Pull-up circuit for altitude encoder tester needed.

[Earls61]

I am building a altitude encoder tester. Altitude encoders are
installed in aircraft and work in conjunction with the aircraft's ATC
transponder to report an aircraft's pressure altitude via ATC's radar.
(ATC = Air Traffic control)The basis of my tester is this:

http://www.shadin.com/products/alt_mgt/ams-2000/index.html

I came across one for free! My problem is this. The encoder is normally
connected to the transponder in the aircraft, which provides a hi
voltage to the encoder's relevant pins. At specific altitudes, the
encoder provides a ground to selected pins at the transponder to
trigger a corresponding altitude code in the transponder's reply
signal. (known as greyscale or Gillham code)

My tester must operate without the transponder providing the +12 volts
to the selected pins. the display on the AMS-2000 jumps around because
of float. (nothing hooked up to the pins).
Can a simply connect a +12V supply trhough a resistor to each pin for
this? will the encoder still pull it to ground? If so, what value of
resistor? Thanks for any help with this.

Earl

 

[John Fields]

I am building a altitude encoder tester. Altitude encoders are
installed in aircraft and work in conjunction with the aircraft's ATC
transponder to report an aircraft's pressure altitude via ATC's radar.
(ATC = Air Traffic control)The basis of my tester is this:

http://www.shadin.com/products/alt_mgt/ams-2000/index.html

I came across one for free! My problem is this. The encoder is normally
connected to the transponder in the aircraft, which provides a hi
voltage to the encoder's relevant pins. At specific altitudes, the
encoder provides a ground to selected pins at the transponder to
trigger a corresponding altitude code in the transponder's reply
signal. (known as greyscale or Gillham code)

My tester must operate without the transponder providing the +12 volts
to the selected pins. the display on the AMS-2000 jumps around because
of float. (nothing hooked up to the pins).
Can a simply connect a +12V supply trhough a resistor to each pin for
this?

 

[RST Engineering \(jw\)]

www.rstengineering.com and then to the bibliography for the altitude encoder
tester article I wrote for Kitplanes about five or six years ago.

Jim

 

[RST Engineering \(jw\)]

I have a difficult time believing that Shadin didn't put pullups in their
product, but perhaps they did not.

Jim

 

[TaxSrv]

...
Can a simply connect a +12V supply trhough a resistor
to each pin for this? will the encoder still pull it to ground?
If so, what value of resistor? Thanks for any help with this.

Transponder mfrs use various, including the voltage "to which pulled
up." 10Kohm pullup should be just fine.

Fred F.

 

[Earls61]

I have a difficult time believing that Shadin didn't put pullups in their

product, but perhaps they did not.

Perhaps. I didn't call them or anything. I lost confidence in Shadin
because they coudn't repair this unit after I removed it from the
airplane and sent it to them. I got it back and fixed it myself. (bad
solder joint at the display connector)

When I power up the unit, with open pins, the display reads random
numbers. Grounding them quiets things down. (and changes the altitude)
I haven't checked it while connected to a xpdr. I think I will try the
10k resister idea next.

Thanks for the link to your website. Very interesting reading. I
thought I'd make one using the shadin unit since it was free and it
also reads out serial altitude from an encoder so equipped.

Earl