Phone wire, RJ-11 jacks and optocouplers build a bus


Posted on Nov 18, 2012

This version of the familiar current loop offers a simplified and somewhat novel implementation in which optocouplers serve triple duty as level converters, isolation/protection devices, and bus interfaces. Galvanic isolation avoids ground loops, increases the effective communications range, and adds a measure of protection for attached hardware. However, inexpensive optocouplers introduce delays that can cause communications timing errors. Faster optocouplers can minimize errors at the expense of component cost and overall complexity, but using conservatively sized data packets at rates of 4.8 kbps or less allows the use of less expensive components. The H11A-817D optocouplers provide 5-kV isolation and current-transfer ratios of 300 to 600%.






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