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NOTE - TM-9-1270-212-14-P0057
TM-9-1270-212-14-P Fire Control Subsystem Helmet Directed XM128 P/N 2277716-00 NSN 1270-00-122-9449 Manual
BIT failure isolation cont'd
TM 9-1270-212-14&P
The high at U7-11 is also connected as the third enable to
low, allowing the U10 decoder on sequencer card A9 to
U2-10, causing U2-8 to go low. The low is inverted to a
continue  operating. If EIA power is abnormal,
high at U3-10, turning on Q3 and energizing K2. K2-7
XA10/XA15-31 will be low; this low is inverted to a high.
removes the 28 volts from K6, which deenergizes. When K6
With. U6-4 and -5 high, U6-6 is low, changing the state of
deenergizes, K6-6 goes low and chassis relays K1, K2, and
flip-flop U4. U4-11 goes high, causing U4-3 to go low; this
K3 deenergize. The helmet sight subsystem is again
low is connected to U8-4 and -5. The resulting high at U8-6
turns on Q7, producing an EIA fail signal. The high at
U4-11 is also connected through XA10/XA15-32 to U10-18
(k) While A10/A15K2 was deenergized, resistor
on sequencer card A9. The high stops the operation of
R4, connected to -6 volts, kept Q1 pinched off. When K2
decoder U10. The system remains in the stopped condition
energizes, it removes -6 volts from R4 and capacitor Cl
until the power is turned off momentarily, to reset the
begins to charge slowly toward +5 volts. In the meantime,
circuitry, or the BIT switch is reactuated.
K2-7 has applied 28 volts to K2-6, lighting the GO indicator
in the helicopter through XAl0/XAl5-13 and J1-x. After 5
(h) If EIA power is normal, at 0.1 second (foldout
to 10 seconds, the voltage at Al0/Al5Q1-G reaches about
FO-2), A9U10-1 goes high and U10-2 goes low. The low at
-1 volt and Q1 conducts. The high at the base of Q2 causes
U10-2 is connected to inverter U2-3; the resulting high at
the collector to go low. This low is connected to U1-13 and
U2-4 is comected to U3-1. The skip input at XA9-24 is
causes the flip-flop to change state. The high at U1-11
low, so no change takes place. The low at U10-2 is also
causes U1-8 to go low. The low at U2-11 causes Q3 to cut
connected to U5-5, causing U5-6 to go high. The next
off and K2 deenergizes. K2 and K6 again apply a high to
strobe from U8-2 to U5-12 causes U5-11 to go low for 5
the base of Q8 and the reset signal goes low. All circuits are
ms. The pulse is inverted at U8-8 to become a
now back to steady-state conditions. The helmet sight
positive-going, 5-ms pulse called enable A. XA9-10 is
system is operative as soon as K6 deenergizes, which occurs
connected to XA10/XAl5-37 and the pulse provides the
when the helicopter GO indicator lights; that is, the
fourth enabling signal to U9 on logic card A10/Al5.
circuitry associated with Q1, Q2, and Q3, which turns on
the GO indicator, has no effect on system operation.
(i) As soon as A10/A15K6 was energized, the
pilot linkage was connected to the comparator circuits, and
(l) If desired, the BIT switch can be reactuate as
if all three outputs were normal, pins 5, 4, and 2 of
soon as the GO indicator lights. The high BIT initiate signal
A10/Al5U9 were high. The enable A pulse causes U9-6 to
will turn on Q8 and cause the reset signal at
go low. This low is connected through K1-2 and 4 to
XA10/XAl5-25 to go low; this low resets the previously
U7-10. The low changes the state of the flip-flop and causes
discussed circuitry on the logic card and sequencer card A9.
U7-8 to go high. The high skip signal is connected through
On the logic card, the low at U2-9 from reset flip-flop U7
XA10/XAl5-29 and XA9-24 to U3-2 on the comparator
turns off Q3, deenergizing K2 and turning off the GO
card. The two highs at U3 cause U3-3 to go low, activating
indicator. Of course, if system operation is normal, the GO
the parallel enable not (PE) at U4-9 and the preset inputs at
indicator will light again in 0.3 second. This operation can
U4-3, -4, -5, and -6. The signals are respectively low, high,
be used to observe waveforms, which, in this system, are
high, low (0110, or a binary 6). The preset causes the U10
not continuous.
count to jump (or skip) to binary count 6, causing U10-7 to
go low (foldout FO-2).
(4) BIT failure isolation is discussed below.
(j) The low at A9U10-7 is connected to U3-9 and
(a) Refer to foldout FO-7/FO-10.1. When the
to U5-4. The low at U3-9 causes U3-8 to go high; this
enable A signal is applied through XAl0/XAl5-37 to U9-1,
causes U3-12 to be high and U3-11 to go low. The low is
if one or more of the comparator inputs to U9 is low, the
inverted to a high at U2-10, causing Q5 to conduct and
output at U9-6 remains high and the output at U7-8
energize relays K1 and K2; this, in turn, connects the
remains low. Something is wrong with the pilot linkage
gunner linkage to the three comparator circuits. The low at
outputs. The low at XA10/XA15-29, which is connected
the collector of Q5 is also connected through XA9-22 and
through XA9-24 to the skip logic, disables the skip logic,
XAl0/XAl5-28 as relay drive 5 to K1 on the logic card. K1
and the decoder continues generating gates at its outputs
energizes and transfers the comparator U9 logic to the
(foldout FO-2). Note on this timing diagram that all U10
gunner flip-flop. If all inputs to U9 are high, the low output
outputs are high except for the short time when they go
at U9-6 is connected to U7-13, where it causes U7-11 to go
low in sequence. Refer to foldout FO-6 and observe that
high. This high is connected through XA10/XA15-30 and
U10 output pins 3 through 6 and 8 through 11 are
XA9-13 to decoder U10-19, where it stops the decoder.
connected to NAND gate U6 or to extender gate U7. If all

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