Problems with long cables and common solutions demonstrated

Bertho was inspired by a recent forum discussion about driving SPI signals over long cables, so he headed to the workshop and did some test. His epic analysis of transmission lines shows real life example of how signals degrade in cables of various lengths, and how to tackle some of the problems associated with transmission lines.

Over at Dangerous Prototypes I got involved in a forum post about problems with an SPI bus, where the signals would not function anymore after some distance. There were two things that my experience told me: 1) check the power supply, 2) check the cabling for reflection. Then I realized, this must be a common problem for all hobby hackers. Cabling is a difficult topic and it is time to demystify some magic.

This is an great writeup, don’t miss it.

Via the forum.

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  1. For square wave transmissions a rule of thumb is to terminate your cables in their characteristic impedance and ensure your cables pass at least five times the bandwidth of your transmission symbol rate.

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