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Multiplexing with NMEA 0183 You Don’T Need State-Of-The-Art Gear to Build a Cost-Effective Onboard Network

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Multiplexing with NMEA 0183 You Don’T Need State-Of-The-Art Gear to Build a Cost-Effective Onboard Network ELECTRONICS The MiniPlex Lite multiplexer allows you to link wind instruments (above) and AIS data (right) to your computer. Multiplexing with NMEA 0183 You don’t need state-of-the-art gear to build a cost-effective onboard network. n the last three years, PS has looked autopilots, wind sensors, Automatic NMEA 0183 PRIMER Iat several ways to integrate per- Identification System (AIS) devices, NMEA 0183 is a serial protocol nor- sonal computers into our navigation etc.—still use it to communicate. New mally transmitting at 4,800 baud. For routine. In July 2006, we looked at and expensive devices meeting the comparison, this is about one-tenth of building a custom mini-computer for NMEA 2000 standard (see “Market the speed of a basic dial-up Internet onboard duties. In September 2006, Notes,” page 29) are readily available, connection. we carried out a broad review of nav but in the spirit of doing more with NMEA 0183 is based on the Elec- software with several updates in 2007 less, this article looks at ways we can tronic Industry Association’s RS-422 and 2008. get the most of our existing electronic standard, a close cousin of the RS-232 This is a fast-moving target, and equipment. standard. RS-422 uses differential several recent technological advances When it comes time to add more voltages while RS-232 is single-ended. are changing the playing field. The sensors to your navigation system This means that RS-422 is much more mainstream introduction of solid (such as an AIS unit) or integrate your resistant to electrical noise and can state memory (no more spinning hard PC into the system, making connec- transmit over much longer wire runs. drives!) in today’s netbooks and smart tions is a challenge. Few computers or NMEA 0183 components (RS-422 de- phones, for example, takes us one step chartplotters have more than one or vices) can be attached to a serial port closer to more stable onboard comput- two ports that can accept NMEA 0183 on a computer (actually an RS-232 ers. Nevertheless, we approach the topic signals. Because you can’t just solder device), but this is neither recom- of onboard computing warily. In our the NMEA 0183 signal wires together, mended nor reliable. The best practice view, a PC or Mac can be a powerful NMEA 0183 signals need to be com- is to connect a NMEA 0183 device to ally at sea, but dedicated marine-grade bined using a multiplexer. a computer’s serial port via an RS-422 electronic gear earns our higher trust. For the purposes of this article, we to RS-232 adapter. There are also many The question then becomes, how scouted basic, inexpensive NMEA 0183 NMEA 0183 multiplexers that take in can we integrate the PC and all its multiplexers with good track records. RS-422 (differential signals) and out- potential into our existing marine Ultimately, we zeroed in on the easy- put RS-232 (single-ended signals). navigation equipment? to-install MiniPlex Lite from Holland- In the last few years, an alternative More than two decades after the Na- based ShipModul. This multiplexer connection option has become avail- tional Marine Electronics Association was one of the first high-speed NMEA able: universal serial bus or USB. Soft- (NMEA) adopted the 0183 communi- 0183 multiplexers capable of handling ware drivers for USB devices emulate cation standard for marine electronic the data rates required for some of the RS-232/RS-422 serial ports, so there’s devices, most nautical devices—GPS, newer electronics like AIS receivers. no need for adapters. As far as your 28 JANUARY 2010 WWW.PRACTICAL-SAILOR.COM MARKET NOTES A pioneer in the NMEA 2000 game, Maretron (www.maretron. com) supplies a range of sensors, cabling, and displays (left) for NMEA networks. NMEA 2000 Standard Creeping into Mainstream hat about the National Marine Electronics Association’s Will NMEA 2000 finally mean an end to the proprietary Whighly touted communication standard NMEA 2000? network protocols (SimNet, SeaTalkHS, etc.) that confound the Basically a variation of the Society of Automotive Engineers mixing and matching of different brands of marine electron- Standard J1939 (itself an evolution of the Controller Area ics? Don’t bet on it. While networking NMEA 2000 devices Network protocol developed by the German corporation Rob- from different makers has become easier, all the major play- ert Bosch), NMEA 2000 is a true serial data communications ers in the recreational market—Raymarine, Navico, Furuno, network. It allows for much, much faster data transmission and Garmin—continue to release sophisticated