Transponder Encoder Help

[bjb3013]

I have a new installed Garmin GTX327 and a Terra encoder. All tested OK on the bench and on the first flight. Now I have a hole in the mode c altitude at several spots. The altitude is off (not showing) from 3500 to 4000, off between 5500 and 6000, off from 7400 and 8100, and I think its off from 9100 to 10400. These are all ground checks using the displayed altitude on the 327. I think I have a loose pin but would like to narrow it down to which one before pulling everything apart. I have compared my missing altitudes with a break down of the Grey Code and can not narrow down the problem.
Any help would be appreciated.

Thanks,
BB

[flickroll]

Originally Posted by bjb3013

I have a new installed Garmin GTX327 and a Terra encoder. All tested OK on the bench and on the first flight. Now I have a hole in the mode c altitude at several spots. The altitude is off (not showing) from 3500 to 4000, off between 5500 and 6000, off from 7400 and 8100, and I think its off from 9100 to 10400. These are all ground checks using the displayed altitude on the 327. I think I have a loose pin but would like to narrow it down to which one before pulling everything apart. I have compared my missing altitudes with a break down of the Grey Code and can not narrow down the problem.
Any help would be appreciated.

Thanks,
BB

If the error with the altitude reporting is repeatable then it is likely the encoder that you have an issue with. FWIW Terra avionics have been trouble prone, although I don't know if that statement applies to their encoders as well. If you have an avionics shop near by they will be able to verify the encoder for you pretty inexpensively.

[Brantel]

This is most likely not just a one pin issue. For you to totally lose the altitude between those points many pins would have to be bad. Some of which are used at other altitudes that you say are working so something more is wrong here....

What Efis do you have if any? Most of them can output the pressure altitude to the 327 over one serial wire....

Blind encoders are not that expensive so before I would waste a bunch of time and effort on that one, I would replace it...

[az_gila]

Originally Posted by Brantel

This is most likely not just a one pin issue. For you to totally lose the altitude between those points many pins would have to be bad. Some of which are used at other altitudes that you say are working so something more is wrong here....

What Efis do you have if any? Most of them can output the pressure altitude to the 327 over one serial wire....

Blind encoders are not that expensive so before I would waste a bunch of time and effort on that one, I would replace it...

...well, old and unused but still in it's original package, Terra encoder if you want to make an offer for it...

[noelf]

...and are looking at the GTX readout. Does this mean that the encoder is completely disconnected from the txponder? If so, are you using a bit-switch box to toggle the gray code high/low signals? Is it possible that the bit-switch box is mis wired?

Have you tried this in the air from ground level to 10,000 ft with the same results as the ground test (encoder hooked up to txponder)??

I'm not sure what you mean by first flight ok, but ground checks are off.

[bjb3013]

Sorry for the confusion. Let me try to explain.

Once I found the problem, I started trouble shooting on the ground with a vac on the static system (like a transponder check) to change the altitude displayed on the GTX 327. Thats how I narrowed the dead zones down.

I have since checked the pins on the encoder and the 327 and they look good. They at least pass the wiggle check with the 327 on and the connector shells off. In the morning I will check pin for pin one more time with a meter.

Is the normal failure mode for an encoder? All the dead altitudes never change. Always 100% repeatable. Thats strange if something is slowly degrading or something is intermitent.

Thanks
BB

[noelf]

...Not that that is wrong or anything, but how do you correlate a value of vacuum with altitude and the value of gray code and the value that the txponder is displaying.

As Brantel has suggested, your description does not point to any one or two gray code bits being at fault. I do not believe there are any combinations that can result in the altitude display value just disappearing.

Do you have access to another GTX327 that you could plug into your aircraft for a quick test??

As another test, you could remove the txponder, but leave the blind encoder and the static line hooked up. Re establish a test altitude that gives a failure, and measure the voltages on ALL of the pins at the txponder's airframe connector. See if the high / low values are proper for the altitude that is being tested.

[Brantel]

I seem to remember that most encoder/transponder signal lines are active while low. If you disconnect the transponder, you might need a pull up resistor and power source in order to test the encoder output lines.

Take this with a grain of salt though because I am going off of memory and it will depend on how your encoder/transponder is made inside....

[DakotaHawk]

BB,

The Terra encoder uses "Gray Scale" binary output to convert altitude to a binary code, which is read by your GTX 327 transponder.

Looking at this chart http://www.airsport-corp.com/modecascii.txt it looks like pin "B4" is the common factor at all of these altitudes. Pin "B4" is supposed to be a "1" at all of the altitudes that you listed, and is a "0" for the altitudes that you said were OK. It looks like you're probably also not getting good output at 1300 - 2200 ft, in addition to the altitudes that you described. There are also blocks of altitudes even higher than the 10,000 that you experimented with, but this should give you something to investigate.

If you have a Dynon, MGL, or other EFIS, you can take the serial data stream as a single wire from the EFIS to the Transponder, instead of the eight(?) wires required for the Terra encoder/Gray Scale. Another bonus of the EFIS serial data stream is that there is no warm up time required to get a good altitude, and you can get rid of the altitude encoder entirely (use less space behind your panel).

[Brantel]

Some of the Alt's that B4 is used for are not reported by him as being missing....

Also he states that his transponder shows no pressure altitude at the missing alt's....just missing B4 would not cause this....