GPS-VHF interface

[Dick Locke]

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.

My puzzlement is because the Uniden shows one wire for GPS Data In and
the Furuno is expecting two wires.

Looking at the Furuno connection diagram

The Furuno has connections labelled SD and RD that go only to the PC.
It has another connection labelled TD-C/SG which the diagram says to
connect to both the PC and the "RD-C" connector of another device like
a radar (and I presume a VHF.) I'm guessing that C/SG means
common/signal ground The fourth Furuno connection is to be connected
to the "RD-H" connector of the other device. I bet that means
Receive-High.

The Uniden connection cable has one wire for "GPS Data In." It also
has a ground wire. The RD-H connector on the GPS obviously goes to the
"GPS Data in" wire on the Uniden.

How does the signal return to the Furuno? Does it go through the -12v
wiring? Or should I connect the Uniden ground wire to the TD-C/SG
connector?

Thanks for any advice

 

[Dennis Pogson]

Link the input wire from the Furuno to your PC also to the Uniden, since it
is obviously working and transmitting NMEA sentences.

 

[Keith]

It doesn't just hook the ground wire to the 12V electrical ground.

--


Keith
__
'the' is a registered trademark of Microsoft Corporation

 

[Dick Locke]

Link the input wire from the Furuno to your PC also to the Uniden, since it
is obviously working and transmitting NMEA sentences.

In the interest of brevity, I didn't mention that the two outputs
(data 1 and data 2) are also labelled "Current Loop" and "RS-232C" so
they must have different electrical characteristics. They share a
common return terminal on the GPS.

I've sent the question to Uniden also.

 

[Dennis Pogson]

In the interest of brevity, I didn't mention that the two outputs
(data 1 and data 2) are also labelled "Current Loop" and "RS-232C" so
they must have different electrical characteristics. They share a
common return terminal on the GPS.

I've sent the question to Uniden also.

RS232 refers to a data-carrier, the term Current Loop could well refer to a
power loop, i.e. 12-volt positive. If you were to continue the data wire and
extend it from the input at the PC end, your VHF would receive the signals
without interuption from your PC.

Remove "nospam" from return address.

 

[Dick Locke]

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.

 

[Jack Painter]

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

 

[Dick Locke]

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.

My puzzlement is because the Uniden shows one wire for GPS Data In and
the Furuno is expecting two wires.

Looking at the Furuno connection diagram

The Furuno has connections labelled SD and RD that go only to the PC.
It has another connection labelled TD-C/SG which the diagram says to
connect to both the PC and the "RD-C" connector of another device like
a radar (and I presume a VHF.) I'm guessing that C/SG means
common/signal ground The fourth Furuno connection is to be connected
to the "RD-H" connector of the other device. I bet that means
Receive-High.

The Uniden connection cable has one wire for "GPS Data In." It also
has a ground wire. The RD-H connector on the GPS obviously goes to the
"GPS Data in" wire on the Uniden.

How does the signal return to the Furuno? Does it go through the -12v
wiring? Or should I connect the Uniden ground wire to the TD-C/SG
connector?

Thanks for any advice

To follow up, the right solution was to hook the "high" output from
the GPS "current loop" to the GPS input of the VHF, and not to hook up
anything else.

 

[Dick Locke]

Auch! That means that the return for the signal goes through the common
power ground. You want to make sure in this case that the main ground from
the battery goes to the VHF and from there on to the GPS. Otherwise the
voltage drop due to high current might influence the datasignal.

Makes me wonder everytime why manufacturers want to save on a $0.20
optocoupler. The NMEA standard is very clear on the REQUIREMENT for galvanic
isolation....
Too bad...

Meindert

Good idea. I still have to hook up a radar and possibly an autopilot
to the GPS too, so things will probably get more confusing. If I get
an expander, will that also avoid one device "contaminating" the
other?

The VHF is Uniden, I got it because of the WHAM wireless remote
microphone.

I guess this is why they call it a "current loop", the current makes a
tremendous loop.