[aprssig] NMEA ISSUE HELP

[Andrew Rich]

Thank you Stephen,

I own several GPS-18's and I have never done a fan out on the data

This is interesting to know.

Given that the GPS is TTL type logic, can u suggest a TTL style chip apart 
from optos, that might be able to provide say 8 streams of "repeated" NMEA 
TTL data ?

We strike this all the time at work - there is "true" RS232 and there is 
"suido" RS232

Real RS232 swings the plus and negative volts

Fake RS232 swings just high enough to trick the device into a high.

The BEST way to deal with this is a scope. That is THE only way you tell 
without a doubt.

- Andrew VK4TEC -

----- Original Message ----- 
From: "Stephen H. Smith" <wa8lmf2 at aol.com>
To: <joe at dellabarba.com>; "TAPR APRS Mailing List" <aprssig at tapr.org>
Sent: Sunday, February 06, 2011 4:53 AM
Subject: Re: [aprssig] NMEA ISSUE HELP


> 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. ]
>
>
> -------------------------------------------------------------------------------
>
> --
>
> Stephen H. Smith    wa8lmf (at) aol.com
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