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

Although cutting-edge technology reaps publicity, the real world often runs on modest hardware that's just "good enough" for home automation, alarm systems, and equipment-monitoring applications. Figure 1 shows a low-speed, multidrop digital data network that uses inexpensive optoisolators, telephone jacks, and two-pair wiring.

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%.

A wall transformer/rectifier provides 12V-dc power for the isolated bus, and devices attached to the bus provide a few milliamperes of 5V power for communicating with isolated-side devices. You can implement RS-232, TTL, or inverted-TTL interfaces by configuring a single jumper and altering connections as appropriate. Figure 1 illustrates representative examples of each interface.

When adding an RS-232 device, you can ensure compliance with the RS-232 standard by using a control line, such as DTR or RTS, to provide pull-up power, and negative voltage from the TX (transmit) line to passively pull the RX (receive) line low. You can devise a suitable master/slave-communication and error-detection protocol to meet your requirements. Using separate transmitter and receiver lines helps simplify the required software. The node-point hardware easily fits inside an ordinary surface-mount telephone jack, thus facilitating quick and easy RJ-11 hookups to master and slave devices. At a 4.8-kbps or lower data rate, the bus can extend as far as 500 ft. Inexpensive dual twisted-pair telephone wire forms the bus.

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