Why is the aircraft transponder uses only as codes the numbers 0-7, Why no 8 & 9?

For reply code designation purposes, the 12 information pulses are divided into four groups of three.The four groups are designated as A, B, C, and D. Each pulse in a lettered group is further designated by a number, 1, 2, or 4. The code digit assigned to the lettered group (A, B, C, or D) is the number equal to the sum of the numbers (1, 2, or 4). The sums are then combined in the order A, B, C, D to form a four-digit code number. For example, code 2435 is represented by information pulses A2, B4, Cl, C2, Dl, and D4.

Emergency code 7700 uses A1, A2, A4, B1, B2 , B4

That is why there is only 0-7.

Yes, it is octal (goes from 0-7). Each digit can be represented by 3 bits (4th bit used for check digit). It is the same way all computers work and it is easy to transmit in binary.

When the ground (secondary) radar hits the aircraft, the transponder will start to send the info. The ground station knows to expect a signal every xxx milliseconds. It can then read the code on the ground. Newer systems not only transmit the number back, but altitude and location info as well.

The system only needs a few thousand codes as the transponders are local only and not transmitted very far. You can have different aircraft in different areas with the same code.

They could have used regular 0-9 digits, but in the early days of computers most geeks stuck with octal. Now it's probably just tradition.

0-7 is eight bits. Computer memory is octal (multiples of eight). As in the old Commadore 64. 80 gigabyte hard drive. Angela is confusing it with hexidecimal code. 0-7 with A, B, C, & D in place of 9 to 12. 10, 11, & 12 would need an extra bit & key stroke otherwise.

There is one input wire and three output wires from each digit's rotary switch. Each output wire can either be hooked to the input or not, usually through a diode.

0 -> no, no, no

1 -> no, no, yes

2 -> no, yes, no

3 -> no, yes, yes

4 -> yes, no, no

5 -> yes, no, yes

6 -> yes, yes, no

7 -> yes, yes, yes

There are no wiring combinations left for 8 or 9.

they're octal: 12 bits in 4 groups of three. hence 0 to 7.

Only 4096 permutations.

if 00005 and stuff like that is allowed, then it is 5 billion i think