74147 IC 10-Line to 4-Line Line Priority Encoders

The 74LS147 TTL encoders feature priority decoding of the inputs to ensure that only the highest-order data line is encoded. The ’147 and ’LS147 devices encode nine data lines to four-line (8-4-2-1) BCD. The implied decimal zero condition requires no input condition, as zero is encoded when all nine data lines are at a high logic level. Cascading circuitry (enable input EI and enable output EO) has been provided to allow octal expansion without the need for external circuitry. For all types, data inputs and outputs are active at the low logic level.

Encode 10-Line Decimal to 4-Line BCD

BCD Encoder

BCD Encoder circuit diagram and truth table in digital electronics- An encoder is a digital or logic circuit, which converts a decimal or octal input to a binary output. In other words, a logic circuit that operates exactly inversely with respect to a decoder and which converts non–binary numbers (i.e. decimal or octal numbers) to binary numbers, is called an encoder. As an encoder converts an active input signal to a coded output signal (i.e. one or more than one input is applied to it and it produces a binary output code greater than 1 – -bit), it functions precisely oppositely to a decoder (because a decoder converts a specific code into a decimal one by means of reading or identifying it, whereas an encoder generates specific codes by itself and then converts them into binary). In Figure 4.44, a general diagram of an encoder has been illustrated. This diagram comprises “n” decimal input lines, of which only one line is active. The inner logic of the encoder converts this active input line to a binary “m” bit-coded binary outputs line. Output lines produce a binary code with respect to 2ⁿ input lines. Remember that encoders are constructed with OR gates and encoder ICs may be expounded as follows:

74147 … decimal to BCD encoder

Figure 4.44 – encoder

In Figure 4.45, a common decimal––to–BCD encoder, which comprises four OR gates, has been illustrated. The switches mounted on it are just similar to the push buttons of a pocket calculator. The operational mechanism of this encoder is as follows:

Figure 4.45 – Decimal – to – BCD encoder

When push button 3, which represents decimal 3, is pressed, inputs of C and D OR gates become high (as an output of OR gate becomes high in case any of its input is high). Thus, the following BCD code is obtained on output.

ABCD = 0011

When push button 5 is pressed, inputs of B and D gates become 1 or high, while in such a scenario, inputs of A and C are low or zero. Resultantly, the following binary-coded output is received.

ABCD = 0101

Likewise, when switch 9 is pushed, the A and D gates are high whereas C and B gates are low. Thus, the following output results.

ABCD = 1001

In Figure 4.46, the logic symbol of a decimal – to – BCD encoder whereas in Figure 4.47 decimal digits (0 – 9) and their equivalent BCD code (or 8421) have been illustrated, with the help of which reciprocated relationship between BCD bit and decimal digits can be understood.

Figure 4.46 – logic symbol for a decimal–to–BCD encoder

Figure 4.47: decimal to BCD encoder truth table

Decimal – to BCD Encoder

In Figure 4.48, a logic diagram of a basic BCD encoder has been illustrated, in which 0-digit input has not been displayed. This is because of the fact that in case of two inputs, entire BCD outputs are low. The operational mechanism of this type of circuit is as follows:

When any one of the decimal digits lines is high, then appropriate output levels result on four BCD output lines. For example, if line number 9 is high (assuming that all other input lines are low), then A₀ and A₃ output become high, whereas A₁ and A₂ output treads low. Consequently, 1001 BCD code is obtained for decimal 9.

Figure 4.48 – basic logic diagram of a decimal–to–BCD encoder. A 0-digit input is not needed because the BCD outputs are all low.  When there are no high inputs.

When line No. 1 is high, only A₀ line is high whereas rest of the lines are low. Thus, a 0001 BCD code (i.e. 0001 = A₀ A₁ A₂ A₃) is obtained on output.

Similarly, when line No. 5 is high, then A₀ and A₂ lines are high whereas A₁ and A₃ lines are low. Consequently, BCD code is obtained in 0101 form, which is obviously equivalent to decimal 5.

Applications Include:

− Keyboard Encoding
− Range Selection

Specifications:-

74147 Datasheet