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GPS 175 Wiring Question

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farmrjohn

Joined: 31 Dec 2018
Posts: 57

Posted: Thu Jan 09, 2025 4:23 pm Post subject: GPS 175 Wiring Question

I'm in the process of making the wiring harness for a GPS 175 installation and would like to have the automatic power on feature.  I am confused by the information in the installation manual though and have a question into Garmin.  While waiting for a response I thought I'd ask here as well.



On the pin-out diagram and the table referring to that pin, #61, it is listed as an "in".  However, on the one wiring diagram that shows pin 61 it is connected to the aircraft ground and is diagramed as an "out".  The footnote associated refers back the previous section that has it as an "in".  I've attached the portions from the installation manual in question.



Thanks, John

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david(at)carter.net
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Posted: Thu Jan 09, 2025 6:11 pm Post subject: GPS 175 Wiring Question

This is very straightforward as documented. If pin 61 is connected to ground, auto power on is enabled and the unit will startup when power is provided on  21 & 42. This is sensing a logic level 0.  

If 61 is not connected to ground (floating), auto power on is disabled & you have to power on the unit manually. 





---

David Carter

david(at)carter.net (david(at)carter.net)



On Thu, Jan 9, 2025 at 7:24 PM farmrjohn <faithvineyard(at)yahoo.com (faithvineyard(at)yahoo.com)> wrote:



 	  Quote: 	
		  --> AeroElectric-List message posted by: "farmrjohn" <faithvineyard(at)yahoo.com (faithvineyard(at)yahoo.com)>

 

 I'm in the process of making the wiring harness for a GPS 175 installation and would like to have the automatic power on feature.  I am confused by the information in the installation manual though and have a question into Garmin.  While waiting for a response I thought I'd ask here as well.

 

 On the pin-out diagram and the table referring to that pin, #61, it is listed as an "in".  However, on the one wiring diagram that shows pin 61 it is connected to the aircraft ground and is diagramed as an "out".  The footnote associated refers back the previous section that has it as an "in".  I've attached the portions from the installation manual in question.

 

 Thanks, John

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=515201#515201

 

 

 

 

 Attachments: 

 

 http://forums.matronics.com//files/gps_wiring_199.pdf

 

 

 

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farmrjohn

Joined: 31 Dec 2018
Posts: 57

Posted: Thu Jan 09, 2025 7:38 pm Post subject: Re: GPS 175 Wiring Question

Thank you.  I guess my confusion comes from the terminology used on the pin-out diagram and description table where the pin is labeled "in".  



An additional confusion point is the GTX 327 transponder wiring that is installed.  In that instance there are only 5 wires in the harness, but not connected as in the wiring diagram, attached as PDF  (transponder works).  As wired, there is an airframe ground connected to pin 1, and a power in through a circuit breaker to pin 15.  The other three wires go to a Trans-Cal altitude encoder: pin 13 to encoder ground, 14 to encoder power, and 19 to encoder's RS-232 transmit.  The encoder itself has a total of four wires, the fourth goes to airplane ground.  



Reading note 5, it sounds like transponder pin 1 should be connected to pin 15 for the auto-on to function.  As wired, it does not.  Is the transponder receiving its ground through the encoder or pin 1 or through the encoder?  If thought pin 1, should the auto-on be functional?



Thanks again,

John 



 	  david(at)carter.net wrote: 	
		  This is very straightforward as documented. If pin 61 is connected to ground, auto power on is enabled and the unit will startup when power is provided on  21 & 42. This is sensing a logic level 0.  

If 61 is not connected to ground (floating), auto power on is disabled & you have to power on the unit manually. 





---

David Carter

david(at)carter.net (david(at)carter.net)



On Thu, Jan 9, 2025 at 7:24 PM farmrjohn <faithvineyard> wrote:



 	  Quote: 	
		  --> AeroElectric-List message posted by: "farmrjohn" <faithvineyard>

 

 I'm in the process of making the wiring harness for a GPS 175 installation and would like to have the automatic power on feature.  I am confused by the information in the installation manual though and have a question into Garmin.  While waiting for a response I thought I'd ask here as well.

 

 On the pin-out diagram and the table referring to that pin, #61, it is listed as an "in".  However, on the one wiring diagram that shows pin 61 it is connected to the aircraft ground and is diagramed as an "out".  The footnote associated refers back the previous section that has it as an "in".  I've attached the portions from the installation manual in question.

