"Dick Locke" wrote in message
...
On Sat, 1 May 2004 08:22:44 +0200, "Meindert Sprang"
wrote:

"Dick Locke" wrote in message
.. .
Hi, I have a Furuno GPS-32 that has been successfully interfaced to a
PC for navigation for quite some time. Now I want to hook up the GPS
output to a Uniden Polaris VHF.

Dick,

The TD-C/SG is indeed the common return for both outputs. The difference
between both outputs is that Data 2 (SD/RD/SG) is at RS-232 levels and
Data1
(TD-H/TD-C) is a current loop output, meant to drive an input with an
optocoupler for galvanic isolation. This output can drive more current
than
the RS-232 can. If the VHF only has one signal in, connect this to TD-H
and
connect the ground of the VHF to SG.

Meindert

Thanks Meindert and Keith.

I'm getting slightly conflicting advice on this as to whether to
connect the ground wire from the VHF to the signal ground of the GPS
or to the chassis ground of the GPS. I suspect the ground wire from
the VHF is boat ground, because it's bare wire. (I'll confirm that
when I get to the boat.) Also because the VHF accessory cable has a
+13.8 v wire and the manual states it and the ground are for a GPS
Antenna.

I'm thinking ahead to other interfaces to the GPS. I have a manual for
the radar here and it's expecting a GPS signal to come in on two
wires, TD/RD1-H and TD/RD1-C. If the VHF wire is boat ground, and I
parallel the radar with the VHF, I will be taking the TD/RD1-C output
down to boat ground. Is that OK? I've heard the term "ground loop"
enough to be wary.

Besides the VHF and the radar, I have an autopilot and a satellite
e-mail device to interface to the GPS. The dealer tells me a GPS can
drive four devices without an NMEA expander. Does that sound right?

While an expander won't help my confusion about the one-wire VHF and
two-wire GPS, if it has isolated outputs that are not grounded it will
avoid this concern about bad interaction between the VHF and the other
devices.

Be very wary!

A VHF radio installed on a boat requires no coax-shield ground, which is the
only thing a "bare wire" from a radio could be for. The VHF radio ground is
direct path to the battery grounding block, and it would most certainly be
insulated (black). VHF marine radios are designed to be isolated from any
and all grounding except that of the DC grounding system of the boat. You
never separately ground the cases of equipment that are DC powered.Consult
only the manufaturers installation plans for interconnected equipments!

Jack
Virginia Beach, VA