NMEA Repeater

meridian

Member
I have read some of the archives on NMEA handling and been boggled by the brilliance of some contributors.

What I am wondering if there is some way to have one NMEA talker feeding more than 3 listeners, ie through a Picaxe of some description. With parallel hardwired listeners, 3 is the limit before it goes wrong. I want to be able to send GPS signals to VHF radio, radar, autopilot and laptop for navigation and AIS plotting. Thus need GPGLL, GPRMC, GPAPA and GPAPB, AIVDM at least.

paulr
S/V "Meridian of Sydney"
 

hax

New Member
Hi Paul,

You could use one MAX232 chip for each receiver.

Feed the NMEA signal into each MAX232 input (through current limiting resistor as per download circuit), and then the output of the MAX232 can be brought out into separate serial connectors.

I hope none of your "listeners" don't send any information to the GPS as well?

-Haxby
 

meridian

Member
Hi Haxby,
Thanks for the reply. No, the listeners just listen. Thanks for the suggestion, I should have a MAX232 in the parts box somewhere.

paulr
 

meridian

Member
NMEA and VHF

Hi Tom,
The radio has DSC (Digital Selective Calling) and it can transmit the current location lat/long. It can also be requested by another DSC radio. I haven't actually done it yet ...

paulr
 

Tom2000

Senior Member
That's pretty cool, Paul. Radios have obviously gotten much fancier since my sailing days!

Many thanks!

Tom
 

hax

New Member
With parallel hardwired listeners, 3 is the limit before it goes wrong.

paulr
S/V "Meridian of Sydney"


I'm surprised you are having trouble with three units. I would have thought you could connect at least 5 to 10 before the internal resistances deteriorated the signal... I bet one of your components is more greedy than the others...(lower internal resistance)
 

MPep

Senior Member
Repeaters

Hi there,

Haxby - the protocol in use is NMEA. Electrically it uses RS422 for transmission. However, as used in NMEA0183 (to be precise), you can commonly connect 2 listeners, sometimes 3, because all receivers MUST be opto-isolated. No ground-loops.

This is where the load comes from, as each input has an LED and resistor.

As a marine electronics engineer, I have come across this situation before. The only way out of this is to use a repeater box. In NZ, many engineers use the DD-20 from Pacific Micro Systems, www.pacmicro.com. I have also seen these in overseas vessels, both great and small, fishing boat and container carrier.

In NZ, DSC is not commonly used. Unfortunately, some radios that accept NMEA (usually RMC) for DSC must have a GPS connected as otherwise alarms will constantly alert you to the fact that there is no GPS information. Frustrating.

Where are you from Paul?

Regards,
Mark
 

meridian

Member
Hi Mark,
I have seen commercial units advertised on the web, but being tightfisted, I'd rather roll my own if possible.

Currently in Cairns after having sailed from Newcastle via New Caledonia, Vanuatu, Solomons and Louisiades (PNG) for the past 15 months.

paulr
 

hax

New Member
See attached. This is the recommended diagram for the MAX232 chip.


In my first post I said to use the recommended download resistor circuit to input to the chip, but I totally forgot that this would invert the signal.

A much nicer solution in any case is to use one of the RS232 inputs on the chip, and feed the inverted output to both two inputs of the same chip. (no resistors required)


So referring to the attached diagram, you should connect your GPS to pin 13.

Then connect pin 10, 11, and 12 together. As you can see this will route the 232 signal to two outputs, pins 7 and 14 which you can connect to your gear.

Leave pins 8 and 9 disconnected.

If two devices can operate off each output, then you can have a total of 5 devices connected together with just one MAX232 chip. (the fifth device can connect directly to the GPS output)

Hope this helps. (but don't blame me if you get lost in the Pacific) :)
 

Attachments

meridian

Member
Hi Haxby,
Many thanks for that cct diagram, I will give it a go as soon as I can put my hand on a MAX232 chip. We are currently in the marina and been unloading and storing "stuff" prior to painting.

Cheers
paulr
 

hax

New Member
No problems.

Have you ever had to deal with pirates? I hear "new age" pirates are becoming quite a risk these days.

I guess it depends on the size of the boat.

Cheers, Haxby
 

meridian

Member
Pirates

Hi Haxby,
Pirates are a pretty much over-rated hazard, particularly in the Pacific. It's a question that we often get asked though. Sure there are incidents around Philippines, Red Sea, Somalia, etc. The main targets are big commercial shipping which carry cash for wages.
That said, if I were travelling in those areas, I would do it in company with other yachts and preferably a patrol boat.

paulr
 

MPep

Senior Member
Another method would to use a non-inverting buffer chip.
Input should use an opto-isolator, and tie all inputs together.
All outputs would then be used per load. As each load should have an opto-isolated input, no problems should occur.

It isn't trcky stuff, just that on vessels, ground loops must be avoided, electrolysis etc.
 
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