 

 Thanks, John

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=515201#515201

 

 

 

 

 Attachments: 

 

 http://forums.matronics.com//files/gps_wiring_199.pdf

 

 

 

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david(at)carter.net
Guest

Posted: Fri Jan 10, 2025 1:42 am Post subject: GPS 175 Wiring Question

I no longer have the 327 in my panel, and don't recall how the previous owner had wired it. From the diagrams, I think you are seeing a difference in approach to logic signaling (auto power on enabled or not) over the course of 17 years of technology evolution (2002 vs 2019). The 327 used an active high (12v) signal on pin 1, the newer gear uses an active low signal on its respective pin. This seems safer to me - one less wire carrying 12v that could short, especially for a non-critical convenience feature like auto on. 

As wired, I would not think your 327 would come on automatically. Wiring it to ground is like explicitly disabling auto on (versus leaving it floating). It's probably redundant to an internal pull-down that defines the default behavior if nothing is wired to it. 

Not having a ground wired directly to the unit ground pin is certainly not best practice. I'd closely inspect any wiring done by whoever wired this unit. ---

David Carter

david(at)carter.net (david(at)carter.net)



On Thu, Jan 9, 2025 at 10:39 PM farmrjohn <faithvineyard(at)yahoo.com (faithvineyard(at)yahoo.com)> wrote:



 	  Quote: 	
		  --> AeroElectric-List message posted by: "farmrjohn" <faithvineyard(at)yahoo.com (faithvineyard(at)yahoo.com)>

 

 Thank you.  I guess my confusion comes from the terminology used on the pin-out diagram and description table where the pin is labeled "in".  

 

 An additional confusion point is the GTX 327 transponder wiring that is installed.  In that instance there are only 5 wires in the harness, but not connected as in the wiring diagram, attached as PDF  (transponder works).  As wired, there is an airframe ground connected to pin 1, and a power in through a circuit breaker to pin 15.  The other three wires go to a Trans-Cal altitude encoder: pin 13 to encoder ground, 14 to encoder power, and 19 to encoder's RS-232 transmit.  The encoder itself has a total of four wires, the fourth goes to airplane ground.  

 

 Reading note 5, it sounds like transponder pin 1 should be connected to pin 15 for the auto-on to function.  As wired, it does not.  Is the transponder receiving its ground through the encoder or pin 1 or through the encoder?  If thought pin 1, should the auto-on be functional?

 

 Thanks again,

 John 

 

 

 david(at)carter.net wrote:

 > This is very straightforward as documented. If pin 61 is connected to ground, auto power on is enabled and the unit will startup when power is provided on  21 & 42. This is sensing a logic level 0.  

 > 

 > 

 > If 61 is not connected to ground (floating), auto power on is disabled & you have to power on the unit manually. 

 > 

 > 

 > 

 > 

 > ---

 > David Carter

 > david(at)carter.net (david(at)carter.net) (david(at)carter.net (david(at)carter.net))

 > 

 > 

 > 

 > 

 > 

 > On Thu, Jan 9, 2025 at 7:24 PM farmrjohn  wrote:

 > 

 > 

 > > --> AeroElectric-List message posted by: "farmrjohn" 

 > >  

 > >  I'm in the process of making the wiring harness for a GPS 175 installation and would like to have the automatic power on feature.  I am confused by the information in the installation manual though and have a question into Garmin.  While waiting for a response I thought I'd ask here as well.

 > >  

 > >  On the pin-out diagram and the table referring to that pin, #61, it is listed as an "in".  However, on the one wiring diagram that shows pin 61 it is connected to the aircraft ground and is diagramed as an "out".  The footnote associated refers back the previous section that has it as an "in".  I've attached the portions from the installation manual in question.

 > >  

 > >  Thanks, John

 > >  

 > >  

 > >  

 > >  

 > >  Read this topic online here:

 > >  

 > >  http://forums.matronics.com/viewtopic.php?p=515201#515201 (http://forums.matronics.com/viewtopic.php?p=515201#515201)

 > >  

 > >  

 > >  

 > >  

 > >  Attachments: 

 > >  

 > >  http://forums.matronics.com//files/gps_wiring_199.pdf (http://forums.matronics.com//files/gps_wiring_199.pdf)

 > >  

 > >  

 > >  

 > >  ===========

 > >   -

 > >  Electric-List" rel="noreferrer" target="_blank">http://www.matronics.com/Navigator?AeroElectric-List

 > >  ===========

 > >   FORUMS -

 > >  eferrer" target="_blank">http://forums.matronics.com

 > >  ===========

 > >  WIKI -

 > >  errer" target="_blank">http://wiki.matronics.com

 > >  ===========

 > >  b Site -

 > >            -Matt Dralle, List Admin.

 > >  ="noreferrer" target="_blank">https://matronics.com/contribution

 > >  ===========

 > >  

 > >  

 > >  

 > >  

 > 

 

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=515203#515203

 

 

 

 

 Attachments: 

 

 http://forums.matronics.com//files/xponder_pins_114.pdf

 http://forums.matronics.com//files/xponder_diagram_157.pdf

 

 

 

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