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G3X non-touch GSU73 discrete active high not available?

pecanflyboy

Well Known Member
G3X non-touch
GSU 73
G3x Installation Manual Rev. AM, Page 26-71
DISCRETE IN* 1 J731 Pin 9

I'm configuring a DISCRETE IN* 1 to give me a notification when the smoke pump is turned on (similar to an electric boost pump). I've taken the GSU 73 J731 Pin 9 to the side of the relay that is energized with 12Vdc when the pump switch is turned on. I've configured the DISCRETE IN* 1 to Active High.

DISCRETE IN* 1 always shows active and the message is displayed, regardless of power to the pin. I’ve pulled the circuit breaker and checked that no power is going to Pin 9, and it still shows Active. In fact, Pin 9 is being powered with 12Vdc from the GSU 73. This is problematic as it would be putting 12Vdc to the pump with the switch off.

As I read the installation manual closer, the asterisk on page 26-71 says, *Indicates Active Low. This asterisk carries over the interconnect drawings. Does this mean that the DISCRETE IN’s are only usable in Active Low configuration? It seems counter to the explanation on page 26-71 on parameters for Active High.

Does the GSU 73 allow ACTIVE LOW and ACTIVE HIGH for the DISCRETES? If so, why is my ACTIVE HIGH DISCRETE IN *1 always active?
 
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Hopefully I don’t steer you wrong on this but I’ve dealt with the same thing on 3 different G3X installs. I’m no electrical engineer so some of my terms may be slightly off but should steer you in the right direction. The problem with the active high is that often times there’s a floating voltage on the line running to the GEA And it always think that the pin is powered. The solution is to run a pull down (don’t remember what I used but 10k rings a bell for some reason, someone much smarter than me can probably chime in) resistor from the power line to ground. There’s two ways to do it both virtually the same. In this example we’ll say the light or CAS message is “BOOST PUMP ON”. Options 1- somewhere on the power wire to the pump Splice in the resistor and run it to ground. Option 2- At the back shell splice in to the active high input wire (similar to a CANBUS splice) with the resistor And run it to ground. Either to the back shell or to an unused ground. I hope this points you in the right direction. Biggest takeaway for me on this subject is NEVER use an active high input if you can trigger the input with a LOW, but many times that’s not possible. I had to do this on five or six discretes in the last three installs I’ve done. Good luck And PM me if I can explain further in the caveman electrical terms I know.

Disclaimer: mine was on a GEA24 but I imagine if active highs are available on the GSU73 it should work the same

Kirk
 
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Kirk,
We both understand the same level of electric engineering, and I think you are spot on. The floating voltage from the GSU73 pin needs to be kicked down below the ACTIVE HIGH trigger voltage with a resistor. I will install this resistor from the switched power location to ground. Without switched power, the floating voltage will be well below the trigger. When switched on, the voltage will be above the trigger. When I get a response from Garmin, I will confirm this plan and post the suggested resistor value.

J
 
I spoke with Garmin, and they confirmed the need for a resistor. They said I should use a 5 kOhm resistor. I had a 5.6 kOhm which works well. It kicks the voltage down to around 4.5 Vdc. I tried a 10kOhm resistor, and it did not switch the pin on and off.

My only concern is the 4.5V still on the power post of the pump. Will this cause an issues with the pump trying to run at low voltage?
 
Nope you’ll be fine. There is no current there just a few milliamps. Probably not even enough to light up an LED.
Kirk
 
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