Encoders

ENCODERS:-

An Encoder is a device, circuit, transducer, software program, algorithm or person that convert information from one format or codes to another, for the purposes of standardization.

OR

An encoder is a multiplexer without its single output line. It is a combinational logic function that has

2n (or fewer) input lines and n output lines, which correspond to n selection lines in a multiplexer.

The n output lines generate the binary code for the possible 2n input lines. Let us take the case of an

octal-to-binary encoder. Such an encoder would have eight input lines, each representing an octal digit,

and three output lines representing the three-bit binary equivalent. The truth table of such an encoder

is given in Table 8.8. In the truth table, D0 to D7 represent octal digits 0 to 7. A, B and C represent the binary digits.

Rotary Encoders

Austria micro systems offers a range of Integrated Magnetic Rotary Encoders, from 8-bit incremental up to 12-bit absolute outputs. Or in other words, from 256 up to 16384 steps per revolution with serial, analog and PWM outputs for absolute position and quadrature. A simultaneous commutation output for Brushless DC (BLDC) motors is also available. These Magnetic Rotary Encoders are an ideal solution for replacement of both absolute and incremental optical encoders, potentiometers or rotary switches in a variety of industrial, consumer or even auto motive applications requiring precise angle sensing.

Priority Encoder:-

A priority encoder is a practical form of an encoder. The encoders available in IC form are all

priority encoders. In this type of encoder, a priority is assigned to each input so that, when more

than one input is simultaneously active, the input with the highest priority is encoded. We will

illustrate the concept of priority encoding with the help of an example. Let us assume that the octalto-

binary encoder described in the previous paragraph has an input priority for higher-order digits.

Let us also assume that input lines D2, D4 and D7 are all simultaneously in logic ‘1’ state. Inthat case, only D7 will be encoded and the output will be 111.

THE TRUTH TABLE OF SUCH A PRIORITY IS;

D0	D1	D2	D3	D4	D5	D6	D7	A	B	C
1	0	0	0	0	0	0	0	0	0	0
0	1	0	0	0	0	0	0	0	0	1
0	0	1	0	0	0	0	0	0	1	0
0	0	0	1	0	0	0	0	0	1	1
0	0	0	0	1	0	0	0	1	0	0
0	0	0	0	0	1	0	0	1	0	1
0	0	0	0	0	0	0	0	1	1	0
0	0	0	0	0	0	0	0	1	1	1

THE DECIMAL TO BCD PRIORITY ENCODER:

This type of encoder performs the same basic encoding function as previously discussed. A priority encoder also offers additional flexibility in that it can be used in applications that require priority detection. The priority function means that the encoder will produce a BCD output corresponding to the highest-order decimal digit input that is active and will ignore any other lower-order active inputs. For instance, if the 6 and the 3 inputs are both active, the BCD output is 0110.

LOGIC SYMBOL OF BCD TO BINARY ENCODER:

Bit A2 is always a 1 for decimal digit 4, 5, 6 or 7 and a=can be expressed as an OR function as follows:

A2=4+5+6+7

Bit a1 is always a 1 for decimal digit 2, 3, 6, or 7 and can be expressed as

A1=2+3+6+7

Finally, A0 is always a 1 for decimal digit 1, 3, 5, 7, or 9. The expression for A0 is

A0=1+3+5+7+9

TRUTH TABLE:-

THE 74LS148 8-TO-3-LINE ENCODER:-

The 74LS148 is a priority encoder that has eight active low inputs and three active low binary outputs,as shown in figure below. This device can be used forbconverting octal inputs to a 3-bit binary code. To enable the device the enable inputs must be low. It also has the enable outputs and gs outputs for expansion purposes. The enable output is low when the enable input is low and none of the inputs is active. This device may be available in other TTL or CMOS families.

LOGIC SYMBOL FOR THE 74LS148 8-LINE-TO-3-LINE ENCODER:-

The 74LS148 can be expanded to a 16 line to 4 line encoder by connecting the enable output of the higher order encoder to the enable input of the lower order encoder and negative-ORing the corresponding binary outputs as shown in figure describe below. The enable output is used as the fourth and most significant bit.

This particular configuration produces active-high output for the 4-bit binary number.

APPLICATION:-

A classic application example is a keyboard encoder. The ten decimal digits on the keybord of a computer must be encoderfor processing by the logic circuitry. When one of the key is pressed the decimal digit is encoded to the corresponding BCD code.

Below describe figure shows a simple keyboard encoder arrangement using a 74Hc147 priority encoder. They keys are representedby ten push-button switches,each with a pull-up resistor to +v. The pull-up resistor ensures that the line is high when a key is not depressed. When a key is depressed, the line is connected to ground, and a low is applied to the corresponding encoder input. The zero key is not connected because the BCD output represents zero when none of the other keys is depressed.

The BCD complement output of the encoder goes into a storage device, and each sucssecive BCD code is stored until the entire number has been entered.

•SIMPLIFIED KEYBOARD ENCODER:-