 1. Differential Transmitter and Receiver TIA/EIA-485-A allow for data rates of up to 10 Mbps. If Line A is more positive than line B (VIA VIB > 200 mV). Devices meeting the TIA/EIA-485-A specifications do not on the input of the receiver, the receiver output is a logic high have to operate over the entire range and are not limited (RO = 1).

3 If Line B is more positive than Line A (VIB VIA >. to 10 Mbps. 200 mV) on the input of the receiver, the receiver output is a The purpose of this application note is to discuss the imple- logic low (RO = 0). mentation of RS-485/RS-422 in an industrial environment. Figure 1 shows that a differential signaling interface circuit Applications for RS-485/RS-422 include process control consists of a driver with differential outputs and a receiver with networks; industrial automation; remote terminals; building differential inputs. This circuit has increased noise performance automation, such as heating, ventilation, air conditioning because the noise coupling into the system is equal on both (HVAC), security systems; motor control; and motion control. signals. One signal emits the opposite of the other signal and TIA/EIA-485-A, the telecommunication industry's most widely electromagnetic fields cancel each other.

4 This reduces the used transmission line standard, describes the physical layer of electromagnetic interference (EMI) of the system. the RS-485 interface and is normally used with a higher-level protocol, such as Profibus, Interbus, Modbus, or BACnet. This allows for robust data transmission over relatively long distances. The RS-422 physical layer is described in TIA/EIA-422-B. The TIA/EIA-485-A standards are similar to those described in TIA/EIA-422-B, and the values used to specify the drivers and receivers in TIA/EIA-485-A standards are specified so that it can meet both standards. Rev. 0 | Page 1 of 12. AN-960 APPLICATION NOTE. TABLE OF CONTENTS. Introduction .. 1 AC Why Use Differential Data Transmission? .. 1 Stub Length ..5 RS-485 or RS-422? .. 3 Data Rate and Cable Length ..5 RS-422 .. 3 Fail-Safe Biasing ..6 RS-485 and the Unit Load Concept .. 3 Differential Input Threshold Voltage of a Receiver.

5 6 Half-Duplex RS-485 .. 4 Open Fail-Safe ..6 Full-Duplex RS-485 .. 4 True Fail-Safe Receivers ..7 Termination .. 5 Isolation ..8 No Termination .. 5 Transient Overvoltage Stress Protection ..9 Parallel 5 References ..9 Rev. 0 | Page 2 of 12. APPLICATION NOTE AN-960. RS-485 OR RS-422? Similarly, if an RS-485 receiver is specified to have UL, RS-422 is specified as a simplex multidrop standard, which the receiver only loads the bus by of the standard UL and, means that only one driver and up to ten receivers can be therefore, 8 times as many of these receivers can be connected connected to the same bus. If more than one driver needs to to the bus (8 32 = 256 nodes). See Table 1 for UL and receiver be connected on the same bus, then RS-485 is recommended. input impedance details. RS-485 is specified as a multipoint standard, which means up Many of the RS-485 transceivers characteristics are the same as to 32 transceivers can be connected on the same bus.

6 For RS-422. The common-mode voltage range for RS-485 is Figure 2 shows a typical RS-422 interface circuit. Although expanded to 7 V to +12 V. The RS-485 transceivers must an RS-485 circuit may appear similar, the main difference is withstand this common-mode voltage range while tristated in the bus architecture. Figure 3 shows a typical RS-485 appli- (disconnected from the bus). cation circuit. An RS-485 system must have a driver that can be disconnected from the transmission line when a particular node is not RS-422. transmitting. The DE (RTS) pin on the RS-485 transceiver The RS-422 standard specifies data rates up to 10 Mbps and enables the driver when a logic high is set to DE (DE = 1). line lengths of up to 4000 feet. A single driver can drive a Setting the DE pin to low (DE = 0) puts the driver in a tristate transmission line with up to 10 receivers. The common-mode condition. This effectively disconnects the driver from the voltage (VCM) is defined as the mean voltage of A and B bus and allows other nodes to transmit over the same twisted terminals with respect to signal ground (VCM = (VIA + VIB)/2).

