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On 2/5/2011 4:55 PM, Andrew Rich wrote:
<blockquote cite="mid:2DB452705F25499188F22BC99430C356@MacBookPro"
type="cite">If you want the GSP-18 to put out "real" RS232 you
need to invert the data and put it through some like a MAX232
chip, that goes TTL to true RS232
<br>
<br>
</blockquote>
<br>
Or you can use ordinary op-amps in non-inverting mode. Power off a
small split +/- 5 to +/-12 VDC power supply. Connect the
inverting input through to the center point of a voltage divider
make from a 100K and a 150K resistor. Connect the 100K to the
positive supply and the 120K to the negative supply. (This biases
the inverting input slightly positive with respect to the PS center
point/ground, thus making the output of the op-amp rest at -volts
with no signal on the non-inverting pin. Connect the GPS-18
signal to the non-inverting input of as many op-amps in parallel as
you need for the number of outputs. Connect the simple voltage
divider to the inverting pin of all op-amps in parallel. I use
LM124/224/324 dual op-amps for this application. About USD .50
each. <br>
<br>
Essentially, the op-amps running open-loop (no feedback) are acting
as voltage comparators. At the data rate of GPS (4800 baud) the
op-amps have no trouble reproducing a clean squarewave. The limited
slew rate and ;bandwidth is actually a bonus in this application, as
it acts as a free low-pass filter to clean high-frequency noise off
the incoming waveform. (The MAX-232 type devices have almost too
much bandwidth and will pass noise readily.)<br>
<br>
In my mobile installation, the input to the non-inverting pins
consists of a 4.7K resistor to neg power, and an NPN opto-isolator's
output to plus power. The input of the opto is driven directly by
the GPS-18 signal. This scheme can provide complete ground
isolation between the GPS and multiple loads. <br>
<br>
For the split power supplies, I use tiny low-power isolated
switching DC-DC converters made by Astrodyne. These devices are
about the size of a standard 16-pin .3-inch-wide through-hole IC,
and solder to a PC board. The particular model I usually use is
here:<br>
<br>
<a class="moz-txt-link-rfc2396E" href="http://www.astrodyne.com/ecatalog/usa/dc-converter/DUP75-12D12"><http://www.astrodyne.com/ecatalog/usa/dc-converter/DUP75-12D12></a>
<br>
<br>
Or browse the full range of offerings here:<br>
<br>
<a class="moz-txt-link-rfc2396E" href="http://www.astrodyne.com/ecatalog/usa/dc-converter"><http://www.astrodyne.com/ecatalog/usa/dc-converter></a> <br>
<br>
Some of the versions of these devices can produce a constant +/- 5
or +/- 12 VDC on an input voltage ranging from 9 to 18 VDC, making
them ideal for mobile radio applications. <br>
<br>
<br>
<hr size="2" width="100%"><br>
--<br>
<br>
Stephen H. Smith wa8lmf (at) aol.com <br>
EchoLink Node: WA8LMF or 14400 [Think bottom of the 2M
band]<br>
Skype: WA8LMF<br>
Home Page: <a class="moz-txt-link-freetext" href="http://wa8lmf.net">http://wa8lmf.net</a><br>
<br>
===== Vista & Win7 Install Issues for UI-View and Precision
Mapping =====<br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/aprs/UIview_Notes.htm#VistaWin7">http://wa8lmf.net/aprs/UIview_Notes.htm#VistaWin7</a><br>
<br>
*** HF APRS over PSK63 ***<br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/APRS_PSK63/index.htm">http://wa8lmf.net/APRS_PSK63/index.htm</a><br>
<br>
"APRS 101" Explanation of APRS Path Selection & Digipeating <br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/DigiPaths">http://wa8lmf.net/DigiPaths</a> <br>
<br>
<br>
<br>
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