GI 275 - How to wire the external GPSS/HDG- switch

[NicoN]

I want to have the external switch to toggle between GPSS and HDG.

I found all options hpw to configure.

 

For me the question is: On which device do I have to connect the switch to one of the Discrete inputs?

PFD ? sounds logic as the toggling happens here and the PFD has the AP interface

I think it is a switch between Discrete in and GND

[jetdriven]

If it's discrete low, then grounding it triggers the discrete function

[NicoN]

Thank you. My posting was not precise enough.

The "pull to ground" is a typical feature of all microcontrollers. INternal pullup drwas the pin to "high". If you short it to GND - fine.

Both Gi 275 have some free Discrete IN- pins free for use. So, the question is - on which device do I use/wire the switch?

On the PFD? Or on the HSI?

 

[PT20J]

I'd try positing on BeechTalk. Several avionics installers hang out there. Terry Markovich would probably know.

[PeteMc]

Is it just a "switch" you need to install????  I'd think there'd also be data to go back and forth.  

 

[NicoN]

Oh yes!

GI 275 and GNS430W are playing in a team.

This works already.

At the moment we are using the touch button on the PFD to switch between HDG mode and GPSS.

But you can also put this function to an external switch.

[NicoN]

Okay, mission accomplished.

Even with a newer firmware, switching between HDG and GPSS mode is not perfect. Using an external switch is way better. But the necessary details are not so easy to get.

Her is what I did.

1. The switch needs to be wired to  the PFD (the GI275 with ADRHS and Ap interface), not the HSI; which is nice because it is the top device

1a It needs to be a switch, not a pushbutton. The swithc connects to GND in GPSS mode, HDG mode is done when teh switch is open with a pullup-resistor. You do not need an external one.

1b I suggest, install a DPDT version, just to have something ready to drive a indicator lamp; the GI 275 has AFAIk no pin to drive a indicator lamp.

2. Find a discrete-IN pin which is not in use.

2a The discrete pins are spread over the J2751 and J2752 connector.

2b I recommend preferring the J2751 connector over the other. J2751 is the upper connector and therefore easier to access.

2c in my installation the discrete-IN on pin 62 and 63 were not in use, which makes it easier. Pins 60 to 78 are located in the upmost row and pin 60 is the rightmost (easy for people like me who have problems with numbers greater five  )

3 one pin from your switch needs to be installed in location 62 or 63.

3a make sure the pin is inserted all the way (it should not be shorter than the others)

4 the other wire from your switch can be connected to the backshell from your connector or to one of the "Aircraft ground"-pins not in use. But this is way more complicated (number greater five, you know !)

5 Next start your PFD in config mode and go down to DIAGNOSTICS - DISCRETE IN

5a There you should see something happening when flipping the switch

6 NExt go to your autopilot settings (it is way down in the INTERFACES menu) and 

6a select DISCRETE instead of BUTTON

6b select the appropriate DISCRETE IN pin

7 leave the menus and power down both GI 275

7a wait 30s without power

8 power up in normal mode

9 You should see your heading bug changing from full to hollow if you flip your switch

10 install the switch.

10a I recommend to install it with the plunger to the left ( to the PFD) indicating HDG mode on and right (to the GNS430 or so) indicating GPSS.

 

Have fun; maybe someone finds that helpful

 

A word to the HD-pins for the 78 pin D-SUB:

I was not able to find the turned pins which need this crazy expensive crimping tool. Instead, I found only those stamped ones .

They can be crimped with the cheap crimping  tools offering an option fro D-Sub. But it takes a good portion of experience before you make acceptable pins which can be inserted correctly.

So, under all circumstances, use the turned ones!!!!

 

[PT20J]

I believe Honeywell makes alternate action lighted square pushbuttons. Should be lots of sources for the machined crimp contacts. Aircraft Spruce lists them.

EDIT: Steinair has the pins and a crimper less costly than the DMC. 

https://www.steinair.com/product/contact-high-density-dsub-pin/

Edited September 2, 2024 by PT20J

[Fly Boomer]

On 9/1/2024 at 4:56 AM, NicoN said:

Her is what I did.

Someone will be delighted to read these details.  Thanks!