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“The space shuttle solid rocket boosters were designed the way they were because of a horse’s ass,” is the way the legend goes. There is actually some basis of fact in this legend. For the details, check out the article in snopes.com.
Even if only partially true, there is insight in this story that can be applied to many other cases of the origin of specs.
The story goes that the width of Roman chariots was defined in terms of the width of two horses side by side- basically the width of their asses. As the Romans conquered most of Europe and England, they brought their chariots, and their roads with them.
Over time, the wheels dug ruts. As new wagons were introduced, they were built with the same axle spacing because this is what the infrastructure supported and to be backward compatible with the existing ruts. After all, there is still a fundamental limit to how close you can get a couple of horses, so this axle size is not so unreasonable.
When the first railroads were built, the cars used were based on current carriage technology, which had an axle spacing of 4 ft, 8.5 inches. The early trains were even called “iron horses.” Once established, this standard proliferated and the railroad car axle and railroad tracks were standardized at 4 ft, 8.5 inches.
When Thiokol was designing the diameter of the solid fuel booster, so the legend goes, they had to limit its OD based on fitting on a railroad car, which is limited by the axle pitch, which is limited by a pair of horses asses.
Is 4 ft, 8.5 inches the ideal axle span spec? I have no idea. This spec is used to be backward compatible with the existing infrastructure. Unless you want to start from scratch, all rail car axles need to be this length. If there is no compelling reason otherwise, this spec may be acceptable.
When considering a new product, you should always ask two important questions: is it important to be backward compatible, and is there a compelling reason to consider alternative values, as in a better cost-performance balance?
There are two specs in the circuit board industry where it is important to consider these questions; board thickness and differential impedance.
I recently read a really interesting piece Lee Ritchey wrote in Circuitree Magazine about the origin of the board thickness spec of 063 mils.
He points out that the original “circuit boards” were made of plywood and were designed to provide a mechanical support for vacuum tubes and other large components. As the leads were short, the thinnest plywood was used, 1/16th of an inch, which is 063 mils. Plywood evolved into Bakelite, which evolved into fiber glass. The same thickness of 063 mils was used in each generation because that was what was used before. As edge card connectors were introduced, they were design for 063 which locked the standard in for compatibility.
With the introduction of many layers, they all would not fit in 063 mils so a new standard was introduced at 1.5 x 063 or 93 mils.
He rightly asks, is there a performance reason for 063? If you don’t need the thickness for mechanical support or edge connector compatibility, is it still the right thickness to use in your application? Not if it costs more.
The same question should be asked about 50 ohms single-ended or 100 Ohm differential impedance. As I wrote about recently, 50 Ohms had its origin 70 years ago as a way of minimizing attenuation in coax cables for radar and radio applications.
If your application is high speed serial links, you still have a need for optimized design for low loss, but in PCB geometries, 100 Ohms is not the optimum value. As we describe in some of our classes, lower impedance has lower loss. Intel has recommended 85 Ohm differential impedance for PCI gen II operating at 5 Gbps. It has lower loss and is a better match to the typically 70 Ohm differential impedance of through vias in a thick circuit board.
Many connector companies, such as Molex in their Impact series, are offering connector options at 85 Ohms. It will become the new standard.
This may not be the right answer for every design, as the lower differential impedance may mean higher power consumption. All of life is a trade-off and that’s why engineers need to be empowered to make their own decisions about their specific, custom products.
Sometimes it is useful to take a step back and look at why the specs we use are the value they are, and if there is a compelling reason, change them to help find a better cost-performance balance.