BIT failure isolation

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

operative.

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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