The main culprits are frequency dependent losses of the transmission cable, predominantly increasing at high frequencies. These losses can degrade a signal to the extent that it becomes undetectable by the receiver. Counteracting these losses, high-frequency compensation methods are needed to restore the signal quality. The two most commonly applied compensation techniques are driver pre-emphasis and receiver equalization.

Aimed at helping the design engineer to increase networking performance, this article discusses cable losses and their impact on signal quality. The paper describes two compensation methods, and suggests solutions for typical application scenarios.

Cable Losses

Typical RS-485 cabling is of the 120 terminated, unshielded twisted pair (UTP) type. Because of relatively long propagation times, the cable is modeled as a transmission line (Figure 1a), with its characteristic losses R, L, C and G that yield the familiar line length versus data-rate dependency shown in Figure 1b.

While at low frequencies, the line resistance R determines the maximum cable length. At high frequencies, R and C components dominate, causing the cable to act as an R-C, low-pass filter.

Figure 2 shows the frequency responses of several UTP cables of varying lengths with the reactive losses occurring predominantly at higher frequencies. To ensure sufficient signal amplitude reaches the receiver, cable losses must be compensated through compensation techniques, adding high frequency content to the transmission signal.

Cable Loss Compensation

The principle of cable loss compensation is to approximate the inverse of the cable's transfer function as accurately as needed, and to implement it into a functional compensator stage in series to the transmission cable. The summation of the compensator gain and the cable loss ideally results in a net gain response close to unity across the frequency range of interest (Figure 3a).

Two compensation techniques most often applied are pre-emphasis and receiver equalization.

When the compensation stage is implemented in the driver (Figure 3b), the amplification, or emphasis of high frequency components, occurs before the signal is sent across the data link " hence, the term Pre-Emphasis. If the cable response is equalized at the receiver end of the cable (Figure 3c), it is called receiver equalization.

While both techniques aim for the same result, their functional principals differ, yielding a number of benefits and drawbacks between the two methods.

Pre-emphasis

Pre-emphasis adds high-frequency components to the original signal by boosting the driver output for a fraction of the bit period at each signal transition. The theory behind the signal boost is demonstrated in Figure 4.

When transmitting a data stream of alternate zeros and ones, conventional RS-485 drivers transmit signals using a 50 percent duty cycle, whose frequency spectrum purely consists of odd harmonics of the original square wave (Figure 4a). A 10 percent duty cycle pulse, however, shows an almost equal level of even and odd harmonics (Figure 4b). Combining both waveforms to one (Figure 4c), provides a signal enriched with high-frequency content and also with higher signal levels.

Boosting the driver output for a short time interval creates the necessary pre-emphasis pulse that adds odd and even harmonics to the frequency spectrum of the original square wave, thereby counteracting the low-pass filter response of the cable. The results are sharper signal edges at the receiver input and less displacement in time, thus yielding reduced (spell out ISI here) ISI.

Pre-emphasis, however, has several limitations: 1) The pre-emphasis pulse exceeds the nominal signal levels typically by 90 percent. This clearly violates the RS-485 standard, which allows less than 10 percent overshoot during signal transitions.

2) The pre-emphasis pulse generates high-frequency emissions, which does not permit the use of transceivers with pre-emphasis in applications with high restrictions on electromagnetic interferences.

3) Pre-emphasis requires more power per bit due to the square-law relation between signal voltage and signal power. For example, boosting a nominal driver output of 1.5V by 90 percent nearly quadruples the instantaneous power consumption. Receiver Equalization
Receiver equalization amplifies the high frequency components of the transmitted signal via active high-pass filters, located between the cable end and the receiver input (Figure 5). Filter design complexity depends on the required accuracy of the cable attenuation approximation. Low filter orders provide accurate approximation only up to a few megahertz, while higher filter orders provide the same amount of accuracy at frequencies in the tens of megahertz.

A main concern in filter design is noise sensitivity, as the increased filter gain makes the receiver sensitive to high-frequency noise. To counteract the raised sensitivity, narrow the filter's high-pass characteristic to the frequency range of interest. As another measure, raise the receiver input thresholds to prevent noisy transients from triggering the receiver's detection circuitry.

