invested heavily in an alternative protocol.
Some feel that the expense involved is too
much of a burden, which may explain why
marine electronics equipment manufacturers
are not jumping on the NMEA 2000 band-
wagon. In fact, as of this writing, the NMEA
0183 standard is still widely used.
Further, in spite of the significant improve-
ment in data transfer rates (up to 250,000 bps
per second versus 4,800 bps for the earlier 0183
system, or at best 38,400 bps for the high-
speed version of NMEA 0183), the system still
can’t handle the system requirements of some
of the latest equipment.
It should also be noted that this 250,000
bps data transfer rate is based on a 200-
meter-long network. Theoretically a 25-meter-
long CAN bus—a more typical length for
many recreational boats—could transfer data at
as much as 1,000,000 bps per second (1 Mb),
but the NMEA 2000 standard has fixed the
value at 250,000 bps.
One example of how this thinking may be
flawed is with video data transfer. On a modern
boat, it’s quite conceivable that an owner may
want to have satellite TV broadcasts available at
the helm station, visible on the same screen on
which the radar image and chartplotter displays
appear. Another example is video monitoring
of an engine room area, a common practice on
larger yachts. NMEA 2000, which like the
SmartCraft system is based on CAN protocol,
can’t handle the data load these systems require
because it doesn’t have enough bandwidth.
Typically, vendors revert back to an
Ethernet-based addition to the NMEA 2000
system to accommodate needs of this nature.
Depending upon the version of Ethernet used,
data transfer rates can be as high as 1 gigabit
(Gb) per second. So, in the end, we still see
installations that use two or more network
protocols to accommodate the needs or desires
of today’s boatowner.
Troubleshooting
The bottom line is that in spite of efforts to
standardize data transfer, no one is there yet,
and this puts the burden on systems and
network installers to be really up to speed
with the equipment from the vendors in
question. The only way to achieve this is
through factory training, which is not some-
thing a boatowner, or even a typical marine
electrician, has access to, nor would such a
high level of product-specific knowledge
make sense (especially as it becomes obso-
lete so quickly).
In the end, about all a boatowner or
marine electrician can expect to be able to do
with any of these networked systems is to con-
firm the integrity of the electrical connections
and wiring that tie the system components
together. It doesn’t matter if the network is
wired with twisted pairs, CAT 5 cable, or a
shielded harness; they are all basically a group
of electrical conductors inside a sheath. In
other words, although components are linked
with specialized data cables made of very small
gauge wire (typically 18 AWG and smaller)
and special connectors with tiny contact
points, it is still “just wire.” And while this
very small wire is primarily carrying data, it is
often also used for low-power distribution.
So many of the troubleshooting methods
I’ve described up to now apply here as well.
We still need to confirm the following:- all the points in the circuit are properly
connected - there is no signal degradation due to EMI
or RFI - excessive voltage drop does not exist
where electric power is being distributed
through the network cabling - appropriate overcurrent protection is
provided
onboard networks 193