Star Tech Journal

S~~TECH _ _ _ _ _ _ _ __ _ g
Output Line D. In a TTL circuit, an open output from one stage
allows all inputs driven by that open output to float to a level of
about 1.5 volts de. This level, which is in the indeterminate
state, may allow all TTL inputs connected to that line to act as
though a static logic high signal were applied. Therefore, if
you observe a logic zero with a logic probe at the output pin of
ICA and an indeterminate level at the input to IC 8 , you should
be suspicious of an open ICA output lead. This condition can
be verified by observing the logic levels at Inputs A and B. If
both of these are high (and the output pin's voltage is
indeterminate), the chip is defective because of an open
output lead.
Figure 5A shows a similar open circuit condition, but in this
instance the open is located at the input to IC 8 • Under these
conditions, that particular input to IC 8 will float to approximately
1.5 volts, which will appear as though it were a logic 1 input.
However, due to the location of the open condition, the
correct output from ICA would be applied to the other circuits
connected to that point. A check at the input pin to IC 8 would
indicate an indeterminate level at that particular pin, with
both the high- and low-level indicators of the logic probe
turned on.
If an open occurs inside the chip, as indicated in Figure 5B, all
logic probe measurements will give the same results as for
Figure 5A, except for the measurement at the input to IC 8 .
Since the open is inside the chip, the input pin is still
connected properly to the output pin of ICA, and the logic
probe will show a low logic level at the input to IC 8 . Thus, if a
faulty output is obtained when all input levels appear to be
correct, it is a fairly safe assumption that the internal circuitry
of the IC is defective.
D
E
A
D
E
B
FIGURE 5
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REPRINTED BY PERMISSION OF BELL & HOWELL EDUCATION GROUP, INC. " COPYRIGHT 1980 BELL& HOWELL EDUCATION GROUP, INC. ALL RIGHTS RESERVED .
NEXT MONTH: SIGNAL INJECTION AND CURRENT TRACING
Midway
"Pac-Man"
Option Switches
Atari
"Battlezone"
Warning
SUBJECT:
DANGER: THE FOOTSTEP
INSIDE THE UPRIGHT
BATTLEZONE CABINET MUST
BE INSTALLED FOR CABINET
STABILITY. WITHOUT THIS
FOOTSTEP INSTALLED, THIS
GAME CABINET CAN TIP OVER
ONTO AN AGGRESSIVE
PLAYER.
Dip switch setting (7 & 8)
1 .
Switches 7 & 8 must be in the off
position for proper game
operation.
2.
When switch #7 is on, a test of
all racks is possible.
3.
When switch #8 is on, the
picture will lock up.