[Design Application]
Sorting Out Backplane Driver Alphabet Soup
A Necessary Evil, The Plethora Of Backplane Drivers Can Greatly Improve System Performance/Cost Ratios--If Applied Correctly.
Our expectations of what technology can deliver are changing at an ever-increasing rate. Consumers today expect flawless performance, higher bandwidth, and better form factor from the systems they interface with—regardless of whether the system in question is a personal notebook computer or a worldwide communications network. These expectations are forcing many designers to reconsider all areas of their system designs, including the backplanes. While new, high-end, application-specific devices are being introduced every day to help in this matter, upgrading the backplane remains a far-from-simple task.
To date, the semiconductor industry has offered a confusing array of alternative technologies, in addition to the inexpensive, garden-variety, 245 Octal bus transceivers. Each of these technologies is accompanied by an abstract multi-letter acronym, such as ABT, BTL, GTL, BLVDS (see the table). Although there are significant differences between these technologies, there exist large areas of overlap in their application. Choosing the right technology is a matter of custom fitting the capabilities of the driver to the specific needs of the system design. Loading, power, bus configuration, and speed requirements must be weighed to meet today's needs and also provide a long-term upgrade path for the system.
The engineer's definition of a backplane can be divided into two general perceptions. The first is that of a generic pc board with multiple connectors. The second is that of a special-purpose, transmission-line network that provides high data-signal integrity, control of potentially damaging crosstalk, minimization of radiated emission (EMI), distribution of data, and even distribution of power and ground references to all cards in the backplane. Both backplane definitions are correct; which one applies to your design depends on the length of the backplane, the data rate, and the signal characteristics of the selected backplane-driver technology. The most common mistake when analyzing the backplane and its cards is to consider them only as a lumped-capacitance load. Because there are two types of backplane models, a determination of which model applies to the application must be made.
The first model applies to backplanes that typically run at a lower speed, such as 5 MHz or less. At these lower speeds, the bit width is relatively long and the edge rates (transition times or rise or fall times) are slow. The key here is the rise time, as this is the main parameter that determines if a lumped-load or a transmission-line model should be used. If the edge rates are slow, then the cards inserted into the backplane may be treated as one lumped load, since the transmission-line effects (reflections) that occur will die out in a short period of time compared to the signal's pulse width (unit interval). This gives sufficient time for the signal to settle out into a stable state before sampling occurs.
A general guideline is to compare the unit interval to six flight times (or to three round trips). A flight time is the electrical length of the backplane; in other words, it is the time it takes the signal to travel from one end of the backplane to the other. A round trip is simply two flight times. Six flight times should be less than 30% of the unit interval to generate a stable state at the 50% point.
Another way to avoid transmission-line problems is through the use of specially designed trapezoidal drivers such as the DS3862 Octal bus transceiver. These drivers feature slow edge rates which are greater than the electrical length of the backplane; thus, the backplane can again be modeled as a lumped load.
A common TTL backplane driver (F245 Octal bus transceiver) driving a 21-slot unloaded backplane is shown (Fig. 1). Transmission-line problems are evident. The waveforms show overshoot, undershoot, and reflections. However, at a relatively low speed (1 MHz), these problems may be ignored because the unit interval is very large and settle out relatively quickly, but can cause other system issues such as EMI (because of overshoot and undershoot, and ringing). As the unit interval is decreased (20 MHz), the width of valid sample area also is decreased. Now the transmission-line effects take up a significant portion of the unit interval.
The second model is for higher-speed applications (>33 MHz) in which the backplane must be treated as a transmission line. If incident-wave switching is desired and the round-trip delay is greater than the edge rate (rise time) of the signal, then you have a transmission line. The simple, lumped-capacitance model no longer applies, and now a distributed model must be used. Incident-wave switching is generally desired at higher data rates.
Such switching requires a clean signal environment to allow the receivers to properly detect the correct state as the signal travels down the backplane. There is not enough time to wait for reflections to step up the voltage, as the pulse widths are very short. For these reasons, a properly terminated bus is very important, as it will prevent the generation of undesired reflections. If you are not sure which model to apply, treat the system as a transmission line.
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