YOn Tue, 21 Oct 2003 16:51:16 GMT, "Gabriel Latrémouille"
tempted fate with:

Has anyone connected several NMEA devices together?
Here is what I am planning.

GPS NMEA output to
1-PC-serial port for navigation software
2-radar to display next waypoint on radar

multiple NMEA capable instruments to
- PC for display within navigation software

Any comments suggestions appreciated

Gaby

Gabriel -

You've lumped several questions together. Rather than try to tell you
what to buy, let me suggest some things to think about first.

1. List all your equipment needs. For instance, you don't mention an
autopilot, which would be a critical consideration.

2. List all your display needs, for instance, do you need instrument
repeaters at the nav station? Is your GPS a chart plotter? How
important is it to have access to charts on the computer? Do you have
a flybridge?

3. If you have an autopilot, will it be driven by a chart program, by
the GPS directly, or do you want to have a choice? For instance,
after having used several PC charting packages, I would never
willingly go back to entering waypoints on the GPS. Nevertheless,
since PCs aren't that reliable compared to a good GPS, I feel it is
critical to be able to fall back to GPS waypoints if the PC fails.
Other people no doubt have other preferences.

4. Would you like to use the PC itself as a repeater display of NMEA
data? Will there be other downstream NMEA data consumers?

5. Be sure you understand the nature of the instrument interfaces you
are using. Are all the devices really using NMEA, or are there some
instruments using SeaTalk in the mix? For instance, my Raymarine
autopilot can run off NMEA or SeaTalk data, but the remote is a
SeaTalk device.

6. Are all the NMEA instruments actually following the specification
exactly? The spec is supposed to be built on RS-422, which has
several benefits. Computer serial ports use RS-232, which is similar
enough to RS-422 for the two to work together, usually. But it is a
bad idea to do this, in my opinion. You really should use a convertor
between the two types of circuits. RS-422 to RS-232 convertors aren't
all that expensive when compared to a fried motherboard on your PC.
The critical feature to look for is isolation, often referred to as
opto-isolation or galvanic isolation. A simple convertor goes from
one NMEEA circuit to one serial port. A multiplexor combines several
NMEA circuits into one serial port. If you select a multiplexor, make
sure it has the isolation feature. I believe that Meindert's have
this. Their specifications generally seem superior to other units on
the market, but I haven't actually used one. I went with a software
solution instead.

To make it more complex, not all NMEA instruments are actually using
RS-422. For instance, my Garmin handheld actually uses RS-232 and has
a typical serial port connector, so no convertor is necessary. My
depth sounder and knotlog seem to be using some sort of hybrid. My
autopilot is an honest to gosh RS-422 interface. When you look at the
wiring instructions, if you see a +, a ground, and a signal wire, you
aren't looking at RS-422. If you see a +, a ground, a signal +, and a
signal -, you have an RS-422. Again, I would never hook an RS-422 and
an RS-232 circuit together directly. Your mileage may vary.

7. To send the GPS signal both to the PC and and the radar, do you
want to split the signal before the PC, or forward the data to the
radar from the PC? Technically, you are supposed to be able to drive
up to 4 NMEA "listeners" from a single NMEA "talker" but I have never
actually done this. You can also buy an NMEA expander which is
designed to drive multiple NMEA circuits, but I think this would be
overkill. Some multiplexors have an NMEA talker port as well as a
serial port, which should work well for you. Don't forget to feed the
autopilot, though.


Just for fun, this is how my NMEA network is rigged.

My GPS (Garmin 48) is plugged directly into a serial port

My DS/KM/Wind is wired to the input side of a B&B NMEA-Serial
convertor.

My autopilot is wired to the output side of the convertor.

The convertor is attached to my laptop USP port via a serial-USB
adapter.

The data from the two serial ports is multiplexed and displayed by my
software. (You could replace this with a hardware multiplexor
attached to one of the ports). Itt displays the data on the PC screen
in nice big numbers that I can see all the way out to the cockpit.
It logs the data to disk.

That's the normal setup. When I want to use the ECS (charting
program) and the autopilot, the wiring stays the same, but the GPS is
attached directly to the ECS. The ECS passes the GPS data and the
autopilot strings it generates through a virtual serial port to my
software, which multiplexes the data with the DS/KM/wind data and
passes the autopilot commands out the USB port to the convertor and
thence to the autopilot.

If the PC was to die, I would unplug the GPS from the PC, and plug it
to the convertor using the gender changer thoughtfully secured to the
convertor.

I tell you all this not as a recommendation for you to follow, but to
illustrate the thought process behind the network configuration. I
hope I haven't confused the situation unnecessarily. It's a beautiful
thing once you get it all working. If you think it through
beforehand, you can save yourself a lot of rewiring, not to mention
you can buy the right equipment the first time.


__________________________________________________ __________
Glen "Wiley" Wilson usenet1 SPAMNIX at worldwidewiley dot com
To reply, lose the capitals and do the obvious.

Take a look at cpRepeater, my NMEA data integrator, repeater, and
logger at http://www.worldwidewiley.com/