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Electronic Labs_87

Electronic Labs_87 - Performance Objectives A Identify a...

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Unformatted text preview: Performance Objectives A. Identify a BCD to Decimal DECODER, demonstrate that a combination of logic gates can be used to decode BCD to deci- mal numbers and verify the DECODER operation. B. Analyze some trouble symptoms of a de- coder circuit and signal trace the circuit to verify erroneous outputs. Basic Concepts 1. A decoder circuit is used to recognize the various combinations of an input word and provide an output for each com- bination. 2. If an input word contains N “bits” then the decoder can have 2” outputs. 3. A combination of gates can be used to construct a decoder circuit. 4. A BCD to Decimal decoder is used to pro— vide decimal information from a binary coded input. Introductory Information A decoder can be thought of in the opposite sense as an encoder. You should recall from the encoder exercise that a decimal number was coded so that digital operation could be performed using binary numbers. Now we will see that binary in— formation which was processed in a coded form by digital circuits can be recognized or decoded by a decoder circuit. The resulting outputs will represent the original input code. A typical ex- ample of this is a BCD to Decimal decoder. Table 6-1 shows the decimal equivalents of a four “bit” binary word. In addition it shows the states of the four inputs ABCD to a decoder required to represent decimal numbers 0 through 9. Notice that the table shows a four bit binary word can produce up to 16 distinct outputs. A general rule for decoders is that when the number of inputs is equal to a number N, the decoder will produce 2” outputs. For Table 6-1, N equals 4 and there will be 24 or 16 outputs. In this Laboratory Exer- cise, however, we need only produce one digit of a decimal number or the decimal symbols 0 through 9. Laboratory Exercise 6 THE DECODER OUTPUT DECIMAL NUMBER DIGITAL LOGIC STATE 4 2 I mmewlmwmlmlmwwmw UUUUUOUUOIUIOIUICIUIUIUIUW monomololoioooomoloiolo >>|>>l>>l>>lﬁ>>l>>l LDCONIOEU‘ob-OJNHO >D>|3>Il>l mmwlw Table 6-1 Fig. 6-1 shows a typical application for a BCD to Decimal decoder. The functions shown will pro- vide a two digit numeric readout using Nixie ® tubes as display devices. The requirement is to provide asignal to energize the proper number,zero through nine of the Nixie tube, when a certain number of input pulses are counted in Binary Coded Decimal form. We will discuss the counter and readout de- vices in later exercises and concentrate on the decoder requirements. That is, a four bit BCD word exists as inputs to the decoder and outputs zero through nine are required for Nixie tube drivers. The circuitry for a second digit is shown in block form, is identical to the first digit and accepts a pulse following each zero through nine sequence of the first digit. Simultaneously, a reset pulse within the counter will initiate the BCD 6-1 ...
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