New-Tech Europe Magazine | June 2019

UARTs Ensure Reliable Long-Haul Industrial Communications Over RS-232, RS-422, and RS-485 Interfaces Art Pini, Digi-Key

Industrial sensing and control poses many challenges for a communications bus. For example, cabling runs of hundreds or thousands of feet are common while the industrial environment itself is noted for often harsh operating conditions. Industrial equipment can be exposed to a wide range of temperatures, high electrical noise on both power supply and data lines, and fault events like electromagnetic interference (EMI), electrostatic discharge (ESD), or short circuits. The solution to these problems is to use a robust serial interface based on a universal asynchronous receiver/ transmitter (UART), also called an asynchronous communications element (ACE) by some suppliers. UARTs are available as standalone devices, such as the Texas Instruments TL16C752D, or may be within a microcontroller like the Microchip Technology PIC16F688T-I/SL.

With appropriate line drivers, a UART can work over long distances: from 15 meters (m) for the RS-232 serial data bus to 1000 m for RS-485 or RS-422 interfaces. All three of these protocols serve to provide control to remote machines and controllers in factory automation applications and are designed to minimize the effects of EMI and ESD in the harshest settings. This article will provide background on these commonly used industrial control interface protocols and show how they can be implemented using UARTs and line drivers. RS-232 The RS-232 serial communications standard is also known currently as EIA/TIA-232-F, which is an Electronic Industries Association/ Telecommunications Industries Association standard. The letter F indicates the latest revision. The standard is identical to the International

Telecommunications Union (ITU) standards V.24 and V.28. This interface was the original serial bus on personal computers. It was used originally to connect the computer—termed data terminal equipment (DTE)— to a modem, referred to as data communications equipment (DCE). EIA/TIA-232-F defines a physical layer standard, including signal levels and timing, control signals, connectors, and connector wiring. It does not define character encoding, framing, and other aspects of the protocol level. A typical asynchronous serial bus will include a UART or ACE, line drivers, connectors, and cables (Figure 1). The UART/ACE converts the internal parallel bus of the computer into a serial data stream. It also supplies the input and output first in first out (FIFO) memory buffering, an interface clock (generally referred to as a baud rate generator), and interface timing and handshaking signals. The UART/

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