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RS-Slide11

The term “electrically short” means that a cable’s length is short enough that it can be considered a lumped element rather than a transmission line, and thus does not require termination to prevent reflection problems. The first rule listed here is based on the idea that if the transition time is slow relative to the round-trip propagation delay down the cable, then the driver will continue to drive the cable to the final value after the reflections have died out. This typically happens within two to four round trip delays, so most sources recommend using two or three round trip delays depending on how conservative the designer wants to be. The second rule is based on the idea that a cable becomes a transmission line when its length becomes 10 to 16% of the wavelength corresponding to the signal’s transition frequency. Cables longer than 10 to 16% of the wavelength must be terminated, so they cannot be considered electrically short stubs. Obviously, calculating the allowed length using the first rule is much easier than with the second, since the latter requires converting the signal transition time to its frequency, and then calculating the wavelength. Using rough numbers of 1.5 ns/foot or 5 ns/meter however, one can quickly generate an estimate of allowed stub length using the first rule. The table on this slide gives stub length guidelines for numerous data rates based on reasonable transition times of transmitters used at those rates. Stub lengths are given using two and three round trip delays as the transition time limit. The RS-485 standard’s application guidelines define a short cable as one having a single round trip delay shorter than the signal’s fastest rise time, so customers can be as conservative or as aggressive as they like. Looking at this table the user can understand why they do not want to use a high data rate transceiver for a low data rate application: the high edge rate device is much less forgiving of longer stub lengths, and similarly, of less than ideal terminations.

PTM Published on: 2016-01-04