No 8's or 9's on a transponder

n2o2diver

n2o2diver

New Member
Just thought I would throw this useless piece of trivia out there.

Ok there are 10 possible single didgit numbers.
0, 1, 2, 3, 4, 5, 6, 7, 8, and 9

So why is it that on a transpoder we only have 0 thru 7?

Why no 8's or 9's?
 
feeble answer

i just searched the web to try and find a substantive answer... but I think it has to do with old 8-bit technology, 8-bits being 0-7. Someone help me. SOrry I cant help more, but I know I read something like this somewhere!

I did see this on some site....

"The aircraft transponder replies on 1.09 GHz with 12 pulses giving the requested information, with each pulse providing a "0" or "1" bit giving a total of 4,096 possibilities."

This rings a bell for me because I remember from computer days that 8-bit color had 4,096 possible colors.
 
oops..

i forgot to add the part that actually kinda clarifies this. 4 digits X 8 possibilities =... 4096.
 
Using Binary language (0 for off, 1 for on), Each digit doubles in value as you move left. Example:
000=0
001=1
010=2
011=3
100=4
101=5
110=6
111=7

That is why the Transponders range is 0-7.
 
subpilot said:
Using Binary language (0 for off, 1 for on), Each digit doubles in value as you move left. Example:
000=0
001=1
010=2
011=3
100=4
101=5
110=6
111=7

That is why the Transponders range is 0-7.
Click to expand...

Winner!

Sorry if I gave the impression I was looking for an answer, I already knew. Just wanted to throw it out there and see how many others could come up with the correct answer. Do you have an avionics background? I always ask other instructors and pilots when we are hanging out and debating things and thats one question knowbody ever gets,
 
I've never researched transponder encoding, but I would guess that as far as a trransponder is concerned:

000=0
001=1
011=2
010=3
110=4
111=5
101=6
100=7
 
n2o2diver said:
Do you have an avionics background?
Click to expand...

No, Navy electronics background. Binary is just basic logic. Actually, Transponders use Octal (0-7) but my explanation just broke down how octal works. Everything with computers and electronics is 1's and 0's when you break it down far enough.
 
Dude that's not octal representation, octal uses the digits 0-7.

Using the standard binary encoding, binary 111 = octal 7.

Decimal 44978 = octal 127662.
 
Ralgha said:
I've never researched transponder encoding, but I would guess that as far as a trransponder is concerned:

000=0
001=1
011=2
010=3
110=4
111=5
101=6
100=7
Click to expand...

Very interesting to say the least. Actually:
000=0
001=1
011=3
010=2
110=6
111=7
101=5
100=4


Ralgha said:
Dude that's not octal representation, octal uses the digits 0-7.

Using the standard binary encoding, binary 111 = octal 7.

Decimal 44978 = octal 127662.
Click to expand...

So you are saying that a transponder does not use digits 0-7? Please explain to us non-engineering types what octal is then since I don't have a clue. I thought I knew but I could always be wrong I guess. BTW, which is it:
is 111=7 or is 100=7 since you have stated both? Thanks Dude.
 
Octal notation is simply a method of representing values, because binary is inefficient for humans. A computer would not actually use octal internally, because computers know only 0 and 1. Octal would be used by a person to make the code more readable (hex is preferred though), in the case of a transponder, the number on the front of the box is actually in octal representation, not decimal. If the dials were marked with what the transponder is actually using, you would see groups of 1s and 0s, not 0-7.

Take a transponder code of 4076. In (standard) binary, this would be 100000111110, and this is what the transponder understands. The decimal equivilant would be 2110.

The VFR transponder code of 1200 is actually 640 in decimal notation. A transponder could be set using decimal notation (valid codes would then range from 0 to 4095), but it would make the internal mechanism more complicated since it would have to convert from decimal to binary, which is more involved than from octal to binary.

The alternative encoding that I provided is Gray code. The advantage of Gray code is that, when increasing the value by one, only one digit ever changes at a time. This reduces the error potential and makes the system more efficient. Like I said, I've never researched transponders, so I don't know if they actually use Gray code or not, I just guess that they do.

Actually, to get super technical, computers don't even know 1 and 0, they only know high voltage (commonly referred to as 1), and low voltage (commonly referred to as 0).
 
I still like telling a new copilot or sometimes even "experienced" copilots that our squawk code is 2835 or some other arbitrary number and wait to see how many times they will spin the wheel looking for the 8 ... :)

Jim
 
Much better response, Ralgha. I was afraid we were getting into a battle of the wits. I have never heard of Gray code before and as for your reasoning to use it... if you look at binary only one digit changes at a time as well, so I do not see the value of the Gray code system. Either way, I don't want to beat on a dead horse anymore. The mystery of the transponder has been solved.
 
Sorry, you're wrong. In standard binary, multiple digits change at the same time.

Standard:
000
001 : first digit changed
010 : first and second digit changed
011 : first digit changed
100 : all three digits changed
101 : first digit changed
110 : first and second digit changed
111 : first digit changed

Gray:
000
001 : first digit changed
011 : second digit changed
010 : first digit changed
110 : third digit changed
111 : first digit changed
101 : second digit changed
100 : first digit changed
 
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