Created it, 06/10/19
Update it, 06/10/29
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4. - SECOND EXPERIMENT : OPERATION OF THE DECODER MM 74C154
The use of a decoder can be necessary to facilitate the comprehension of the data.
Indeed, we are accustomed to employ a decimal representation and not the binary code, much more tiresome to use because of the high number of figures 0 and 1.
In this experiment, like in the following one, you will learn how to use a decoder allowing to translate the binary code of a number of 4 bits into a decimal representation of this number thus more explicit.
This decoder can be useful, for example, to immediately know the state of a meter modulo 16.
The integrated circuit MM 74C154 allows this decoding.
It comprises 4 principal entries which correspond to the 4 bits of the binary number to decode and 16 exits.
The exit which corresponds to the binary number present on the entries is on the level L, the other exits being on the level H.
If one connects the 16 exits to 16 LED, it is possible to immediately know the binary number present at the entry of the decoder.
The logic diagram and the stitching of this decoder are respectively deferred to the figure 5-a and 5-b.
As lets it appear the figure 5-a, this decoder is only consisted a network of logical doors.
The binary number to decode is applied to the noted entries A, B, C and D (see figure 5-b).
The two entries G1 and G2 are called STROBE. They are two entries of validation of the decoder. This one is validated if these two entries are on the level L.
It is enough that only one of both is on the level H so that the decoder is blocked.
In this case, the 16 exits remain on the level H which is the level of rest of the decoder.
As soon as the decoder is validated, one of the 16 exits passes on the level L which is the active level.
The table of operation located on figure 6 shows well that so at least one of the two entries of validation (G1 or G2) is on the level H, all 16 exit east on the level H.
Remember that symbol X in the table of operation, means that the logical level applied to the entry does not have any influence on the state of the exits.
4. 1. - REALIZATION OF THE CIRCUIT
a) Leave in place the connections and the integrated circuit relating to the preceding experiment.
Introduce the integrated circuit MM 74C154 on the matrix and carry out the connections indicated to the figure 7-a.
b) Introduce into the L7 contact of the group of connectors, the end of a piece of wire of approximately 20 cm length. Leave the other loose lead for the moment.
During the experiment, it will be connected in turn on the outlet sides of the decoder, by introducing it into the holes of the matrix corresponding to the exits.
The LED L7 is thus used like probes logical.
The electric diagram of the assembly is illustrated with the figure 7-b.
4. 2. - OPERATIONAL TEST
a) Put the digilab under tension.
b) Support on P1, then on P0 while maintaining supported a short moment on the two buttons.
The meter is given to zero.
The four LED L0, L1, L2 and L3 are extinct.
c) Slacken the two buttons P0 and P1.
Put two switches SW0 and SW1 on position 0 (G1 and G2 are on the level L). The decoder is thus validated.
d) Insert the loose lead of the driver (probe) connected to L7 in the hole of the matrix which corresponds to pin 1 of the decoder.
Figure 8 indicates the connection to you to be carried out.
Exit 0 (pin 1) is on the level L (extinct LED L7).
Indeed, as you carried out right before a restoring, exit 0 of the decoder is on the level L.
e) Disconnect the probe from pin 1 and check, by successively connecting it with pins 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,13, 14, 15, 16 and 17 of the decoder, that all these exits are well on the level H.
The LED L7 ignites as soon as the probe is connected to the one of these 15 exits.
f) Support once on P0. The meter is incremented of a unit.
The LED L0 ignites.
With the probe, you note that only exit 1 (pin 2) is on the level L.
g) Continue the experiment each time by incrementing the meter of a unit (using the P0 button).
Control that only the exit of the decoder corresponding to the state of the meter is on the level L.
h) Put SW0 on position 1.
All the exits are on the level H. You can check it with the probe.
Put then SW1 on position 1, then bring back SW0 on position 0.
You can note that the exits remained on the level H.
With this test, you checked that it is enough that only one of the two entries G1 or G2 is on the level H so that the decoder is blocked.
i) Put the digilab not under tension.
With this assembly, it would suffice to have for your provision 16 pilot LED to know the state of the meter constantly.
The elevators have an indication which posts their position according to the stage to which they are. These indications are carried out by means of a meter similar to that described here.
If the exits of the decoder are active on the level L and not on the level H, it is that in general, a decoder is connected to another logical circuit whose entries are active on the level L.
A typical case is provided by the meter MM 74C163 which, to be charged, requires a level L on entry LOAD.
In this same decoder, the entries of G1 validation and G2 are also active on the level L.
This is useful when it is a question of decoding binary numbers of more than four bits.
Indeed, these entries are used to invalidate one or more decoders.
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