RS-485: The Workhorse of Industrial Serial Communications Systems

RS-485 is a telecommunication standard describing physical attributes used for serial communications. Two lines are dedicated for a differential pair and one as a reference. A network may be built with the standard and is balanced with one master and up to 32 slave devices. The standard TIA/EIA-485(-A) is jointly published by the Telecommunications Industry Association and the Electronic Industries Alliance. It is suitable for use in noisy industrial environments where lower transfer rates are more than adequate to drive multiple devices.

The standard defines differential pairs for data transmission. Differential pairs support equal and opposite signals to represent communication bits. The signals travel adjacent to one another in a pair between devices. Noise injected onto the pair during transmission is equal and opposite allowing cancellation before arriving at the receiver. This cancellation effect maintains signal integrity of the data. Cables use two-wire twisted pair to maintain optimum magnetic flux characteristics of the signal.

Differential pairs may provide communication on a total of 32 devices along cables as long as 4000 ft. The standard defines up to 100Mbps clock rate for RS-485 although slower clocking is required for longer cable lengths defined in the standard. This is due to impedance requirements to prevent reflections and to terminate signals when not in use.

Defining Characteristics of RS-485

It is customary to configure devices along a linear path for use of the RS-485 standard. Otherwise, impedance will build preventing fidelity of signals expecting defined terminations for proper transmission. Further complications arise due to stubs should any nonlinear configuration be attempted for use with RS-485.

There is one master device in use at any moment in time, as specified for RS-485 configurations. The standard provides the ability to configure more than one master on the bus with the caveat that only one master be active at any one time. It is best to place the master in the middle of the physical configuration of the cables and devices. This allows for a balanced impedance within the network and results in the best signal transfer among devices.

The physical setup defined by RS-485 standard supports multiple protocols, though not all operate on the network at the same time. The system must consistently use the same communication protocol for a network of devices. As stated in the standard, a combination of signal A being low while signal B is high defines transmission of a logic 1. This is called “Mark.” The reverse is true for logic 0, called “Space.” This is illustrated in the figure below. It is interesting to note that many vendor datasheets invert the standard in provided truth tables found in RS-485 part datasheets. It is always a good idea to compare the standard to the protocol programmed into your network to make sure it will play well when hosting third-party devices.

RS-485 physical transmission standard By Royvegard at the English language Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=5498232

Parameters to Consider When Selecting an RS-485 Transceiver

Determine the number of devices to connect on your network and if the system will use more than one master. Master drivers for RS-485 must be placed in the middle of slave devices to take advantage of the standard’s impedance guidelines. Without balance, unnecessary impedance may be added to wires and disrupt signals causing failed communication. Keeping the devices wired in linear formation removes stubs that can also disrupt signals.

Further considerations include whether the system will need full- or half-duplex. Full-duplex occurs when the master is able to both send and receive transmissions at the same time. Half-duplex allows transmission in one direction only. Consider how far devices will locate from the master as this will determine transmission, or baud, rate of your network. The longer the cable, the slower the baud. A rule of thumb in determining cable length states that baud (in Mbps) times cable length (in meters) should not exceed 108.

RS-485 is specified to handle up to 100Mbps and is most suitable in industrial applications for transmitting information from actuators or card readers. The transmission speeds are also ideal for building automation controls, video surveillance, security control panels, and is used in model railway systems for the Digital Command Control (DCC).

The following describes some parts detailed on our website that may be useful in your system. Each vendor offers families of parts so you may choose which works best in your application. To develop ideas, each vendor provides an evaluation board as well. We list each below with links to their listing on the website where you will find sourcing and costing information.

Texas Instruments, THVD14xx family of RS-485 Transceivers

This group of RS-485 transceivers offers either half- or full-duplex operation. Operating from either a 3.3V or 5V rail, the parts meet or exceed requirements of TIA/EIA-485A standard. They offer an extended operational common-mode range of +15V and provide ESD protection of up to +18kV. An evaluation kit for half-duplex is available, the RS485-HF-DPLX-EVM.

THVD14xx is a family of noise-immune RS-485/RS-422 transceivers designed to operate in rugged industrial environments. The bus pins of these devices are robust to high levels of IEC electrical fast transients (EFT) and IEC electrostatic discharge (ESD) events, eliminating the need for additional system-level protection components.

Found on page 3 of THVD14xx 3.3-V to 5-V RS-485 Transceivers datasheet

These parts are suitable for industrial applications such as motor drives, factory automation, and control, grid infrastructure, building automation, HVAC systems, video surveillance, process analytics or wireless infrastructure.

Analog Devices, ADM306x family of RS-485 Tranceivers

This group of RS-485 transceivers offers either half-duplex operation compliant to Profibus standard with increased 2.1V bus differential voltage at VCC > 4.5V. Operating from either a 3.3V or 5V rail, the parts offer full hot-swap support for a glitch-free power-up. An evaluation board, the EVAL-ADM3061EEBZ is available for development.

The ADM3065E is a 3.0V to 5.5V, IEC electrostatic discharge (ESD) protected RS-485 transceiver, allowing the device to withstand +12kV contact discharges on the transceiver bus pins without latch-up or damage. The ADM3066E/ADM3062E features a VIO logic supply pin allowing flexible digital interface capable of operating as low as 1.62V.

Found on page 6 of Analog Devices ADM3061E/ADM3062E/ADM3065E/ADM3066E datasheet

These parts are robust for industrial applications such as use in fieldbuses, process control, building automation, profibus networks, and motor control servo drives and encoders.

Maxim Integrated, MAX22502E Full-Duplex RS-485 Transceiver

This RS-485 transceiver provides high-speed operation over long distances at up to 100Mbps. The part incorporates integrated protection up to 15kV ESD human body model. It is flexible for use in many different types of applications. For development an evaluation board, the MAX22502EEVKIT# is available for testing at the bench.

The MAX22502E full-duplex, ESD-protected, RS-485/RS-422 transceiver is optimized for high-speed (up to 100Mbps) communication over long cables. This transceiver features larger receiver hysteresis for high noise rejection and improved signal integrity. Integrated preemphasis circuitry extends the distance, and increases the data rate, of reliable communication by reducing inter-symbol interference (ISI) caused by long cables when supplied with 5V.

Found on page 1 of MAX22502E 100Mbps Full-Duplex RS-485/RS-422 Transceiver datasheet

With full duplex, this part is applicable for high-speed serial communications such as motion control, encoder interfaces, field bus networks, industrial control systems, and backplane busses.

RS-485 is a great standard for serial transmission networks up to 100Mbps and operating in noisy environments. Differential pairs may host up to 32 devices on the standard that uses twisted pairs and a reference. The low wire count for transmitting RS-485 results in lower weight cables that are ideal for minimizing bulk in wiring environments. Finally, the standard is suitable for use with several protocols that define twisted pair for successful transmission.

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