The trend in industrial networks towards increased performance, be it through network expansion or bandwidth increase, requires the compensation of frequency-dependent losses of the transmission cable, predominantly existing at high frequencies, Figure 1.

Figure 1: a) Principle of cable loss compensation via b) pre-emphasis, and c) receiver equalization
(Click on image to enlarge)

The two most commonly applied (and inexpensive) compensation techniques are driver pre-emphasis and receiver equalization.

Driver Pre-emphasis
Pre-emphasis increases the high-frequency content of the original signal prior to entering the transmission medium (Figure 1b), by briefly boosting the driver output to almost twice the amplitude of the original signal. Figure 2 clarifies the theory behind it.

Figure 1: Harmonics spectra of a 50% duty cycle (a), a 25% duty cycle (b), and the combined signal (c)
(Click on image to enlarge)

While the output signal of a conventional driver with 50% duty cycle consists entirely of odd harmonics of the original square wave (Figure 2a), a 10% duty-cycle pulse presents odd and even harmonics in its spectrum (Figure 2b). The combination of both waveforms then yields a spectrum enriched in high-frequency content and with higher signal levels (Figure 2c).

Thus, boosting the driver output briefly generates the necessary upper harmonics that counteract the low-pass response of the transmission cable. It also creates sharper signal edges at the receiver input with less displacement in time, thus keeping inter-symbol interference (ISI) at a minimum.

Unfortunately, pre-emphasis has serious drawbacks, such as:

1. The violation of the EIA-485 standard by exceeding the specified maximum of 10% signal overshoot by some cool 90%,
2. The inapplicability in electromagnetic interference (EMI) sensitive applications due to the high-frequency emissions caused by the pre-emphasis pulse,
3. Instantaneous power consumption which is approximately four-times higher, due to the high pulse amplitude and the associated current boost.

While this approach is straightforward, noise-sensitivity considerations make the optimization of the internal filter response to certain applications necessary. Thus, lower filter orders are applied for approximating the cable attenuation in the lower megahertz-range, while higher filter orders provide equal accuracies in the tens of megahertz.

Receiver equalization reigns over pre-emphasis by:

1. complying to the EIA-485 specifications for maximum driver-voltage overshoots,
2. avoiding high-frequency emissions, thus making it the preferred solution in EMI-sensitive applications,
3. and keeping the increase in power consumption due to the added filter circuit negligible.

Figure 1: Data rate vs line length characteristics in comparison
(Click on image to enlarge)

The diagram above shows that for 200-meter cable length, the data rate of device A can be 10× the data rate of a conventional transceiver. At 1000 meters, the data rate of device B maxes out at 5× that of a transceiver without equalization. In the extreme case of a 1500-meter cable, device B is still capable of transmitting at 1 Mbps.

Conclusion
Receiver equalization is the preferred technique to extend the usable limits of EIA-485 data communications, while complying with the EIA standard. Increasing the signaling rates, or cable distances over inexpensive cables are easily achieved. Examples include Texas Instruments' SN65HVD23 and SN65HVD24 family of EIA-485 transceivers whihc provide integrated receiver equalization with proven performance advantages.

Reference
1. Use Receiver Equalization to extend RS-485 Data Communications (SLLA169); http://focus.ti.com/general/docs/techdocsabstract.tsp?abstractName=slla169; click here.

About the Author


Thomas Kugelstadt is a Senior Systems 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. You can contact Thomas about this article at: scb@list.ti.com.

Previous installments of this series:

• "SIGNAL CHAIN BASICS (Part 22): Phantom microphone power--the ghost in the machine", click here
• "SIGNAL CHAIN BASICS (Part 21): Understand and configure analog and digital grounds", click here
• "SIGNAL CHAIN BASICS (Part 20): Understand the basics of op amps and speed", click here
• "SIGNAL CHAIN BASICS (Part 19): Exploring and understanding linear voltage regulators", click here
• "SIGNAL CHAIN BASICS (Part 18): The op amp as integrator", click here
• "SIGNAL CHAIN BASICS (Part 17): Hysteresis--Understanding more about the analog voltage comparator", click here
• "SIGNAL CHAIN BASICS (Part 16): Understanding the analog voltage comparator", click here
• "SIGNAL CHAIN BASICS (Part 15): Analog/digital converter--dynamic parameters", click here
• "SIGNAL CHAIN BASICS (Part 14): Analog/digital converter--static parameters", click here
• "SIGNAL CHAIN BASICS (Part 13): Putting the Bode plot to use", click here
• "SIGNAL CHAIN BASICS (Part 12): The Bode plot, an essential ac-parameter display tool", click here
• "SIGNAL CHAIN BASICS (Part 11): Introducing voltage- and power-conditioning circuits", click here
• "SIGNAL CHAIN BASICS (Part 10): Exploring the Delta-Sigma Converter", click here
• "SIGNAL CHAIN BASICS (Part 9): SAR Converter Operation Explored", click here
• "SIGNAL CHAIN BASICS (Part 8): Flash- and Pipeline-Converter Operation Explored", click here
• "SIGNAL CHAIN BASICS (Part 7): Op Amp Performance Specification--Bias Current", click here
• "SIGNAL CHAIN BASICS (Part 6): Op Amp Input Voltage Offset", click here
• "SIGNAL CHAIN BASICS (Part 5): Introduction to the Instrumentation Amplifier", click here
• "SIGNAL CHAIN BASICS (Part 4): Introduction to analog/digital converter (ADC) types", click here
• "SIGNAL CHAIN BASICS (Part 3): Analog and the digital world", click here
• "SIGNAL CHAIN BASICS (Part 2): Op Amp--Basic operations", click here
• "SIGNAL CHAIN BASICS: Operational Amplifier--The Basic Building Block", click here