Electronic Design

Full-duplex optical RS-232 connector

Situations in which the reliability of data communications can benefit from optical isolation are rather common. But the circuit described here was designed in response to an application with the unusual need to optically couple the actual serial connection itself. In the application, microprocessor-controlled apparatuses are deployed at shallow underwater sites in river estuaries and salt marshes. These rigs are designed to collect gas samples from the estuarine sediment over time intervals ranging from days to weeks. Periodically, a field worker will visit the sampling sites in a small boat and use a laptop PC to communicate commands to the sampler microcontroller memory. The problem addressed by the circuit shown here is how to provide a drown-proof means of connecting the RS-232 interfaces of the microcontroller and laptop.

Although connectors are available that can tolerate saltwater submersion while mated, this application needs one that’s compatible with lying open underwater while waiting between data collection visits. The combination of RS-232 voltage levels and water on connector pins will drive aggressive electrolytic corrosion processes capable of digesting even the best gold plating. Plus there’s the fun prospect of trying to clean and dry a wet, salty connector while bobbing around in an open boat. No such difficulties can arise, of course, if the “connector” used has neither voltages to short out nor metal pins to corrode.

This “optical connector” therefore replaces the usual serial electrical connections with LED/phototransistor electrooptical pairs. For example, data originating from the left-hand RS-232 port #1 drives IRLED D3. Positive-going “space” bits forwardbias D3, and the resulting emission causes conduction in Q3. The photocurrent is amplified by Q4 and relayed to the receive side of the righthand port #2. Saturation of the Q3/Q4 pair is prevented by clamp diodes D7 and D8. This greatly speeds up the turn-on/turn-off times of normally slow phototransistors like the PN168, and makes possible reliable 9600-baud operation. D5, Q2, Q1, etc., take over for data flow in the opposite direction.

Assembly of the connector has all active components except for Q2, Q3, D3, and D5 mounted in the electronic enclosures of their respective computers. Therefore, only the optical components themselves had to be separately waterproofed, which was accomplished using heatshrink tubing and waterproof glue. Minor modification of inexpensive AMP Inc. nylon connector bodies provided a mechanical foundation for mounting and holding the LEDs and phototransistors in adequate alignment to provide enough coupled gain—once the submerged connector is fished up and the seaweed shaken out, that is!

The optical connector should prove useful in any application in which a robust weather-and-waterproof, galvanically isolated, field-mateable connection is needed; not only submarine ones.

See associated figure

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