Serial connections are most often
point-to-point between computers, or between computers and NMEA
0183-compatible devices. NMEA 0183 runs at the slow speed of 4800 bits
per second (4800 bps). Generally, serial cables of 10 feet or so will
run up to 56,000 bits per second (56 Kbps).
USB is a branching tree intended to
connect devices to a single computer. It comes in several
versions with increasingly higher speed. Originally intended for slow
devices like keyboard, full-speed USB 2.0 is fast enough to run
external hard disks.
While both technologies are from the
National Marine Electronics Association, they are radically different.
NMEA 0183 is a low-speed serial device oriented to having a
single "talker" and one, or sometimes more than one, "listener".
It runs at 4800 bits per second (bps), although it is often
possible to make it work with faster devices, such as AIS
. AIS normally operates at 38,400 bits per second (38.4 Kbps).
NMEA 2000, physically, is much different, being built around a backbone
cable with drops rather than point-to-point topology. It is much faster
than NMEA 0183, although still not as fast as Ethernet and proprietary
derivatives of Ethernet. Devices can draw substantial power (4 or
8 amps) from the backbone. NMEA 2000,depending on factors such as
current draw, can have a backbone that is 328 feet (100 meters) or 656
feet (200 meters) long.
Modern Ethernet (technically a family
of standards under the IEEE 802.3 label) begins with a speed of 10
million bits per secomd (Mbps), 40 times faster than NMEA 0183. 100
Mbps Ethernet is commonplace. It is possible to power devices
from Ethernet, although relatively low current and with voltages
characteristic of the telephone industry.
Ethernet was once a backbone cable technology, but now, beyond simple
point-to-point connections, it connects devices, typically with
cables up to 100 meters long, to switches; the switch is the center of
a star. Switches can be interconnected, including with optical fiber
cable immune to electrical noise and long enough to service an aircraft
carrier rather than a fishing vessel. While such speeds are
unlikely to be needed in marine electronics, variants go as fast as 10
billion bits per second (10 Gbps).
100 Mbps Ethernet can keep up with most marine displays, but that is
worth testing. There are other graphics-intensive applications, such as
movie special effects, that need the highest speeds.
Marine electronics manufacturers have a
variety of networking technologies that officially are unique to them,
but are often, although not always, based on Ethernet
Some of the earlier versions are daisy-chained.