Specifications

The NMEA 0183 Information sheet - Issue 3
Page 5© 2011 Active Research Limited
Electrical specication
The latest specications for NMEA 0183 (version 2 and
onwards) should, as a minimum, meet the requirements
of the computer standard “RS422” (Standard EIA-422-A).
They use +5 / 0 volt signalling, which is low voltage and
easy to interface to computer equipment.
However, voltage levels present on an NMEA 0183 bus
can be much greater - up to +/- 15 volts, particularly
where older equipment is used, as the original version 1
specication used +/- 12-15 volt signalling.
Thus, all NMEA 0183 inputs conforming to v2.0 and higher
should be capable of receiving +/- 15 volt differential
signals without suffering damage.
The NMEA 0183 specication also requires that all
receiving equipment must be opto-isolated; this opto-
isolation requirement reduces the chances of interference
and removes the problem of ground loop effects.
All connections should be made using twisted pair cabling,
with a shield. To prevent ground loops, the shield should
only be connected at one end - NMEA species that the
shield should be connected to the Talker chassis.
NMEA 0183 v2.x and v3.x Talker
As specied in RS422, version 2 and newer NMEA
connections are labelled “A”, “B” and Shield. Sometimes
the signals are label Data + and Data -, these relate to
NMEA A and NMEA B respectively.
The signal states provided by the Talker are those specied
in EIA-422-A. This states that the logical “1” or STOP bit
state is dened by a negative voltage one line “A” with
respect to line “B”. A logical “0” or START bit is dened by
a positive voltage on line “A” with respect to line “B”.
A pictorial representation of an RS422 type talker can be
seen in the diagram below:
The A is the regular output from the amplier, and the B is
the “inverted” output. The Shield or ground is connected
to the talker chassis as shown.
In practice, this means line A is zero volts and line B is at
5 volts on an NMEA output when a logical “1” is present,.
For a logical “0” line A is at 5 volts and line B is at 0 volts.
These voltages would be present only when no load is
attached.
NMEA 0183 v1.x Talker
NMEA 0183 version 1 talkers used slightly different
connection methods and signal levels. Devices had just
one “NMEA” data line (‘Tx’ ‘Data’ or ‘Out’), and used the
ground as the other line - similar to a computer serial port.
This connection method is referred to as “single ended”
instead of the “differential” method used by NMEA 0183
v2.0 devices.
Version 1 talkers also had signal levels similar to that
used in the computer ‘RS232’ standard. Here, the data
signalling voltage was up to +/- 15v, and the logical “1”
or STOP bit state is dened to be within the range -15
to +0.5v. The logical “0” or START bit was dened to be
within the range +4.0v to +15.0v.
Other Talkers
Many devices are produced that are fully compliant with
the NMEA 0183 standard. Most simply label the outputs
as “DATA” or “TX”, and use the system ground line as the
“B” line, as per the version 1 standard, but often the drive
voltage is 5 volts, as used in version 2. Many of these
talkers are labelled as version 2, but in this case they are
referring to the contents of the data format, not the talker.
Another limitation of these circuits is that they often use
low power driver chips, and thus can often only talk to
one listener instead of many. This means that an “NMEA
Buffer” must be used to drive multiple listeners
NMEA Talker Limitations
NMEA Talker outputs cannot be turned off, or switched
into a receive mode, which means that the outputs are
continuously driven.
This is why only one talker can be present on a single
B
Shield
NMEA 0183 Talker
v.2.0 onwards
A
Data
Ground
NMEA 0183 Talker
before v.2.0