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Figure 2-5. Timing Diagram, Decode Cycle (SIF Reply Code 7777)
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TM-11-4920-296-14-P Transponder Set Test Set AN/APM-378 (NSN 4920-00-134-1533) Manual
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SIF SELF-TEST MODE


TM 11-4920-296-14&P
------
to clear all register stages. Basically, this
CAUTION
actions ends the decode-cycle. PRF circuits
------
within the U3 module will determine when the
Use only the cable provided; use
next encode-cycle (interrogation) is to begin,
of other cables may result in
and will at that time change the CLOCK rate
damage to the AN/APM-378.
to 1 MHz and generate a P1 pulse on the DATA
Additionally, during M4 operation, the front
line (at U3-TP1), to start the next encode-
panel FUNCTION switch is placed in the
cycle. Encode (Interrogation) and decode
SYSTEM position, and the MODE switch is
(reply) cycles and actions then repeat, in the
set for MODE 4. During M4 Operation, the
manner described thus far for one encode/
CLOCK rate in the test set is 555 kHz (1.8-
decode cycle. The PRF circuits of U3 control
microsecond period). In the same manner
the rate of encode/decode repetition such
as for the SIF modes discussed previously,
that iterrogation/reply-checking occurs at
the encode-cycle of operation begins with a
a rate of 257 cycles per second.
counted-down CLOCK signal generating the
2-34.  Each time a correct reply is obtained
To Pulses (U2-TP4).  This action sends the
from the transponderundertest, the result-
M4 PRETRIGGER to the KIR-1A, which then
ing SIF GO strobe is fed to the evaluator U4
returns the M4 VIDEO interrogation word.
circuits, as mentioned. An integrator/
The M4 VIDEO pulses are shaped in U4 cir-
comparator circuit of U4 measures the per-
cuits, and are then sent to the U2 circuits
centage of times that correct replies are
(U2-TP10). From U2, the pulses are ap-
received (for the number of times the trans-
plied as Interrogation pulses (U2-TP8) to
ponder is interrogated), and causes the
the rf module to modulate the 1030-MHz
ACCEPT lamp to light. if the correct-reply
t r a n s m i t t e r stages.
rate is greater than 80 percent of the inter-
2-37. In response to the M4 PRETRIGGER,
rogation rate.  The 80-percent correct-
the KIR-lA equipment also applies an M4
reply criteria applies for testing of single-
channel and diversity transponders. When
GTC signal to the test set. This signal trig-
gers an approximately 78-microsecond gate
testing lobing-type transponders, a lower
to yield a GTC GO signal (U3-TP9), which in
threshold voltage is applied to the U4 com-
turn provides a VIDEO GATE (U3-TP3) that
parator circuit, to cause the ACCEPT indi-
cation to be indicated for correct-reply
enables a 3-pulse decoder in the U4 module.
rates greater than 40 percent of the inter-
2-38.  As for SIG mode operation, the trans-
rogation rate.
mitter rf pulses during the encode-cycle
2-35.  MODE-3 AND MODE-TEST EMER-
cause the transponder under test (and its
GENCY. Operation of the test set circuits
associated Interrogator Computer equipment)
for Mode-3 and Mode-Test Emergency is
to generate reply pulses, These reply pulses
similar to that described for the SIF modes.
are checked for correct power and frequency
in the rf module circuits, and if correct, are
However, for these M3 and MT Emergency
operations, the code is always 7700. The
detected and sent as the REPLY SAMPLE
proper comparison code states for the par-
data to the digital section (U3-TP10). Upon
ity comparison are then generated by the
receipt of the REPLY SAMPLE data, CLOCK
A&B ENABLE Ul-TP9) and the C&D) DIS-
is desynchronized and the data is loaded into
ABLE code selector (which bypasses the
shift-register stages. At this time, the 3-
front panel CODE SELECT switches). In
pulse decoder that was enabled in response
Mode C, there is no emergency function. Also,
to the GTC GO signal (U3-TP9) checks for
in Mode C, the test set will give an ACCEPT
coincidence of To (U2-.TP4), the 1-micro-
indication, either with or without a SPI pulse.
second tap signal (U2-TP2), and the 2-micro-
second tap signal (U4-TP2). If correct coin-
2-36.  MODE-4 OPERATION. For Mode-4
dence occurs for this check, a pulse is
applied to one input of an AND circuit of U4.
(M4) operation, the test set is used in
conjunction with an Interrogator Computer
2-39.  The REPLY SAMPLE data, after
KIR-1A. The KH3-1A is connected to the
test set via the M4 cable supplied with
regeneration, is also applied to the KIR-lA
the test set.
Interrogator Computer as the M4 REPLY


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