17 Instrument Networks
Sailing instrumentation is much more useful if the output from multiple measurements can be shared and combined to record and display the results using some sort of communications network. The National Marine Equipment Association (NMEA) defines proprietary network standards that are widely used in the industry. NMEA-2000 is a high speed network that is electrically compatible with the Controller Area Network (CAN) bus, standard in the automotive industry, but using different messages specifically for marine applications. NMEA-183 is an older, slower standard, electrically compatible with RS-232 serial communications and more easily interfaced to microcontroller serial ports. Although proprietary, there are many public sources describing NMEA-183 messages. As a dominant manufacturer, RayMarine uses proprietary connection hardware and similar standards in their SeaTalk (NMEA-183) and SeaTalkng (NMEA-2000) interconnection systems.
CAN Bus
CAN bus is the international automotive communications standard built into all cars since 1996. It allows bi-directional communications among multiple devices over a single cable. Fusion is a small independent company that makes a CAN bus adapter for Teensy 3.5/3.6 boards. https://www.tindie.com/products/fusion/single-can-bus-adapter-for-teensy-35-36/
“This tiny backpack includes one SN65HVD230D transceiver for connecting to virtually any CAN network. Includes 120Ohm termination resistor. The unobtrusive design allows you to easily access surrounding pins if needed.
An extra Rs pin for provides switching to high speed mode (strong pull down to GND) or low power mode (strong pull up to VCC). The board can also be used on Teensy 3.1/3.2 on pins 2,3,4 instead of 28,29,30 (see pinout), but short wires will need to be lead to 3.3V/GND pins. It can be breadboarded too.
The recommended library for this board is the FlexCan lib included with Teensyduino installation.”
NMEA-2000 and SeaTalk ng
These fast communications networks based on CAN bus are the mature standard for modern marine systems. They are beyond the scope of simple microcontroller projects, and inserting our own hardware into the network could compromise the functioning of instruments required for safety. A commercial adapter like the Actisense NGW-1 can pull out the signals we would like to use as NMEA-183 sentences, while isolating the main network from our experimenting.
N.B. Spur cables to SeaTalk1 carry only red, yellow, and shield conductors and are thinner than regular spur cables. When plugged into a standard T connector, there is no signal on the yellow ST1 conductor for either the ST1 cable or the STng cable. On the STng cable the red conductor is at 12 volts relative to the black ground cable. The blue and white conductors both average about 2.5 volts from ground. The white CAN H conductor shows pulses on the scope going about 1 volt higher, minimum pulse length about 4 microseconds or 250 kHz. The blue CAN L conductor shows