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Steady-state BIT logic is discussed below cont'd
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
TM 9-1270-212-14&P
pilot and gunner linkages and transfers the linkage outputs
junction drop, or about -0.7 volt. R7 and R8 limit the
to the comparator circuits. The linkages should be in their
current through 5.1-volt zener diodes CR19 and CR32. The
magnetic BIT brackets.
lows at U2-3 and U2-5 are inverted to highs. The 5-volt dc
power is checked on the logic card. Since a 5-volt failure
(c) On sequencer card A9, the high reset signal at
could disable all logic circuitry and prevent a failure
XA9-6 acts as an enabling signal for oscillator U11, clocked
indication, the circuitry attached to A10/Al5Q7 does not
flip-flop U9, and four-bit counter U4. At U3-13, the high
require 5 volts for operation. A10/Al5U8 receives its
excitation from a divider consisting of 5.1-volt zener diode
has no immediate effect except to enable a change of state
CR6 and R8, which is connected to 28-volt dc input power.
when the high on U3-9 changes.
If the 5-volt power fails, U8-1 and -2 go to ground through
R7, and the low inputs provide a high at U8-6; this turns on
(d) The high on A9U11-4 causes this oscillator to
Q7 and energizes K5. The 28 volts dc connected to K5-3 is
generate 5-ms-wide pulses at 100-ms intervals at U11-3; this
connected through XA10/XAl5-23 and J1-w to the EIA
is a pulse rate of 10 Hz (foldout FO-2). The 10-Hz signal is
fail indicator in the helicopter.
connected to U9-2 and to U5-1. U9 is a clocked flip-flop
that is triggered on the trailing edge (down slope) of each
oscillator pulse. The result is a 5-Hz square wave (clock) at
U9-6 (foldout FO-2). At U5-1, the 10-Hz pulses act as
The EIA fail indicator in the AH-lS(Mod)
momentary enables to the NAND gate.
helicopter is labeled INTFC; in the AH-1S heli-
, this indicator is labeled
(e) The square-wave output at A9U9-6 is
connected to the clock input at U4-2 and to NAND gate
U5-2. At the NAND gate, coincidence only occurs between
the oscillator pulses at U5-1 and the clock pulses at U5-2
(j) The  reset  signal  at  XA10/XAl5-25  is
before the down slope of the clock pulses. The
connected through XA9-6 to U11, U9, and U4 of sequencer
negative-going pulses at U5-3 are inverted at U8-2 and
card A9 and also to flip-flop U3. At U11, U9, and U4, the
appear as a positive-going train of 5-ms strobe pulses
low inhibits operation, respectively, of the oscillator, the
(foldout FO-2). The strobe pulses are connected to U5-12
clocked flip-flop, and the four-bit binary counter. At U3,
and U5-10.
the low sets the flip-flop; this cuts off transistor Q5, causing
its output to go high through relay coils K1 and K2 and
(f) A9U4 is a four-bit binary counter with a
X49-18 to 28 volts.
master reset at pin l; a preset at pins 3, 4, 5, and 6; a preset
enable at pin 9; and outputs at pins 11 through 14. The
(3) BIT operation is discussed in the following
counter can be stopped, started, reset, and preset. The four
outputs, each with two states (high and low), provide 16
different outputs (24). In this application, the presets are
(a) Power to initiate BIT comes from the BIT
used to jump the output count to a binary 6 if certain logic
switch in the helicopter to J1-f of the EIA and to XAl0-9.
conditions are met. The 5-Hz square-wave input at U4-2
When the BIT switch is actuated, 28-volt dc aircraft power
causes the counter to provide binary outputs on pins 11
is connected through XAl0XAl5-9 and R10 to the base of
through 14. These outputs are connected to 1-in-16
Q8 and through CR7 to K6. K6 energizes and the 28 volts
decoder  U10  and  drive  the  decoder  to  produce
dc on K6-2 is transferred to K6-3, latching the relay in the
negative-going, 200-ms-wide sequential pulses at pins U10-1
energized condition. When the BIT switch is released, the
through U10-13 (foldout FO-2). The inputs at U10-18 and
28 volts dc is removed from the base of Q8, and R11 pulls
-19 must remain low for the decoder to operate. If either
the base to near ground potential; this cuts off Q8, causing
input goes high, the decoder stops decoding.
the reset signal to go high. On card A10/Al5, this high
becomes one enabling signal to NAND gates U1, U4, U5,
and U7. Refer to foldout FO-2 and note that, as the BIT
(g) As indicated in b, the first test checks power.
switch is released, the reset signal at XA10/XA15-25 goes
At time zero, when the reset pulse goes high (foldout
FO-2), A9U10-1 (foldout FO-6) is already low. The low,
inverted to a high, is connected through XA9-23 and
(b) When K6 energizes, relay contacts K6-7 and -6
XA10/XAl5-41 (foldout FO-7/FO-10.1) to provide one
apply 28 volts dc through XAl0/XAl5-10 to energize
high at U6-5. If EIA power is normal, XA10/XA15-31 will
chassis relays K1, K2, and K3. Refer to foldout FO-1 and
be high; this high is inverted at U6-3 to provide a low at
observe that this action applies fixed inputs to both the
U6-4. The output at U6-6 remains high and U4-11 remains

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