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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
41


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