connecting gps to serial computer cable

[thomashoebus]

I want to connect my gps to my laptop using a serial cable.
Can anyone tell me how i need to connect the output of the Gps: NMEA + data out and 0V (-ve) cable to the serial cable. So which pins to use from the serial cable.

 

[earlybird]

For a 9 pin computer serial connector,pin nos. are:- 2 receive; 3 transmit; 5 signal ground.

 

[thomashoebus]

why is there a transmit pin?
There are only 2 cables leaving from my gps.
Is this transmit pin for other equipment?

 

[chewi]

when traffic goes over the serial comms it goes in to a buffer and the receiver sends back a signal to say "enough, hang on a minute".
I have tried GPS to GPS comms eg to send waypoints from one to another. Unless you connect the other end up it will not complete its comms andf you will get only a proportion of the traffic until the buffer fills up and it will not recover and carry on...

It will also not do comms in the other direction eg from laptop to GPS for when you want to treansfer waypoints from a PC chartplotter to the GPS.


You need to connect transmit to recieve and vice versa as well as a common -ve. The transmit and recieve are typically labelled nmea + or out and nmea - or in.

 

[earlybird]

On many GPS's, it's possible to load waypoints from the laptop. Furuno is one example.

 

[thomashoebus]

correct there is a data + data - and -ve coming from my gps but i never had to use the data in before only data out (vhf autopilot, navtex), but i understand that i can upload data from the laptop to the gps that way. I will start soldering and test everything during the weekend thanks and keep you informed

 

[Danny]

[ QUOTE ]
when traffic goes over the serial comms it goes in to a buffer and the receiver sends back a signal to say "enough, hang on a minute".

[/ QUOTE ]While this is true for most serial communications protocols it is not true for NMEA. NMEA sentences are broadcast by the 'talker' to all 'listeners' and require no acknowledgements or pacing responses.

The reason for having both transmit and receive terminals on the GPS receiver is, as Earlybird said, so that waypoints and routes can be loaded into the GPS. Be aware that different manufacturers have various methods for uploading/downloading routes and waypoints.

 

[chewi]

Thanks for that danny, youre quite right as I remember it garmin/garmin comms not nmea when I had that. Either way it would be better practice to connect it so he could do just that.

btw elswhere you have replied suggesting daisy chaining nmea..
Are nmea "talkers" supposed to echo their received sentences?

I never tried that, I just paralleled them.

 

[Danny]

I think I've misled you, chewi. All I meant by daisy chaining was to connect from the output of the talker to the input of the first listener, then from the input of the first listener continue the cabling to the input of the second listener, etc. In other words it's electrically identical to a fanout of cables from the talker.

It's best to think of an NMEA network as a simplex network consisting of just one talker and one/several listeners.

So if you feed the NMEA input on your GPS from, say, your PC then this is a second NMEA network.

And if you use an NMEA buffer (some A/Ps also have this capability) to combine two NMEA talkers then the output of this is yet another NMEA simplex network.

The important thing to remember is that you can only have one talker on each network.

 

[chewi]

Thanks Danny thats what I thought, so I have a multicore from dashboard to cabin with "GPStalk" "Windtalk" and "auxtalk", power and common returns

GPStalk talks to VHF and PC
Windtalk talks to Autohelm
Autohelm can also swich to connect to GPS instead
Auxtalk takes Depth data to GPS but if I swithch off the depth I can conncet a handheld GPS in there and up/download waypoints between.

It works for me.

 

[Danny]

That sounds pretty well sorted!