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NMEA Ports
The NMEA ports are the inputs/listener ports and outputs/talker ports on the MiniPlex-2 that are used to
connect to navigation instruments, chart plotters etc. There are many interpretations and variations of
NMEA ports so we’ll explain a few things first.
NMEA Signals
Although the NMEA 0183 standard specifies the signal names, voltage levels and connection methods
very clearly, the reality is far from this ideal world.
The most important property of an NMEA port is that the connections or wires are labelled A and B and
that it uses a differential signalling scheme. This means that data is transmitted on both wires, but in
“opposite direction”. Both wires are driven between 5V and 0V and opposite of each other. This means
when wire A is at 5V, then wire B is at 0V and when wire A is at 0V, then wire B is at 5V. The advantage
of this signalling scheme is that it is very insensitive to electrical interference.
NMEA A and B are often labelled as NMEA + and - respectively. When connecting devices, simply connect
NMEA A to NMEA A or NMEA + and NMEA B to NMEA B or NMEA -.
Things get complicated when manufacturers don’t follow the NMEA 0183 standard, which is very often
the case. Many devices have an NMEA interface, which is actually an RS-232 interface. The only
resemblance with the NMEA standard is the format of the data transmitted. Electrically, they are an
entirely different world. The used signal names differ wildly and often lead to confusion. When a device
has a NMEA input with connections “Data In” and “Data Return” it is often not clear whether this input is
galvanically isolated or if “Data Return” is simply another name for “Signal ground”.
Instead of being fully compatible with the NMEA standard, many devices use a single-ended signalling
scheme where data is transmitted on one signal wire while the power ground presents the return path for
data. Single ended devices often have connection names like TX and Gnd (transmit and ground) on the
NMEA output and RX and Gnd (receive and ground) on the NMEA input. Also used are Data Out, Data In
and Signal Ground. Mix these with standard NMEA connections and confusion is imminent!
In general it is easy and safe to connect a single-ended output to a differential input. How to connect a
differential output to a single ended input however is less obvious. When NMEA B of an output is
connected to the signal ground of an input, the B signal is actually short-circuited to ground. A properly
designed output can handle this abuse but it will result in fairly high currents in cables, which in turn can
lead to severe interference on SSB and VHF radios In the worst case, the output will be damaged.
To overcome this problem, the MiniPlex multiplexers provide a Com terminal on each output in addition
to the A and B terminals. This Com terminal is similar to a signal ground. To connect an output of the
MiniPlex to a differential NMEA input, use the A and B terminals. Use the A and Com terminals to connect
to a single-ended NMEA or RS-232 input.
NMEA Inputs
The MiniPlex-2 has four NMEA inputs called In1 to In4. Each input should be connected to one output
only. These inputs are galvanically isolated from the multiplexer, as specified in the NMEA 0183 standard.
The default communication speed of the NMEA inputs is 4800 Baud (NMEA standard) and it can be set to
any value from 4800 to 57600 Baud using MPX-Config. Set the speed of an input to 38400 Baud if it will
be connected to an AIS receiver or transponder.
Connect the A and B terminals of the NMEA input on the multiplexer to the A and B terminals of the
NMEA output on the instrument as shown in Figure 22 on the left. These terminals may also be labelled
as Data+/Data-, TX+/TX-, Out+/Out– or ve+/ve-.
Some instruments have single ended outputs with only one data terminal. Connect this terminal to the A
terminal of the multiplexer input, and connect the ground of the instrument to the B terminal of the
multiplexer output as shown on the right in Figure 22. The instruments data ground is often combined
with its power supply ground. In that case, connect the power ground of the instrument to the B terminal
of the multiplexer input.
