NMEA oddities

[wiggy]

I have magelan gps with NMEA output conected to repeater in cockpit and laptop. Ive just installed DSC VHF and when its also conected to nmea the repeater looses its input signal but VHF recieves one ,this is with laptop off.
Why can GPS run 2 products (laptop and repeater) but not reapeater and VHF? is laptop still taking NMEA output although its off?
VHF is Standard Horizon and repeater is Silva.

 

[iangrant]

fire up hyperterminal - accessories, communications and point it to the port on the laptop, set 4800 BPS should see the data stream and any holes in it.

Ian

 

[qetoo]

The NMEA output from the Magelan should feed at least three listeners, provided there is no "grounding" problem. If the Magelan is like my Garmin 128, the NMEA 0183 output is not isolated from ground, i.e. the 12-volt negative. Thus the power supply for the laptop probably needs to be isolated from "ground". Mine uses a dc-dc converter so it maintains isolation. I don't know if the VHF requires an isolated input or not.

The NMEA-Out terminal of my Garmin goes to pin-2 on the laptop COM1 connector. The common terminal of the laptop, pin-5, goes to ground.

 

[wiggy]

with the laptop turned off could it still effect the NMEA of the others, it does seem that its the VHF causing the problem as all has worked fine up to now.

 

[Danny]

[ QUOTE ]
is laptop still taking NMEA output although its off?


[/ QUOTE ]If the laptop port is still connected to your NMEA bus it'll probably present about the same impedance whether it's turned off or on.

 

[wiggy]

Thanks looks like I need to do some investigating

 

[alan]

Try disconnecting the plug (serial/USB) from laptop; it could be that it is still presenting an impedance even though it is off.

Alan.

 

[qetoo]

Some laptops might echo the NMEA input (which comes in on pin-2) out on pin-5. So if you are using the laptop to echo the NMEA signal, whatever is connected to pin-5 won't receive the NMEA signal when the laptop is turned off.

 

[qetoo]

I show my NMEA connection at:

http://carlten.tripod.com/Navigation/navigation.html

 

[qetoo]

Is there any way you could show us your wiring diagram?

 

[Bilgediver]

It is not good practice to expect a nmea device to drive more than one other. ALthough in practice we do sometimes get away with this we should certainly consider using a device which is sometimes called a multiplexor to interlink two or more devices together. There are several on the market and I believe companies like Mailspeed sell them . Thes allow up to 5 or more NMEA devices to be connected including serial devices as well

I suspect your problem is due to some sort of missmatch caused by connecting 3 devices.

 

[pau1gray]

I'd only use a multiplexor to join additional strings together or if the baud rates were different. if all you want to do is increase the number of listeners you would only need an RS232 amplifier which is far cheaper than a multiplexor.

Do you have any of the Tx lines connected? Or are you only using 2 wire connection (Rx and 0V). You can generally run a number of listeners with no problems.

 

[Danny]

[ QUOTE ]
Some laptops might echo the NMEA input (which comes in on pin-2) out on pin-5.

[/ QUOTE ]Why would they do that? Pin 5 is signal ground. Was that a typo for pin 3 (xmit data)?

 

[wiggy]

Just got nmea+ and 0V conected no tx

 

[qetoo]

Yep, you're right. Sorry about the typing. Should have been pin-3, not pin-5. Pin-5 goes to ground.

Thanks.

 

[hightech]

Too much load, you need a buffer. All your connected devices will present a load to the output whether they are on or off.