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by Robin Minnear
Conversions:
A letter
Negative Logic
When first learning about Logic or
when first starting to repair logic
boards, most people eat dirt when
they come across a negative logic
situation.
You may come across a weird
looking symbol on your schematic
or go way off track when applying
Positive Logic functions to a
Negative Logic situation . The truth
is that the same logic gate will
perform a different logic function in
a Negative Logic situation .
The reason Negative Logic and
Positive Logic are used together is
that by their combination , the
amount of chips needed to perform
a task are usually reduced .
The best way to understand
positive / negative logic is to apply
them to a couple of common logic
gates.
Figure 1 is the truth table and
symbol for the Nand gate (74OOl.
According to the truth table, if
inputs A and B are high at the same
time, the output Y goes from HI to
LO.
This is the positive logic
situation.
It is important to realize that this is
the basic" And" function . A and B
are high to make the output change.
All of this is assuming that the Nand
gate is looking for His coming in.
What if the Nand gate is looking for
LOs to come to inputs A and B7
Figure 2 is the symbol for the
Nand gate when it is hooked up in a
negative logic situation . Notice that
the zerks are on the inputs instead
of the output. It is the same device,
same truth table but he is going to
do his thing, or function when lows
appear on the inputs.
Looking at the truth table in
Figure 2, if A is low or B is LO, or A
and B are low at the same time, the
output Y is high .
Now we are
dealing with a situation where if
either A or B are low the output will
change . Now we are dealing with
the basic OR function.
Figure 3 is the positive logic
symbol and truth table for the Nor
gate (7402). According to the truth
table if A or B is high, the output Y
is low. Again, this is the positive
logic situation where the Nor gate is
looking for His on his inputs to do
his thing.
Figure 4 is the negative logic
symbol for the Nor gate. This time
58
we'll be concerned with the inputs
going LO . According to the truth
table in Figure 3, if A and B are low
at the same time, output Y is HI. No
other combinations will produce a
HI on the output.
To summarize this mess, "In a
negative logic situation, Nand gates
act like Nor gates and Nor gates act
Most logic
like Nand gates."
schematics show the negative logic
symbols for negative logic situa-
tions. Be aware that there are some
schematics that only use positive
logic symbols.
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",,,,.1: PC BOMd c~
Concerning the Trak 10 conver-
sions discussed in the January
article and the April "Mailbox" :
Mr. Randall of Oceanside Amuse-
ments solved the disable "Crash"
problem one way. Here is another,
somewhat simpler method. Add a
diode (lN914) between F9-S and
C7 -5 so that " A ttract" will disable
the crash op-amp.
(Cathode to
F9-S) .
This method was devised by our
Service Department in the devel-
opment of the "Grand Prix" - Chal-
lenger Customizing Kit for Grand
Traks or Trak 10's, sold by O.B.A.
Inc., in Dallas.
Another modification (which is
incorporated in the Kit) which may
be of interest is improved steering
for Grand Trak and Trak 10's. This
modification improves the sensitiv-
ity and "feel" of steering action.
Changes at and near I. C. chip A4
are made, and a jumper wire added
from A4-13 to a point connected to
V RESET which originates at H1 -S.
(Electrical changes are shown sche-
matically in Figure 1. The mechan-
ics of the corresponding PC board
changes are shown in Figure 2.)
The changes can be executed by
performing the following sequence
of operations:
Step 1. To change "P" from
A4-13 to A4-12, perform two
operations:
Sever, between the
chip and the PC board and as near .
the board as is practical, I.C. chip
lead A4-13, then bend the severed
lead up and away from chip &
board . Jumper between PC board
PADS A4-12 and A4-13.
Step 2. To change "Rate" from
A4-12 to A4-11, perform three
operations: On the back (non-chip
side) of the board , cut the etch at
A4-12. On the back of the board,
cut the etch near A4-11 . Add a
jumper from A4-11 to the etch
which led to A4-12.
Step 3. To connect the now
isolated A4-13 to "v RESET' which
originates at H 1-S, first, check for
continuity between H1 -S and B1 -11,
then perform the final operation:
Connect a jumper wire to the
uplifted lead A4-13 and to B 1-11
(H1 -S or V RESET) .
Nathan E. Bush
Steed's
Garland, Texas