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At the output of the encoder (encoder) is set to binary code corresponding to decimal number initiated data input. In the callout boxes using the letters of the CD (from the English. Soder). The encoder can be used to represent (encode) a decimal number binary code and for issuing the specific code (the value pre-selected) when you press the corresponding symbol. When this code system is notified that a specific key is pressed on the keyboard.
Fig. 20.5. Eight-bit decoder
Similarly, the decoders, the encoders are complete and incomplete.
For a complete encoder the condition
n = 2 N ,
where n is the number of inputs, N is the number of outputs.
Fig. 20.6. IC 155ID9
Fig. 20.7. Pair ID9 with led indicators
To convert the code to the keypad in a four-digit binary number is sufficient to use only 10 inputs, while the total number of inputs is 16 (n= 16), so the encoder 10 × 4 (10 to 4) will be incomplete.
an example of designing encoder to convert the eight-bit unit code (decimal numbers from 0 to 7) in binary code.
it is assumed that a signal corresponding to a logical unit at each moment of time is applied to only one input. The table of correspondence of codes is given in table.20.1.
Table.20.1 correspondence table of codes of the encoder
Using this mapping table, we can write a Boolean expression including a logical sum of those input variables that correspond to a unit of a certain output variable.
So, the output Y1 will be logic "1" when a logical "1" is either input X1 or X3 or X5 or X7, i.e.,
, Y1 = X1 + X3 + X5 + X7,
Y2 = X2 + X3 + X6 + X7,
Y3 = X4 + X5 + X6 + X7.
In Fig. 20.8, and the diagram of the encoder, using the elements OR.
Fig.20.8. Encoder elements OR (A) and its symbol ()
In Fig.20.8, shows the findings of the encoder: E – enable input E0 of work and output, logic 0 which indicates that no information input is not excited.
In practice, we often use encoder with priority. In such encoders the code in the binary number corresponds to the highest input number on which the signal is "1". The priority encoder is allowed to give signals to multiple inputs, and it exposes the exit code of the number corresponds to the older entrance.
an Example of the priority encoder is the chip K555IVZ (Fig. 20.9).
Fig. 20.9. The priority encoder K555IV3
the Encoder has a 9 inverted inputs, denoted by PR1 to PR9....
the Abbreviation PR stands for "priority". The encoder has four inverted output B1,B2,B4,B8. The acronym At means "bus" (from the English. bus).
Numbers determine the value of the active level (zero) in the corresponding digit binary numbers. For example, B8 indicates zero this output corresponds to the number 8. Obviously, this is a partial encoder.
If all the inputs of the logical unit, the outputs are also a logical unit that corresponds to the number 0 in the so-called inverse code (1111).
If at least one input a logical zero, the state of output signals is determined by the highest input number that has a logical zero, and does not depend on the input signal, having a smaller room.
for Example, if the input PR1 is a logical zero and all other inputs - logical unit, on the outputs there are the following signals:B1=0,B2=1,B4=1B8=1, which corresponds to the number 1 in the inverse code (1110). If the input PR9 logic zero, regardless of the other input signals at the outputs there are the following signals: B1=0,B2=1,B4=1B8=0, which corresponds to the number 9 in the inverse code (0110).
To obtain encoders with a large number of inputs, i.e. increasing the dimension of the encoder, integrate circuits encoders with additional insights.
this chip K555IV1 (Fig. 20.10) is a priority encoder 8x3, i.e., the inverse has 8 inputs and 3 inverted output. In addition, it has an enable input EI, the output EO of the transfer and the output of G, the defining characteristic of the input information signal.
Fig. 20.10. The priority encoder K555IV1
If all information inputs a logical 1 when the input EI is a logical 0, the outputs 1, 2, 4 and G are such logical 1, and the carry output SW logic 0.
If you activate one of the information inputs (supply logical 0), then on the inputs 1, 2, 4 will be inverse code corresponding to the number of activated input, the input G is a logic 0, that indicates supply of the input signal, and the output EO is a logical 1.
If the chip is not enabled, i.e. the enable input EI filed logical 1, then all outputs of the chip will also be a logical 1 regardless of what will be served on informational inputs.
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