Examination of a rocker
Reset-Set realized with 2 NAND |
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Created it, 06/10/19
Update it, 06/10/24
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In this 4th practice, you will continue the assembly of the digital desk by increasing its employment opportunities thanks to two push-buttons. the latter will make it possible to generate impulses of tension necessary to new experiments. You will cable also a circuit allowing to detect the rising faces of a signal, i.e. the passage of a tension of the level L on the level H.
You will also learn operation from the rockers. The circuits previously seen (AND, OR, NAND, NOR, EX-OR) are used in the combinatory logic.
This term means that the logical level present at exit of the circuit considered is an only function of the logical levels present at the entry at the moment considered. In other words, the logical level of the exit does not depend on the former states of the entries.
The rockers belong to the SEQUENTIAL systems ; the latter make it possible to obtain at exit a logical level which depends on the level of the entries at the time considered, but which also takes account of their former state.
The sequential circuits can be very complex.
The rockers are simplest of them.
We will examine some types of rockers carried out first of all with circuits NAND then thereafter with single specialized integrated circuits.
1. - PREPARATION OF THE MATERIAL
In order to be able to carry out the exercises suggested, please use the following components (that you will take among the material in your possession) :
2 push-buttons reversers
2 integrated circuits MM 74C00
1 integrated circuit MM 74C74
4 resistances of 4,7 kW, 1 / 4 W, 5 % (yellow, purple, red, gold)
2. - FIRST EXPERIMENT : EXAMINATION OF A ROCKER
RESET
- SET
(R-S) REALIZED WITH 2 NAND
Examination of a rocker Reset - Set (R - S) realized with 2 NAND.
A rocker is a bistable circuit, i.e.
characterized by an exit being able to take two stable logical states in time (L
or H) ; the exit rocking as soon as
one acts on a suitable entry. A rocker has two noted exits in general Q
and 
.
The two exits are always on complementary levels: when Q
is on the level H,
is on the level L and vice versa.
Generally, one bases oneself on the exit Q to define the state of the rocker. It is said that the rocker is in a logical state 1 if the exit Q is in a logical state high and that the rocker is in a logical state 0 if the exit Q is in a low state.
In this exercise, you will study the simplest rocker : the rocker RESET - SET realized with two circuits NAND.
One calls SET, state 1 of the rocker and RESET state 0.
Preparation of wire of connection to three drivers :
a) Cut three pieces of insulated rigid wire : one of approximately 10 cm and two others of 5 cm. Strip the ends. Connect three wire between them at an end by twisting them, then by carrying out a welding as represented figure 1.
b) Proceed like above, prepare two others wire of connection to three drivers. You can now begin the exercise.
c) Remove all the connections relating to the last experiment.
d) Remove support I.C.X the integrated circuit used previously and replace it by the integrated circuit MM 74C00.
While referring to you with the figure 2-a, carry out the following connections :
e) Using two pieces of wire of 5 cm, connect pin 1 of the integrated circuit (entered of the 1st NAND) into switch SW0 and pin 5 (entry of the 2nd NAND) into switch SW1.
f) By means of the triple wire prepared previously, connect pin 3 (exit of the 1st NAND) and stitches it 4 (entry of the 2nd NAND) with the LED L0.
g) With the second wire triples that you prepared, connect pin 2 (entry of the 1st NAND) and stitches it 6 (exit of the 2nd NAND) with the LED L1.
h) Put two switches SW0
and SW1 on position 1.
With the connections carried out, you carried out the illustrated circuit
appears 2-b. As you can observe it, the circuit has two entries and two exits.
An entry is indicated by the symbol 
and the other by the symbol 
.
The letters S and R
are the abbreviations of the Set terms (writing
or setting with 1) and Reset (obliteration
or handing-over with 0).
The two exits are noted Q
and 
bus there exists a possibility so that the two exits are not complementary. The
exit Q is the exit of reference of the
circuit.
Connect the food now and observe the LED L0 and L1, one of the LED will be lit and the other extinct one. It is not possible to envisage which of both will ignite because that depends on the physical characteristics of two circuits NAND and more exactly of their response time. They are never perfectly identical.
Since the response time of a logical circuit can vary in a rather broad range, it is not possible to know which of the two NAND constituting the rocker will commutate the first with the powering.
