New-Tech Europe Magazine | June 2019

is limited to just 10 receivers on the bus. RS-485 in full duplex mode requires four wires compared to two for half-duplex operation and RS- 422 (Figure 6). The differential bus wiring uses two conductors for each transmit or receive signal line as shown in the figure. Full duplex operation requires four wires, while half duplex requires only two. Due to the higher speed of both RS-422 and RS-485, the transmission lines must be terminated at each end. For twisted pair, the termination resistors RT are 120 ohms (Ω). As might have been surmised by the dual UART configuration of the TL16C752D interface IC, it has an RS-485 mode. This is why many UARTs and related line drivers employ the dual configuration.

Voltage levels at the transmitter side for RS-422 are ±6 volts while those for RS-485 are -7 to +12 volts. At the receiver the sensitivity is ±200 millivolts (mV) for both standards. Conclusion The three serial interfaces RS-232, RS- 422, and RS-485 offer a range of choices for robust serial communications over both short and long distances. UARTs form the basis for all three standards and make it easy to add serial communications to designs, especially those for challenging industrial environments. bout this author Arthur (Art) Pini is a contributing author at Digi-Key Electronics. He has a Bachelor of Electrical Engineering degree from City College of New York

and a Master of Electrical Engineering degree from the City University of New York. He has over 50 years experience in electronics and has worked in key engineering and marketing roles at Teledyne LeCroy, Summation, Wavetek, and Nicolet Scientific. He has interests in measurement technology and extensive experience with oscilloscopes, spectrum analyzers, arbitrary waveform generators, digitizers, and power meters.

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