Monday, 9 June 2014

GPS receiver for VHF radio

The VHF radio in the boat is a DSC radio which for the uninitiated means "digital selective calling". In layman's terms you can call up another DSC radio if in range by keying in their MMSI number (a bit like a mobile phone number for radios). One of the features of DSC radios is the ability to transmit position if connected to an appropriate GPS receiver.

I  like the idea of having an independent GPS on board in case something happens to the chart plotter. As the VHF in mounted at the chart table, it's handy having the position indicated all the time.

The radio, an Icom IC-M411, needs a NMEA 0183 feed from an external GPS receiver to function according to the manual. I found one of these on Amazon; it's an external GPS receiver that delivers the appropriate electronic string that the radio understands.

It needs a 12V feed to power it and after a bit of research to understand the wiring, it hooked up OK and bingo - position data on the VHF. 

The Icom manual gave the wiring connections as "NMEA+ & NMEA-" The GPS receiver was labelled as RS232 wiring with 6 naked wires half of which aren't required. Apparently the NMEA signal voltage levels are similar but not identical to those used is RS232 networks. The main difference being that the NMEA standard uses a fluctuating voltage from positive to negative to differentiate between signals whereas RS232 uses a positive voltage and 0V to differentiate.

The transmit (Tx) wire from the GPS gets connected to the NMEA+ and the signal ground (or shield) to the NMEA-

I routed the wire through the pushpit rail by drilling a hole at the top and bottom and using rubber grommets to protect the cable.

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