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Mode 1 Transponder Min/Max Values


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I was wondering about the min and max values of the two digits of the Huey's Mode 1 transponder.

 

UH-1H-Mode-1.jpg#

 

The first digit can go from 0-3, and the second digit can go from 0-7; the above screenshot shows the maximum value of 37.

 

However, in other modules (A-10C, F/A-18C) the value range for the first digit is 0-7 and for the second digit it's 0-3 - basically the digit ranges are swapped around in comparison to the Huey, with their max value being 73 (instead of 37 for the Huey).

 

Is that just the way it is with the transponders in these different aircraft, or could it be that the Huey just has the min and max values the wrong way around on the panel?

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Great info, thanks!

 

In the meantime I found this video that seems to support the mixing of the values in the Huey:

 

 

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These are all "octal" meaning 8 values, 0-7. APX-72 is 32 possible codes M1 (4x8, full octal times half an octal) while M3/M2 is 4096 8^4 (four octal). The Mark X standard apparently only had 32 codes while the modern stuff (Mark XII or newer?) has M1 64 codes. At some point they expanded the M1 codes in aircraft. Probably as the tech got better there was no savings in design or something. That's interesting. I wonder why they limited the M1 by a factor of 2 before. The idea was M1 encodes airplane-mission type and they didn't think there were more than 32 types of missions?


Edited by Frederf
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I noticed the 3-7 vs. 7-3. It makes sense to a programmer counting in octal that it would be the first one not the second (zero to thirty-two in base 8). But video and other evidence shows as you say. Either designers of Mark X IFF system put least significant digit on left side or they are doing a weird sequence (1, 2, 3, 4, 9, 10, 11, 12, 17, 18, 19, 20, 25, 26, 27, 28).

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7 minutes ago, Frederf said:

But video and other evidence shows as you say.

 

It's quite possible that the Huey has it the other way around IRL, though it wouldn't make much sense to me.

 

The video I linked above was suggested to me by a former RL German Army UH-1D pilot, though he didn't say whether this was actually the way he remembers it or not. But I guess he wouldn't have given me the link if he remembered it the other way around. 😉

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Yeah, the Hornet as well.

 

The C-101EB allows 0-7 on both Mode 1 digits, but like Kirk66 said, newer devices can handle that, so that seems to be correctly modeled. But the 3/7 combo for the DCS Huey seems pretty unique.

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