In the very next post, I will explain in detail the implementation of a multiplexer (¹) Miniplex-2USB ShipModul's website. But before removing the electrical panel and pull out the tool box, it is better to understand the subtleties of a connection which, at first, seems very simple, and yet ... can quickly become a real headache.
The standard NMEA 0183 specifies that the input and output ports of transmitters and receivers should be Differential. This means that data is transported using an electric current on two wires, separated from the ground. The voltage oscillates between 0 and 5 volts, and both wires are in opposite phase. When one is at 5V, the other is 0V, and vice versa.
Such a system has two advantages :
- Less susceptible to interference : a differential receiver only detects the voltage difference between the two conductors, but not the absolute value of the voltage. Interference did not influence this measure.
- Less interference generator : a differential system produces little interference on systems with high frequency. The magnetic fields generated by each wire are in opposite phase and do not affect other drivers nearby.
All this would be great if all manufacturers adhered to this standard. Well, isn't it !
To save a few bucks, some of them prefer to rely on the quality of the conductors or even the presence of opto-couplers recommended for galvanic insulation of electronic equipment (²).
Thus, their instruments are simply equipped with NMEA inputs and outputs Asymmetric, composed of a single conductor data, where the return signal is transported on the common negative connection (ground). Technically, the difference lies in the current return, common to all instruments and supply, with the risk of severe voltage fluctuations that entails, and hence disruption of transported data.
The boater, his screwdriver in his hand, will then meet with four possible connections to be made with its own instruments :
- Differential to Differential
This is the normal configuration of standard NMEA. Valid in both directions : talker to listener and vice versa.
- Single-ended to Single-ended
This is the worst configuration, negatives are connected, but relies to common supply ground.
- Sigle-ended to Differential
In this configuration, It is important not to link the return of the B receiver to a power ground close to the multiplexer, but directly to the ground connector of the talker, itself connected to its own ground. This prevents any supply currents to disturb the data.
- Differential to Single-ended
Only data connectors are connected (A to A or RX). Connector B return is free. The return through the ground of both listener and talker. The wire must be the shortest and most straight on. Above all, do not connect the talker return connector (B) to ground, it would be short-circuited.
- Differential (Multiplexer) to multiple listeners
If multiple receivers are connected to a talker, as for a multiplexer NMEA OUT output, must establish connections for different types of instruments, following the diagram above. Different combinations can coexist.
Once well understood and integrated these schemes, you can boldly take your screwdriver, NMEA will have no secrets for you.
Sources of text and illustrations : ShipModul website. Translated and freely adapted.
(¹) For beginners in NMEA, start by reading the document NMEA Multiplexers.
(²) A opto-coupler is a device that carries information by means of light instead of electricity, preventing parasitic currents to flow between the instruments and the multiplexer. These currents can damage equipment or interfere with radio signals, galvanic isolation of input is required by the NMEA 0183.