[aprssig] NMEA ISSUE HELP

[Stephen H. Smith]

On 2/5/2011 10:41 AM, Joe Della Barba wrote:
> I just got a Garmin GPS-18-C to replace my Magellan 315. It is on my boat
> connected to the laptop, VHF and TT2. This has always worked fine until now
> with the 315. The Garmin will not run the TT2 and the computer at the same
> time. If I connect the computer, it works and the VHF works, but the green
> light goes out on the TT2. Disconnect the computer and the TT2 works again.
> Very frustrating. Any ideas?????
> Thanks!
>
Once again, the TinyTracks do not have a true RS-232 interface.  They "cheat"
and use a 0/+5-volt TTL-level input/output to approximate the -8 to +8 volt
swing of a true RS-232 signal.     This works when the only load on an RS-232
source is the TinyTrack.

When several devices are connected in parallel across the GPS-18 source, the
output voltage from the GPS drops below the critical threshold the TT requires
(around +3 volts).    (The GPS-18 is a very low-power device with less ability
to source current into several loads at once than the older Magellan.)

I addressed this same issue with my GPS-18 by building a GPS data splitter
consisting of some 50-cent opto-isolators and a couple of op-amps that provides
each of 4 devices with an isolated and amplified version of the original GPS
RS-232 stream.

[ In my mobile SSTV LiveCAM APRN setup, I split the data from a GPS-18 4 ways,
to go into a laptop computer for UI-View, two TinyTracks (one on 2M and one on
30M HF), and into a video character generator that overlays live SSTV pics with
the lat/long they were sent from. ]


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Stephen H. Smith    wa8lmf (at) aol.com
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