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Connections

NMEA Signals

Although the NMEA 0183 standard very clearly specifies the signal names, voltage levels and

connection methods, 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 A is 5V, then B is 0V and vice versa. 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 port, which is really an RS-232 port. 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 listener port with connections ‘Data In’ and ‘Data Return’ it is not clear whether this

input is galvanically isolated or ‘Data Return’ is simply another name for ‘Signal ground’

As opposed to the NMEA standard, many devices use a single-ended signalling scheme where data

is transmitted on one signal wire while a power/signal ground presents the return path for data.

Single ended devices often have connections named as TX and Gnd (transmit and ground) on the

talker port and RX and Gnd (receive and ground) on the listener port. Also used are Data Out, Data

In and Signal Ground. Mix these with ‘standard’ NMEA connections and confusion is imminent!

In general it is safe to connect a single ended talker port to a differential listener port. Connecting

a differential talker port to a single ended listener port however is less obvious. When NMEA ‘B’ of a

talker port is connected to the signal ground of a listener port, the ‘B’ signal is effectively short-

circuited to ground. A properly designed talker port can handle this abuse but it will result in fairly

high currents in long cables, which in turn leads to severe interference on SSB radios and possibly

on VHF radios too. In the worst case, the talker port will be destroyed.

To overcome this problem, the MiniPlex multiplexers provide a Com terminal on each talker port in

addition to the ‘A’ and ‘B’ terminals. This ‘Com’ terminal is similar to a signal ground. To connect a

talker port of the MiniPlex to a differential NMEA listener port, use the ‘A’ and ‘B’ terminals. Use the

‘A’ and ‘Com’ terminals to connect to a single-ended or RS-232 listener port.

NMEA Listener Ports/Inputs

The multiplexer has four listener ports or inputs, In 1 to In 4. Each listener port should be

connected to one instrument only. These inputs are completely floating and galvanically isolated

from the multiplexer, as specified in the NMEA 0183 standard.

Connect the A and B terminals of the listener port on the multiplexer to the A and B terminals of

the talker port on the instrument. These terminals may also be labelled as Data+ and Data-, TX+

and TX- , Out+ and Out– or ve+ and ve-.

Some instruments have single ended talker ports, with only one data terminal. Connect this

terminal to the A terminal on the multiplexer, and connect the ground of the instrument to the B

terminal on the multiplexer. The instrument’s data ground is often combined with its power supply

ground.

NMEA Talker Ports/Outputs

Both talker ports or outputs can be connected to up to four instruments. Connect the A and B

terminals of the talker port on the multiplexer to the A and B terminals of the listener port(s) on

the instrument(s). These terminals may also be labelled as Data+ and Data-, RX+ and RX- ,

Out+ and Out– or ve+ and ve-.

Multiplexer

In A

In B

Instrument

Out A/+

Out B/-

Differential

Multiplexer

In A

In B

Instrument

TX/Out

Ground

Single ended

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