SECTION IV. THEORY OF OPERATION

T.O. 12P4-2APX-142

NAVAIR 16-35C6280-1

TM 11-5841-268-25

SECTION IV

THEORY OF OPERATION

41. INTRODUCTION.

digit number, the "tens" number designates the A group,

while the "units" number designates the B group. In a four-

42. A general description of the transponder set control

digit number, the "thousands" digit designates the A group,

and its operating controls and indicators is included in Sec-

the "hundreds" digit the B group, the "tens" digit the C

tion I. Except for the operation of the Mode 1 and Mode

group, and the "units" digit the D group. Figure 41 shows

3/A code selector switches, this information adequately de-

the standard reply-code pulse trains for each of the four

scribes the purpose and operation of the transponder set

modes of operation.

control. For additional information, the purpose and opera-

46. The control inserts the selected code number into the

tion of the Mode 1 and Mode 3/A code selector switches are

transponder set encoding matrix by grounding appropriate

described in paragraphs 43 through 47.

circuits. Table 41 shows the pattern of "grounds" and

43. CODE NUMBER SELECTION.

"opens" provided by the two Mode 1 switches, while Table

42 shows the pattern of "grounds" and "opens" provided

4-4. The setting of the code selector switches determines

which information pulses are present in a transponder set

by the four Mode 3/A switches. The first column in each

reply to IFF/SIF interrogations. The presence or absence

table identifies the switch by its reference designation (see

of these information pulses is used to convey flight informa-

the schematic diagram, FO1*) and its code position. The

tion to the ground controller. From 1 to 12 pulses are selec-

second column specifies the connector pin associated with

the circuit which is designated in the third (Pulse Position

ted for coded identification. Each of the 12 pulses has a

Controlled) column. The fourth column indicates which

permanent nomenclature and position in the reply pulse-

circuits are grounded for each switch position. In both ta-

train signal. The pulses are divided into four groups of three

pulses identified as the A, B, C, and D groups. Each pulse

bles, an "X" in the fourth column indicates that the lead is

within a group is assigned an identification number which

grounded.

frees its position in the reply-pulse train. The numbers used

47. As shown in Tables 41 and 42, all selector switches

(1, 2, and 4) form a binary code in which their various sums

except the second Mode 1 selector (S212) go through the

give the maximum number of combinations, without repeti-

same sequential pattern of grounds. S212 provides only po-

tion, for any three numbers. Any digit from O to 7 can be

sitions O through 3 (positions 4, 5, 6, and 7 are not used).

obtained within a pulse group; thus, any number between

The tables also show that while code numbers 1, 2, and 4

0000 and 7777 using these digits can be selected for the

can be represented by a single pulse, code numbers 3, 5,6,

transponder reply.

and 7 require combinations of two or three pulses. In these

cases, the sums of the pulse values are equal to the desired

45. The digits of the assigned code number indicate the

code numbers.

code groups to be used and the pulse coding within the

group. Depending on the mode of operation, the assigned

*FO (Foldout) refers to foldout illustration pages which are arranged

in sequence (for example FO1, FO2) and included at the end of

code is either a two-digit or a four-digit number. In a two

this manual.

TABLE 41. MODE 1 CODE NUMBER SELECTION