RS-485: Still the Most Robust Communication Table of Contents
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TUTORIAL RS-485: Still the Most Robust Communication Table of Contents Abstract...........................................................................................................................1 RS-485 vs. RS-422..............................................................................................................................2 An In-Depth Look at RS-485...........................................................................................................3 Challenges of the Industrial Environment.....................................................................................5 Protecting Systems from Harsh Environments.........................................................................5 Conclusion......................................................................................................................10 References......................................................................................................................10 Abstract Despite the rise in popularity of wireless networks, wired serial networks continue to provide the most robust, reliable communication, especially in harsh environments. These well-engineered networks provide effective communication in industrial and building automation applications, which require immunity from noise, electrostatic discharge and voltage faults, all resulting in increased uptime. This tutorial reviews the RS-485 protocol and discusses why it is widely used in industrial applications and the common problems it solves. www.maximintegrated.com 1 of 10 RS-485 vs. RS-422 An examination of the characteristics that make RS-485 the most popular interface protocol for use in harsh, industrial environments. The RS-485 transceiver is the most with factory automation can corrupt popular interface for implementing data transmission or physically destroy the physical layer in harsh industrial the network. and building automation networks for serial port communications. This serial There are many types of data interface interface standard provides single-path protocols commonly used today. differential signaling over a twisted Each was developed with a specific pair of wires for long-distance and application in mind with a particular set high-speed transmission needed for of protocol specifications and structure. industrial applications. The RS-485 These interfaces include CAN, RS- standard provides a robust interface 232, RS-485/RS-422, I2C, I2S, LIN, SPI that can withstand harsh environments. and SMBus, to name a few. Of these, One common problem with industrial RS-485 and RS-422 are still the most and building applications is that large reliable, especially in harsh industrial, electrical transients from fast switching electrical environments such as factory inductive loads, electrostatic discharge, and building automation. and frequent voltage surges associated SLAVE SLAVE R R MASTER D R R R SLAVE SLAVE SLAVE Figure 1. RS-422 multidrop bus 2 of 10 www.maximintegrated.com Although RS-485 and RS-422 are under maximum load. With this similar, they are not the same. There enhanced multidrop capability, you can are some differences between these two create networks of devices connected to standards that need to be noted when a single RS-485 serial port. The greater designing a system. noise immunity and multidrop capability make RS-485 the serial connection of RS-422 is best suited for industrial choice in industrial applications that The RS-485 environments that require only one require multiple distributed devices transceiver is the bus master (driver). It provides a networked to a PLC or other controllers most popular mechanism for transmitting data up for data collection, HMI, or similar interface for to 10Mbps. RS-422 sends each signal operations. RS-485 is a superset of RS- implementing using a twisted pair of wires to increase 422; thus, all RS-422 devices may be the physical noise immunity and improve baud rate controlled by RS-485. layer in harsh and cable length. RS-422 is specified industrial for multidrop applications where only Typical applications for RS-485 are and building one transmitter is connected to, and similar to those of RS-422: process automation transmits on, a bus of up to 10 receivers automation (chemicals, brewing, paper networks for (Figure 1) according to TIA/EIA-422. mills), factory automation (automotive serial port Typical applications include process and metal fabrication), HVAC, security, communications. automation (chemicals, brewing, paper motor control, and motion control. mills), factory automation (automotive Thus, RS-485 is the more commonly and metal fabrication), HVAC, security, used between the two standards due to motor control, and motion control. its enhanced capability. RS-485 offers greater flexibility when more than one bus master/driver is An In-Depth Look at RS-485 needed. It is an improvement over RS- As mentioned earlier, TIA/EIA-485, 422 because it increases the number of commonly known as RS-485, is the devices from 10 to 32 per TIA/EIA-485 most broadly used interface in industrial and has a wider common mode (-7V applications. RS-485 can be used over to +12V versus ±7V) and slightly lower long distances up to 4000 feet and high differential voltage range (±1.5 versus speeds greater than 40 Mbps for shorter ±2V) to ensure adequate signal voltages disances. The differential nature of RS- www.maximintegrated.com 3 of 10 VCC RO RO R A A R RE RE DE B B DE DI DI D D MASTER A B A B SLAVE D D R R RO RE DE DI RO RE DE DI SLAVE SLAVE Figure 2. Multidrop half-duplex transceiver system commonly used in industrial applications 485 will enable long runs, but it will slow operations with a two-pair bus (4- down as the cable length increases. In wire). It can handle up to 32 drivers addition to the distance, data rate will and up to 32 receivers in a half-duplex be affected by the wire gauge and the multidrop configuration. New devices number of nodes on the network. RS-485 have emerged that feature 1/4-unit-load, with preempahsis like the MAX3291 can and even 1/8-unit-load receiver input significantly improve on bandwidth and impedances, such as the MAX13448E, distance for applications that require it. which allows 128 to 256 receivers on the same bus. With this enhanced multidrop The RS-485 interface can be used in capability, you can create networks of half-duplex mode with a single-pair devices connected to a single RS-485 transmission bus or full-duplex mode serial port as shown in Figure 2. for simultaneous transmit-and-receive D +1.5V -1.5V +200mV -200mV R Figure 3. RS-485 minimum bus signal levels 4 of 10 www.maximintegrated.com The receiver input sensitivity is ±200mV. in these systems are subject to harsh This means that to recognize a 1 or a 0 electrical environments. bit, the receiver must see signal levels DC-DC converters used in industrial greater than +200mV for zero and lower applications include those with high than -200mV for one (Figure 3). Noise input voltages and isolated power that falls in the range between ±200mV outputs. Many applications use 24V Maxim’s is essentially blocked. The differential or 48V DC inputs to provide power to RS-485 format produces effective common- distributed applications that are not transceiver ICs mode noise cancellation. Minimum line powered. Once down converted to feature internal receiver input impedance is 12kΩ, 12V or 5V, point-of-load conversion can protection such and the driver output voltage is ±1.5V be applied. Applications that maintain as high ESD- minimum, ±5V maximum. communications with remote sensors immunity, and actuators need protection against fault protection Challenges of the Industrial the effects of transients, EMI, and against large Environment differing ground potentials. voltage spikes and hot- Companies such as Maxim Integrated Designers of industrial systems face swap capability take great care in ensuring that many difficult challenges to assure robust for error-free ICs for industrial applications are operation in the face of environmental data robust and able to withstand harsh conditions that may damage the transmission. hardware or negatively affect the digital electrical environments. Maxim’s RS- communications. 485 transceiver ICs feature internal protection such as high ESD-immunity, One example is automatic control fault protection against large voltage of process machinery in factory spikes and hot-swap capability for error- automation. A process controller free data transmission. monitors and measures operational and environmental variables and sends Protecting Systems actuation commands out to control from Harsh Electrical devices or alarms. The controllers are Environments usually microcontroller-based machines with architecture optimized to meet the The following are protection features needs of the factory and the application. that may be incorporated into RS-485 Point-to-point data communication line transceivers. www.maximintegrated.com 5 of 10 RUPTURED PASSIVATION DIELECTRIC FAILURE & CONTACT SPIKING ELECTROTHERMAL MIGRATION ESD is SPLATTERED a serious ALUMINUM industrial problem Figure 4. ICs with inadequate ESD protection are subject to catastrophic failure estimated to cause Extended ESD annually. ESD events that occur in the billions of Electrostatic discharge (ESD), an field can cause individual component dollars in overvoltage event, occurs when two failure and sometimes catastrophic damages materials with different electrical system failures. potentials make contact, transfer annually. stored