Position the rocker in a quite precise state with the powering, certain installations are necessary. They will be examined in practice following.
Check the operation of the circuit now :
i) Put switch SW0
on position 0. Thus, the entry
is on the level L. You note that the LED
L0 connected to the exit Q is
lit, whereas the LED L1 connected to the
exit ,
is extinct. This means that the exit Q is on
a level H and the exit
is on a level L.
j) Give SW0 on position 1, the state of the exit is unchanged.
k) Put then SW1 on position 0. You note that the LED L0 dies out and that the LED L1 ignites.
l) Give SW1 on position 1. You note that L0 remains extinct and L1 lit.
With this test, when SW1
()
is positioned on 0, you note that the exit Q
(L0) remains unchanged by repositioning SW1
on 1.
m) Carry out same handling with switch SW0 by putting it on position 0. You note that L0 ignites, while L1 dies out.
n) Give SW0 in position 1, L0 remains lit. When SW0 is positioned on 0, the LED L0 ignites and remains lit when SW0 is repositioned on 1.
When SW1 is positioned on 0, the LED L0 dies out and remains extinct when SW1 is positioned on 1.
Simultaneously not to position for the moment the two switches out of 0.
Preceding tests, you can draw the following conclusions :
the two exits Q
and
are always complementary one of the other.
each time entry
(SW0) is carried, even temporarily on the
level L, the exit Q
is carried on the level H (L0
lit); a return to the level H
on
preserve the level H at exit Q.
One can say that the rocker
“remembers” that the entry
temporarily passed by the level L.
each time entry
(SW1) is carried, even temporarily on the
level L, the exit Q
is carried on the level L (L0
extinct) ; the return of the level H
on
does not have an influence on Q which
remains on the level L.
One can say that the rocker
“remembers” that the entry
temporarily went on the level L.
ultimately, the rocker “remembers” which of the
two entries was carried the last on the level L.
This one remains with the state corresponding indefinitely until the other entry
is carried on the level L.
an entry is known as “active” when it determines
a swing of the state of the two exits.
the fact that the two entries are active in a low
state is indicated by the bar placed above the “symbols” R
and S (
and ).
As for all the combinative logical circuits, the operation of the rocker can be described by the table of operation and the truth table.
The table of operation can be drawn up on the basis of test carried out previously.
When the two entries
and
are all the two ranges on the level H,
the
rocker does not change a state.
The state of the exits is determined
by one of the two entries carried before on the level L.
This condition is generally represented by “Q0”
and “0”,
which you find in the first line of the table of operation of the figure 3-a.
In the last line of the table, there is a combination of the levels of entries which does not have, until now, not tested. It is about the case where the two entries are on the level L.
To see what occurs in this case, carry out the following operations :
a) Put two switches SW0 and SW1 on position 0. You note that L0 and L1 are both lit.
This is against the traditional operation of a rocker which, usually, sees its exits taking complementary levels.
b) By simultaneously positioning two switches SW0 and SW1 on 1, you note that one of the two LED dies out, according to the entry which reaches the first the level H.
It results from it that the state of
the rocker is unspecified when the two positioned entries with L
are carried on a level H (at-rest state).
This is why this combination (=
0 and
= 0) of the levels of entry is undesirable.
In the figure 3-b, to you is represented the truth table of the rocker RESET - SET. You pass immediately from the table of operation to the truth table by associating at the logical level L figure 0, at the logical level H figure 1.
In short, the rocker RESET - SET, often indicated by initials R-S, is a storage element which has the following characteristics :
the two entries
and
are with the at-rest state when they are on the level H.
a logical level L
applied to
fact of rocking the exit Q on a level H
:
this state is also known as SET.
a logical level L
applied to
fact of rocking the exit Q on a level L
:
this state is also known as RESET.
a logical level L
applied to the two entries
and
makes rock the two exits on a level H. When
the two entries return to the at-rest state (H),
the rocker passes in an unspecified state. Thus the combination of entry L
is undesirable.
to conclude, rocker R-S
makes it possible to memorize the passage on the level L
of the one of the two entries.
The uses of this circuit are multiple, as you can realize it by continuing handling.
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