Receiver equalization has several advantages over pre-emphasis: 1) Transceivers using receiver equalization conform to the RS-485 specifications for driver voltage overshoot, transceivers using pre-emphasis don't!

2) Unlike pre-emphasis, receiver equalization does not generate high-frequency emissions and, therefore, can be easily applied in applications with tight EMI constraints.

3) The hardly noticeable increase in power consumption due to the added filter circuit is negligible in comparison to pre-emphasis.

While discrete filter designs are easily implemented, they quickly become space and cost intensive. Therefore, transceiver solutions with integrated receiver equalization and pin-compatible to the industry standard 75176 were designed. As an example, the SN65HVD23, optimized for 25MHz at 160m, supports high data rates over medium cable length. This transceiver promotes medium data rates over three times that length and is optimized for 5MHz at 500m.

Figure 6 shows the individual frequency responses for receiver gain, cable attenuation, and the resulting net gain for both transceivers.

Figure 7 compares the devices' signaling rate versus line length characteristics, with the characteristic of a conventional RS-485 transceiver.

For a cable length of 200m, the data rate of the HVD23 can be 10-times the data rate of a transceiver without receiver equalization. While at 1000m, the maximum data rate of the HVD24 is more than five times higher than one of its conventional counterpart. Even in the case of an extreme long distance data link, the HVD24 is capable of transmitting at 1MHz over 1500m cable length.

Applications
Figure 8 illustrates the benefits of receiver equalization in a factory automation network. In this type of application, end-equipment users may require flexibility to install programmable logic controllers (PLCs), sensors and actuators up to 500 meters away from the main human-machine interface (HMI).

Without receiver equalization, signaling rates are limited to about 2Mbps. However, using an RS-485 transceiver with receiver equalization, the signaling rate can be raised to 7.5Mbps using the same cable. This significant increase in data rate can either provide higher data throughput on the same installed network, or enable faster system timing for critical functions.

Another interesting application is building automation for HVAC and security functions, where a network extension at standard signaling rates is beneficial. For a high-speed network running with a signaling rate of 25Mbps, the SN65HVD23 for instance, can extend the maximum network length from about 50 to about 150 meters. Similarly, for a building automation network running with a signaling rate of 5Mbps, an enhanced version (SN65HVD24) can extend the maximum network length from about 150 to about 500 meters.

Finally, in any application where cable cost reduction is a benefit, consider the advantages of receiver equalization. Suppose in a building automation where the installed cable is adequate to support up to 500kbps signaling, the HVAC and security electronics are to be upgraded for increased energy efficiency and better security. These higher performance features require the network to support signaling rates of 1Mbps and higher. While the costs associated with upgrading the cable for higher signaling rates might become prohibitive, the implementation of transceivers with receiver equalization can avoid all these expenses, thus proving the benefits to be substantial.

Conclusion
Receiver equalization is the only technique that, while extending the usable limits on RS-485 data communications, conforms to the RS-485 standard. Extension to higher signaling rates, longer cable distances, and more economical cable are all possible with integrated receiver equalization and proven performance advantages.

References
1. Use Receiver Equalization to extend RS-485 Data Communications (SLLA169): http://focus.ti.com/general/docs/techdocsabstract.tsp?abstractName=slla169
2. Extended Common-Mode RS-485 Transceivers Data Sheet (SLLS552D): http://focus.ti.com/docs/prod/folders/print/sn65hvd24.html
3. Extend Your Reach, Howard Johnson, Signal Integrity Solution Guide, August, 2005.

About the Author
Thomas Kugelstadt is a Senior Applications Engineer at Texas Instruments where he is responsible for defining new, high-performance analog products and developing complete system solutions that detect and condition low-level analog signals in industrial systems. During his 20 years with TI, he has been assigned to various international application positions in Europe, Asia and the U.S. Thomas is a Graduate Engineer from the Frankfurt University of Applied Science.