help: problems with garmin 18 5hz

Hello,
I hope someone can help me. I bought a garmin18 5Hz, but what I didn't know
is that it comes without the serial plug but with a free wire that you have
to connect to a serial plug.
So I looked for the wiring diagram and then I welded the wires to the serial
plug. The problem is that this device can't receive power supply by the
serial port because it hasn't got enough amperage; so, a friend of mine who
is quite skilled in electronics suggested me to use the usb port as power
supply, so i connected the black and the red wires with the ones of the usb.
The result is that gps 18 can't be detected by the specific software that is
provided by garmin; I tried to change the com port, the baud rate both in
the software and in the panel control of the com port, but it seems that the
pc doesn't recognize my gps receiver.
Besides, using a tester I saw that usb supplies 5,5 volts to the device
(which shoud be right) and also the data transmission wires seem to transmit
something.

Does anyone have something similar and can give me an advice on what to do?
Perhaps I am forgetting something important... I am a mechanical engineer
and so I am not very expert about electronics.
Sorry for my bad english and thanks in advance to those who will be so
gentle to answer!

Giorgo, as you staed the USB supplys you with a votage of around 5 Volt. The
Garmin 18 - 5 Hz will work on an voltage between 4.5 and 5.5 Volt.
It will by default trasmit NMEA data, but you need most likely need to
connect a nul modem connection on the serial port. I have a 18 - PC and
withjout the nulmodem it will not work properly. if you do a google search
for a - nul modem cable with loopback handshaking - I am sure you find
something

more

Giorgio wrote:



Serial ports can never be used to power anything in my experience. The
voltages are wrong and not steady, they vary with the signals.


I don't have any experience with the GPS-18, I'm just trying to help.


Do you have it wired as shown here?


http://www.garmin.com/manuals/425_TechnicalSpecification.pdf


Here are some considerations or suggestions.


The GPS 18 does not have a LED or anything that changes states to
reflect the fix/no fix status. As I understand it, it does not output
any data until it has a fix. That can take some time with new receiver
and for a first time fix.


Make sure that there is a common ground between pin 5 (Ground) on the
COM port and all the grounds or power returns (negative or (-) power)
from the GPS receiver. If the receiver has more than one ground wire
(power ground, shield, etc.) bind all of them to the negative (-) or
ground lead on the power source.


Looking at the documentation above, the GPS-18 has two grounds (black
wires) and it also shows a common connection between pin 5 (ground) on
the COM port and the other grounds. If yours is not wired that way, you
can make temporary cconnections to a common round with test leads to see
if that helps.


There can be some confusion on connecting the TXD and RXD wires from the
receiver. In a RS-232 serial port a TXD should connect to a RXD and a
RXD should connect to a TXD. On a DB9P male COM port on a PC that would be:


COM port GPS wire
pin # name
---------------------
2/RXD--------TXD
3/TXD--------RXD
5/GND--------Ground


That appears to agree with way Garmin has it described and shown in the
documentation.


But wires on the GPS can sometimes be misnamed. Sometimes when they
mark a wire TXD it means "this wire is the TXD" other times it means
"connect this wire to the TXD".


So if you've check the grounding issue, and you are sure the receiver is
outputting data but cannot see it in a terminal program (like Hyperterm
or something better), and I've already checked the grounding issue, I
switch the TXD and RXD wires. I never solder anything until I have it
working, I make temporary connections first.




Your seeing a voltage? Or something on a terminal program? I've never
used the sensor configuration software mentioned in the documentation
but it sounds like it will work and is worth trying.


If you use Hyperterm (or a better software) set to the right COM port at
4800 8 N 1, you may see something echoed from the receiver when it is
plugged into the COM port or when you tell Hyperterm to connect. If the
receiver is transmitting NMEA sentences you should see them. If you are
seeing a repeating garbled message you probably have a Baud rate mismatch.


Good luck with it, and your English is fine. Much better than my
Italian for sure.


Jack

--
Jack Erbes in Ellsworth, Maine, USA (jackerbes at adelphia dot net)
(also receiving email at jacker at midmaine dot com)

Thanks a lot to you all for the great answers, they have been all useful :)
Reading the blog of Cynicor I have noticed that the numbers of the pins
were different from mine, because I followed the wiring diagram on the
technical manual, but i lookd at the geometry of the draw and not at the
number of the pin, and on that draw the geometry is probably simmetric
compared to the pins i saw on my db9, because probably in that diagram they
look at the db9 on the side of the connection to the serial port, while i
lookd at it on the opposite site. Tonight Il'll try to solder the wires
using the numbers written on the db9 insted of following the geometry.

Thank you a lot again! You all wrote very interesting things that i didn't
know!

Looking at the specifications, the GPS18-5hz doesn't match the RS-232
standard. The lines are described as "CMOS level with RS-232 polarity"
meaning they switch between 5V and ground, whereas RS-232 switches
between 5V and -5V. You should be careful. Older RS-232 ports switched
between 12V and -12V, which would probably destroy your GPS.

Judging by other people's experience, you might be able to get away
with a direct connection. However, it would be safer to use an adapter
with the proper levels.

FTDI has a nice solution. Their TTL-232R cable provides the signal
levels you want as well as filtered 5V power. It costs $20 from
www.mauser.com. This is a USB adapter cable, so you can use it with
laptops which don't have com ports.

- John Morris

Here's a product sheet:
http://www.ftdichip.com/Products/EvaluationKits/TTL-232R.htm

For other options, look at www.sparkfun.com. They have several
converters which would also do the job.