Specifications

The NMEA 0183 Information sheet - Issue 3
Page 11© 2011 Active Research Limited
Connecting a computer to your
NMEA 0183 system
A very common requirement now is to connect a PC or
Laptop computer to the marine data network. As the
NMEA specication basically uses the same asynchronous
Comms specication in accordance with ANSI standard as
used on a standard PC serial port, most PC’s can read
NMEA directly through their Comms port.
Many installations have been performed where the
NMEA 0183 data signal of a talker is connected to the
computer RS232 “Rx” line, and the ground of the talker to
the Computer “Ground “ connection. Do not connect the
NMEA Talker “B” line to the PC ground.
In our experience, almost 100% of PC’s will show the data
of the NMEA link with this simple connection.
This is dangerous and most denitely not recommended,
as high voltage spikes can be present on the signal cable,
by induction from motors, alternators, radar pulses, high
power radios etc. and will damage the Comms port on a
laptop or PC that has only been designed for domestic
use. Particularly large voltage spikes can occur when
engines switch on and off - outboard motors are often very
hazardous to the quality of electrical signals on board a
boat.
Often, this connection will function for months with no
problem, then fail at the worst possible time. This has
even happened to our lab PCs in controlled conditions.
We now always use opto-isolators on our test benches
(see below).
Connecting directly to the computer RS232 port also
breaks the NMEA 0183 requirement for opto-isolation
between Talker and Listener; therefore there may be
warranty conditions to this effect on equipment sold for
marine use.
This opto-isolation requirement was created to prevent
ground loops within an NMEA 0183 system - this can be
a bigger problem than the high voltage spikes, as ground
currents can destroy a PC by allowing amps of current to
ow through the delicate printed circuit boards within.
It is also often the case that the PC is powered by an on
board mains supply, derived from an inverter. Connecting
an NMEA 0183 data signal to a PC serial port directly may
connect the ground of the inverter to the ground of the
ship’s battery. This would cause a “ground-loop” issue.
Recommend method of connection of
a PC to an NMEA 0183 data bus
To safely interface to the NMEA 0183 system on a boat, it
is best to obtain an opto-isolated NMEA to RS232 adapter
cable (see the Actisense PC-OPTO range).
Simply plug the PC-OPTO into the PC port and connect
the open end wires to the NMEA 0183 lines.
Connection of a PC with no serial port
to an NMEA 0183 data bus
There are two ways to do this - you can purchase a USB
to serial adapter, which creates a serial connection from a
USB port and connect to the PC-OPTO as above.
Better still is to use an Actisense USG USB to serial
Gateway. This creates a totally isolated RS422 driver and
receiver ready to connect safely to any NMEA 0183 data
bus
Shield
NMEA 0183 Talker
v.2.0 onwards
A
Standard
Personal
Computer
9-PIN
‘D-Type’
Serial Port
PIN 5
PIN 2
A functional but not-recommended method often used
to connection a PC or Laptop to a marine network
Shield
NMEA 0183 Talker
v.2.0 onwards
A
Standard
Personal
Computer
9-PIN
‘D-Type’
Serial Port
Recommended connection of a PC or Laptop to a talker
PC-OPTO-1
B
Shield
NMEA 0183 Talker
v.2.0 onwards
A
Connecting a PC or Laptop USB port safely to a marine network
Note the isolated output of the USG-1 allows safe driving of the marine bus from the PC
B