software and speeds (250 kilobytes per second), but even this isn’t enough devices that rely on proprietary data. While some of these to handle high-bandwidth applications such as video. For capabilities are impressive, they are hardly essential to safe these, makers of marine electronics use typically use Ethernet, navigation. Do sailors really need a 3D rendering of Miami’s an open-communication standard. Government Cut? One of NMEA 2000’s most appealing elements is its true NMEA 2000 has so much going for it—plug and play, reli- plug-and-play capabilities. So long as you have the NMEA able rugged connectors and built-in redundancy that we hope 2000 cabling “backbone” (which also supplies power) on your that more sailboat makers sign on. One of the biggest draw- boat, you can use rugged watertight T-connections to add de- backs is the premium you pay for NMEA 2000 equipment. vices. These “plug-and-play” devices should be immediately Bottom line: If you are re-wiring your boat and plan to welcomed into the network. significantly upgrade your electronics, an NMEA 2000 back- Most major marine electronics manufacturers are produc- bone opens up your options and will simplify additions down ing a range of NMEA 2000-compliant devices (finally including the road. However, if you’re merely fascinated by those high the necessary wire adapters as well). Nevertheless, few build- definition chartplotters, be sure to include all the hidden costs ers are incorporating NMEA 2000 cabling into their boats, so of a full upgrade (wires and transducers are just a start) before full adoption will take years. taking the plunge. software is concerned, the USB port sions for two NMEA 0183 talkers on redundant information arriving at an appears as an RS-232 or RS-422 port. the same wire. The only way to allow input port. In Figure 1, the AIS receiver The NMEA 0183 protocol was meant multiple NMEA 0183 talkers (an AIS probably provides data at 38,400 baud, to be implemented with isolated elec- receiver and a GPS, for example) to while the other devices probably pro- trical inputs, which until a few years share the same wire is to use a multi- vide data at 4,800 baud. This means ago limited data speeds to 4,800 or plexer. Figure 1 on page 30 shows how a that both of the multiplexers’ output 9,600 baud. Opto-isolator circuits are four-port NMEA 0183 multiplexer can data streams will need to be set to at now available that easily run up to receive data from four devices (GPS, least 38,400 baud, otherwise the mul- 38,400 baud, also referred to as NMEA AIS receiver, wind sensor, and depth tiplexer will need to filter out some of 0183-HS (high-speed). NMEA 0183- sounder), and transmit the data to the information it receives. HS devices include weather and AIS three other devices (PC, chartplotter, Some NMEA multiplexers have just receivers, two of the most popular nav and autopilot.) serial (RS-422) output; others also system upgrades today. If data is arriving at each of the four provide USB output. USB drivers typi- The NMEA 0183 standard specifies input ports at speeds greater than 1,200 cally emulate a COM port, so your PC that an NMEA “talker” (a device send- baud, then the single 4,800-baud out- navigation software does not care if Photo courtesyPhoto of ShipModul ing data, such as a wind-speed sensor) put of a standard multiplexer can’t keep should be able to have enough power to up with the combined signal rates. As send data to up to four NMEA “listen- a result, information is lost. ers” (devices that receive data, such as The current NMEA 0183-HS mul- a PC, repeater display, or chartplotter). tiplexers fix this by allowing the out- NMEA 0183 does not make provi- put to run faster. They also filter out The ShipModul MiniPlex Lite has outputs for a USB cable (on left) to your onboard computer and individual wires to NMEA 0183 electronics (on right). PRACTICAL SAILOR JANUARY 2010 29 ELECTRONICS computer (if available), or use a high- speed multiplexer. FIGURE 1 To connect your AIS or GPS directly to a computer, you’ll likely need a se- rial port-to-USB adapter. Serial-to-USB adapters are available in a variety of configurations, such as different Figure 1 (above) shows the typical combinations of RS-232 and RS-422 arrangement, with four inputs inputs. Many nav-software makers of- into the NMEA 0183 multiplexer. fer them. Nobeltec (www.nobeltec.com) Figure 2 (at left) uses cascaded and Si-Tex (www.si-tex.com) provide multiplexers to connect more good support for these products. We’ve FIGURE 2 than four devices. Figure 3 used the serial-to-USB adaptors from (bottom) shows the AIS being Sealevel Systems (www.sealevel.com) used as pass-through device. with good results. Each physical input (connected via a DB9, nine-pin serial plug) is presented to the computer as a virtual serial port.
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