multiple device GPS installation - HELP
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I connected a Garmin 176C GPS (that has a 4 wire connector) that I use on my
boat to a PC (sending NMEA data sentences) by connecting the NMEA signal in
and signal out wires from the GPS to two separate bus bars (one for GPS info
in and the other for GPS info out) and connecting the ground wire on the GPS
directly to the rs232 serial cable and to a ground on the boat. The
positive was connected to 12v+. When the GPS and PC are the only units that
are turned on the PC accurately reads the NMEA information from the GPS.
All okay on this end. I then connected the NMEA info in and info out wires
on a Simrad radar unit and on an Omni VHF radio to the corresponding NMEA
signals on the separate bus bars. I was following the directions given to
me when I purchased the radar and VHF radio.
So, here's what's happening. When the PC is on and clearly communicating
with the GPS, so also is the radar unit . So, I know that the wiring is
probably correct. But the VHF radio, which is also connected to the same
bus bars, receives no NMEA data from the GPS. If I turn off the PC,
however, the radar loses the NMEA data from the GPS but the VHF now reads
that data just fine and displays the location of the boat in longitude and
latitude. Both the radar and VHF have only 2 leads for the GPS signals,
although the documentation does not use the same convention(one says in and
out, the other A and B). Do I have to add some sort of switch or otherwise
isolate the VHF from the radar and or PC to get them to work at the same
time?
The following is a rather inept attempt to show how I wired all of these
units together by connecting the 4 wires from the Garmin 176C. In sum, the
PC, radar and VHF radio all work in some combination or other but not all at
the same time. However, the idea is to use the common NMEA information for
all of them without having to add a second GPS unit.
NMEA out from GPS _____/_________/___________/__________BUS 1
PC radar
VHF
NMEA in from GPS _____/__________/____________/__________BUS 2
PC radar
VHF
Ground from GPS to ground on rs232 serial connector to ground on boat
12v + from GPS
Am I overlooking something simple? Needless to say, the problem is
perplexing and any help would be greatly appreciated.
Gene S. Berkowitz
says...
Well, since each device DOES work, alternate with the PC being on/off,
I'd guess that you have a ground loop (different devices are seeing
"ground" at different potentials). A long bus could easily cause this.
As a test, try running a dedicated ground wire from each device to a
common grounding point.
You might then try an optoisolator for the RS232 connection to the
PC. By the way, is it a laptop, or a desk machine? If desk, are you
using any kind of DC->AC inverter to power it? That will mess up
the ground but good. Also, are you using the RS232 ground pin, or are
you assuming that the RS232 _shell_ is the ground (it often isn't)?
--Gene