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 , 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

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 operations. RS-485 is a superset of RS- implementing using a 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 static charges, and generate a External ESD diodes and other discreet spark. ESD sparks are often produced components can be used to protect data by the interaction of people with lines. Many IC devices integrate some their surroundings. These inadvertent degree of ESD protection and require sparks can change the properties of a no further external protection for the semiconductor device, degrading or IC itself. Figure 5 shows a simplified destroying it entirely. ESD can also functional diagram of a common threaten an electronic system when integrated protection scheme. Voltage someone replaces a cable or even touches spikes at the signal input/output (I/O)

an I/O port. Discharges that accompany are clamped to VCC or GND and protect these routine events can disable the the internal circuitry. Many interface port by destroying one or more of the products and analog switches integrate interface ICs (Figure 4). Such failures ESD protection designed to comply with can be expensive—they raise the cost IEC 61000-4-2 standards. of warranty repairs while diminishing the product’s perceived quality. ESD is Maxim Integrated has invested a a serious industrial problem estimated substantial effort in developing ICs with to cause billions of dollars in damages robust internal ESD protection. Starting

6 of 10 www.maximintegrated.com addition, they are protected against VCC ESD strikes while powered up, powered down, and in shutdown mode.

SIGNAL I/O Fault Protection GND The driver outputs/receiver inputs of (OR NEGATIVE Faults, such as VOLTAGE RAIL) IC RS-485 devices in industrial network overvoltages, applications often experience voltage can be caused Figure 5. Simplified integrated ESD faults resulting from stray voltage by wiring protection circuitry spikes. Faults are different from ESD errors, loose events. While ESD events occur within with RS-232 and RS-485 interface ICs, connections, a short time range less than 100ns, Maxim has become a leader in ESD pinched or faults generally occur over a sustained protection for interface transceivers. faulty cables period of approximately 200µs or These devices withstand the application or even solder longer. Faults, such as overvoltages, of IEC 61000-4-2 and JEDEC JS-001 ESD debris that can be caused by wiring errors, loose events directly to their I/O pins. This causes the connections, pinched or faulty cables, approach is robust, readily available, powerline or even solder debris that causes the requires no external real estate, and to contact powerline to contact the data connection costs less than most alternatives. the data on the PCB or in the connector, can be connection All Maxim devices incorporate ESD- exposed to high voltages for seconds or on the PCB protected structures on all pins to even minutes before failure. This can or in the protect against electrostatic discharges be catastrophic since many industrial connector. encountered during handling and power supplies exceed +24V. Any assembly. Transceivers with high ESD contact with a data line ensures the protection, such as the MAX3483AE/ destruction of a standard, unprotected MAX3485AE family, feature ±20kV ESD RS-485 transceiver. protection on their transmitter-output and receiver-input pins. Not only will To protect against these faults, ordinary these transceivers not be damaged by RS-485 devices require costly discrete ESD strikes less than their rated value, external protection circuitry or devices. but they will continue to work normally Fault-protected RS-485 transceivers without the need to cycle power. In offer overvoltage protection as high

