Standards are key to optimizing high-speed data bus communications

Standards are key to optimizing high-speed data bus communications
Analog and mixed-signal product designers are frequently faced with the task of selecting and optimizing an interface for transmitting binary data. While this data transmission function may be secondary to the central purpose of the product, most customers in our increasingly interconnected world expect a reliable, efficient means of linking new products together.

Before making any choices towards a solution for high-speed communication, a designer should identify the relevant requirements and constraints for the end application. These include the data rate, system reliability, the cost of all components, the distance between adjacent connected nodes and end points, and connectivity - the mode of operation (simplex v.s. duplex),and topology of interconnection (daisy-chain, star, ring).

Keep in mind that what is considered "high-speed" for one application might be considered sluggish in another application. The required rate of data communication depends upon the number of nodes connected to the network, the data requirements to support each node, the data latency allowable between nodes, and allowances for future upgrades.

Several organizations are active in setting and maintaining standards for data bus communications. These include IEEE, the Telecommunications Industry Association/Electronics Industry Association (TIA/EIA), the International Telecommunications Union (ITU). Several trade organizations are now organized to promote specific bus standards and a multitude of widely recognized data bus standards exist, covering a variety of requirements — RS-232, RS-485, CAN, TIA/EIA-644 (LVDS), and IEEE 1394 (FireWire).

Unless overriding reasons force the development of a proprietary solutions, designers will benefit from the advantages of using an existing standard: interchangeability, available components, known characteristics, and ready customer acceptance. Choosing the best solution for any specific application begins with a survey of the most common standards.

Once an appropriate standard has been selected, the bus design can be optimized for reliable transmission of data at high rates. Each wireline standard may have different ranges of operation, so that the actual signaling rates considered "high" may vary from standard to standard.

As signaling rate is increased, the effects of electrical noise — especially at high frequencies — become more of a concern. All cables, printed etch, and even component leads, tend to act as antennas, both receiving and radiating electrical energy. The dominant wavelength of the antenna is inversely proportional to the fundamental frequency. Therefore, as the fundamental frequency is increased, even relatively short line lengths become efficient antennas for radiating and receiving electrical noise. The noise coupling (both the magnetic field coupling and electrical field coupling) reaches an effective maximum when the wavelength is four times the line length.

In order to reduce the electrical noise generated by the data transmitter, the fundamental frequency should not exceed what is strictly required by the application. This means that the rate of signal transition (slew rate) should be controlled to reduce emitted noise. Similarly, to reduce the received electrical noise on a data bus, the frequency response of the receiver should be limited to only the frequency range of the valid data.

For any electrical data bus network, the effects of termination impedance at the nodes and at the line extremes must be considered as the signaling period becomes comparable to the propagation time of a signal across the bus. Signal reflections will be generated at any discontinuity in the bus characteristic impedance. If all reflections (primary and secondary) occur during the transition time of the signaling period, they may be neglected.

However, if significant signal changes are caused after the transition time has elapsed, these reflections reduce the margin of the signal compared to a threshold. Along with induced electrical noise and signal attenuation, reflections are a contributor towards data bit errors.

The signal distortion introduced by the electrical characteristics of the wired medium can be compensated for — at least to some extent — at either the transmitter or at the receiver. This can be used to extend the maximum distance over which data may be transmitted, and/or increase the usable signaling rate. The major signal distortion effects introduced by the bus are attenuation due to the line resistance and frequency shaping due to capacitance of the network components.

The capacitance associated with the bus network components has a wave shaping effect — wireline signal distortion — with preferential attenuation of high frequency signals. This imposes a limit on the speed of voltage transitions, effectively constraining the signaling rate. At least two methods exist to compensate for this, pre-shaping (emphasis, boosting) of the signal at the transmitter, and post-shaping (equalization) at the receiver.

Pre-shaping compensation applies to the signal a shaping function that amplifies the portion of the waveform that will be attenuated by the wireline medium. The shaping function can be designed in the time domain, boosting the signal voltage for a specified time during each transition, or can be a frequency function, amplifying the frequency components most attenuated by the transmission medium.

Pre-shaping preparation
In either case, the pre-shaping must be selected to match the characteristics of the wireline, which assumes that the designer has knowledge of the length and characteristics of the wired medium to be used for the application. Another consideration when using pre-shaping: the resulting signal may exceed the allowable signal levels of the applicable standard, limiting interchangeability with other products and applications.

The effect of pre-shaping — boosting the signal at the driver — before transmitting the signal over the data bus can be visualized. The edges of the original signal are enhanced, compensating for the expected high-frequency losses over the wireline medium. If the exact characteristics of the wireline losses were known, an exact compensation can be accomplished. Here, an approximate knowledge of the medium is assumed, and the signal that reaches the receiver is an approximation of the original signal. The amplitude of the boosted signal exceeds the original signal amplitude by over 100%, which may be incompatible with the requirement to conform to the signal levels of the applicable bus standard.

Post-shaping compensation applies a shaping function to the signal at the receiver. This acts to recover the signal as it was originally transmitted, before the effects of the wired medium.

As with pre-shaping, the post-shaping function must approximate an inverse of the frequency-dependent attenuation of the wireline. The post-shaping process is an inexact approximation. The original signal is shown, along with the degraded version, with frequency response set by the resistance and capacitance characteristics of the wireline. This degraded signal reaches the receiver, and the post-shaping function is applied to restore the original signal. If the actual characteristics of the channel are known, an exact restoration may be accomplished by applying an inverted version of the degradation function. Two restored signals A and B may have approximation errors of plus and minus 10%, respectively, of the wireline RC values. These approximations produce the recovered signals, with much of the original frequency content restored.

As with the pre-shaping method, the success of post-shaping is dependent upon accurate knowledge of the wireline characteristics. One advantage of this method, however, is that the transmitted signal is not altered on the bus; therefore no deviation from accepted standards is necessary. Another benefit is that the restoration function can be tailored at each receiver on a multi-receiver network. For widely separated nodes, this allows optimum signal restoration at each receiver; while pre-shaping must compromise in sending all receivers the same signal. Also, post-shaping at the receiver enables an adaptive shaping scheme, in which the restoration function is tuned to produce a known signal, based on the applicable bus standard.

The design and optimization of a data bus must follow the same requirements-driven methodology as any other part of the development of the system. Key concerns are the selection of an appropriate industry standard, and identification of the application requirements and constraints on data signaling rate. The number of nodes, data latency, and node bandwidth will drive these requirements. The constraints will include data integrity/allowable bit error rate, power consumption, and electromagnetic interference concerns.

Good engineering practice as far as component selection, medium specification, and proper termination will get the data bus design off to a good start. Advanced techniques, including pre-shaping at the bus driver and/or post-shaping at the bus receivers, can be considered in cases where the signaling rate and wireline length must be extended beyond typical values.