8 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

8.1 Application Information

When designing a system that uses drivers, receivers, and transceivers that comply with RS-422 or RS-485,

proper cable termination is essential for highly reliable applications with reduced reflections in the transmission

line. Because RS-422 allows only one driver on the bus, if termination is used, it is placed only at the end

of the cable near the last receiver. In general, RS-485 requires termination at both ends of the cable. Factors

to consider when determining the type of termination usually are performance requirements of the application

and the ever-present factor, cost. The different types of termination techniques discussed are unterminated

lines, parallel termination, AC termination, and multipoint termination. Laboratory waveforms for each termination

technique (except multipoint termination) illustrate the usefulness and robustness of RS-422 (and, indirectly,

RS-485). Similar results can be obtained if 485-compliant devices and termination techniques are used. For

laboratory experiments, 100 feet of 100-Ω, 24-AWG, twisted-pair cable (Bertek) was used. A single driver and

receiver, TI AM26C31C and AM26C32C, respectively, were tested at room temperature with a 5-V supply

voltage. Two plots per termination technique are shown. In each plot, the top waveform is the driver input and

the bottom waveform is the receiver output. To show voltage waveforms related to transmission-line reflections,

the first plot shows output waveforms from the driver at the start of the cable; the second plot shows input

waveforms to the receiver at the far end of the cable.

8.2 Typical Application

DIN

D

AM26C31

(One Driver)

D

RT

AM26C32

(One Receiver)

ROUT

Figure 8-1. Differential Terminated Configuration

8.2.1 Design Requirements

Resistor and capacitor (if used) termination values are shown for each laboratory experiment, but vary

from system to system. For example, the termination resistor, RT, must be within 20% of the characteristic

impedance, Zo , of the cable and can vary from about 80 Ω to 120 Ω.

8.2.2 Detailed Design Procedure

Figure 8-1 shows a configuration with no termination. Although reflections are present at the receiver inputs

at a data signaling rate of 200 kbps with no termination, the RS-422-compliant receiver reads only the input

differential voltage and produces a clean signal at the output.

AM26C32, AM26C32C, AM26C32M, AM26C32I

SLLS104M – DECEMBER 1990 – REVISED OCTOBER 2023

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