www.maximintegrated.com 7 of 10 as ±40V, ±60V, and even ±80V on and -200mV. This guaranteed logic- communication bus lines. Maxim offers high is achieved by setting the receiver many fault-protected RS-485/RS-422 threshold between -50mV and -200mV. Fail-Safe transceivers such as the MAX13442E– If the differential receiver input voltage Receivers MAX13444E that can survive high DC (V - V ) is greater than or equal to guarantee A B voltages on the data pins. To reduce -50mV, RO is logic-high. If (V - V ) a logic-high A B system complexity and the need for is less than or equal to -200mV, RO is receiver when the receiver external protection, the driver outputs logic-low. In the case of a terminated inputs are and receiver inputs of these fault- bus with all transmitters disabled, the opened or if all protected devices can withstand voltage receiver’s differential input voltage is transmitters on faults of up to ±80V with respect to pulled to ground by the termination. a terminated ground without damage. Protection is This results in a logic-high with a 50mV bus are guaranteed regardless of whether the minimum noise margin. Unlike previous disabled. device is active, shut down, or without fail-safe devices, the -50mV to -200mV power. This makes them the most threshold complies with the ±200mV Hot-Swap robust transceivers in the industry and, EIA/TIA-485 standard. Circuitry therefore, ideal for industrial applications. eliminates These devices are designed to survive Hot-Swap Capability false overvoltage faults such as direct shorts Hot-swap circuitry eliminates false transitions to power supplies, miswiring faults, transitions on the data cable during on the data connector failures, cable crushes, and circuit initialization or connection to a cable during tool mis-applications. live backplane. Short-circuit current initialization limiting and thermal-shutdown circuitry or connection True Fail-Safe Receivers protect the driver against excessive to a live An important feature of many RS-485 power dissipation. backplane. transceivers is the true fail-safe circuitry, which guarantees a logic-high receiver Inserting circuit boards into a hot, or output when the receiver inputs are powered, backplane may cause voltage opened or shortened, or if all transmitters transients on DE, DE/RE, RE, and on a terminated bus are disabled (high receiver inputs A and B that can lead to impedance). True fail-safe solved the data errors. For example, upon initial problem of a collapsing bus by changing circuit board insertion, the processor the receiver input threshold to a slightly undergoes a power-up sequence. During negative differential voltage of -50mV this period, the high-impedance state of

8 of 10 www.maximintegrated.com the output drivers makes them unable How does this internal hot-swap circuitry to drive the transceiver enable inputs work? At the driver-enable input (DE), to a defined logic level. Meanwhile, there are two nMOS devices, M1 and leakage currents of up to 10µA from the M2 (Figure 6). When VCC ramps from high-impedance output, or capacitively zero, an internal 15µs timer turns on M2 coupled noise from VCC or GND, could and sets the SR latch, which also turns cause an input to drift to an incorrect on M1. Transistors M2, a 2mA current logic state. To prevent such a condition sink, and M1, a 100µA current sink, pull from occurring, devices such as the DE to GND through a 5.6kΩ resistor. M2 MAX3440E–MAX3443E feature hot- pulls DE to the disabled state against swap input circuitry on DE, DE/RE, and an external parasitic capacitance up to RE to guard against unwanted driver 100pF that may drive DE high. After activation during hot-swap situations. 15µs, the timer deactivates M2 while

When VCC rises, an internal pulldown M1 remains on, holding DE low against (or pullup for RE) circuit holds DE low three-state leakage currents that may for at least 10µs and until the current drive DE high. M1 remains on until an into DE exceeds 200µA. After the initial external current source overcomes the power-up sequence, the pulldown circuit required input current. At this time, becomes transparent, resetting the hot- the SR latch resets M1 and turns off. swap tolerable input. When M1 turns off, DE reverts to a standard, high-impedance CMOS input.

Whenever VCC drops below 1V, the input VCC is reset. A complementary circuit for RE 15µs TIMER uses two pMOS devices to pull RE to TIMER VCC.

DE 5.6kΩ (HOT SWAP)

2mA 100µA

M1 M2

Figure 6. Simplified structure of the driver enable pin (DE)

www.maximintegrated.com 9 of 10 Conclusion

Industrial system applications, such as factory automation, are subjected to harsh electrical environments. It is vitally important for the system-level designer to take into account voltage transients from several sources when developing the hardware that will withstand these conditions. Most data communications networks rely on the robustness of the RS-485 protocol standard along with special safety features incorporated into the transceiver ICs to withstand these effects. Structures such as extended ESD protection, high-voltage fault protection, and hot-swap capability safeguard against these events to help maintain system reliability.

References

1. Application note 4491, “Damage from a Bolt or a Spark—It Depends on How Tall You Are!” 2. Application note 5260, “Design Considerations for a Harsh Industrial Environment” 3. Application note 639, “Maxim Leads the Way in ESD Protection”

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