7 Pair cable. The RS-422 receivers can withstand a common-mode voltage (VCM) of 7 V. When all ten receivers are placed on the RS-485 transceivers also have an RE pin that enables/disables bus, a maximum load condition occurs. The input impedance the receiver. The DE and RE pins combined allow some RS-485. of the RS-422 receiver must be larger than or equal to 4 k . transceivers to be put into a low power shutdown mode. This is important for battery-powered applications. RS-485 AND THE UNIT LOAD CONCEPT. The input impedance of the RS-485 receiver is specified as Table 1. UL Receiver Input Impedance larger than or equal to 12 k . This impedance is defined as Unit Load No. of Nodes Min. Receiver Input Impedance having one unit load (UL). The RS-485 specification specifies 1 32 12 k . the capability to sustain up to 32 ULs. 64 24 k . 128 48 k . Some RS-485 receivers are specified as having UL or 256 96 k . UL.

8 A receiver specified to have UL means that the receiver only loads the bus by of the standard UL and, therefore, 4 times as many of these receivers can be connected to the bus (4 32 = 128 nodes). RO RO RO RO RO. R1 R2 R3 R4 R5. DI D RT R10 RO. R6 R7 R8 R9. 07395-002. RO RO RO RO. Figure 2. Typical RS-422 Interface Circuit Rev. 0 | Page 3 of 12. AN-960 APPLICATION NOTE. HALF-DUPLEX RS-485 FULL-DUPLEX RS-485. Half-duplex RS-485 links have multiple drivers and receivers on Figure 4 shows an example of an RS-485 bus connected in the same signal path. This is the reason why RS-485 transceivers a full-duplex bus configuration. This configuration is also must have driver/receiver enable pins enabling only one driver known as a 4-wire RS-485 network connected in a multipoint to send data at a time. See Figure 3 for a half-duplex bus config- master/slave configuration. Full-duplex RS-485 allows for uration. This configuration is also known as a 2-wire RS-485 simultaneous communication in both directions between network connected in a multipoint configuration and allows for master and slave nodes.

9 Data transmission in both directions, but only in one direction at a time. RO R R RO. A A. RE RE. RT RT. DE DE. B B. DI D D DI. GND GND. A B A B. R.. R. D GND D GND. 07395-003. RO RE DE DI RO RE DE DI. Figure 3. Half-Duplex RS-485 Bus Configuration MASTER SLAVE. DE. A Y. RO R RT D DI. B Z. RE. DE. Z A. DI D RT R RO. Y B. RE. GND GND. A B Z Y A B Z Y. GND GND.. SLAVE R R SLAVE. D D. 07395-004. RO RE DE DI RO RE DE DI. Figure 4. Full Duplex RS-485 Bus Configuration Rev. 0 | Page 4 of 12. APPLICATION NOTE AN-960. TERMINATION. DIFFERENTIAL DIFFERENTIAL. In a transmission line, there are two wires, one to carry the DRIVER RECEIVER. currents from the driver to the receiver and another to provide A. DI RO. the return path back to the driver. RS-485 links are a little more RT. 07395-005. complicated because of the fact that they have two signal wires B RE. that share a termination as well as a ground return path.

10 Figure 5. Parallel Termination However, the basic principles of transmission lines are the same. DIFFERENTIAL DIFFERENTIAL. For reliable RS-485 and RS-422 communications, it is essential DRIVER RECEIVER. that the reflections in the transmission line be kept as small as A. DI RT RO. possible. This can only be done by proper cable termination. 07395-006. CT. Reflections happen very quickly during and just after signal B RE. transitions. On a long line, the reflections are more likely to Figure 6. AC Termination continue long enough to cause the receiver to misread logic Table 2. Termination Advantages and Disadvantages levels. On short lines, the reflections occur much sooner and Termination Advantages Disadvantages have no effect on the received logic levels. None Simple, low power Suitable only for short In RS-422 applications there is only one driver on the bus and links with slow drivers if termination is to be used it must be placed at the end of the Parallel Simple High power cable near the last receiver.