A Project Report Ondecimal to bcd encoder the term uncompressed BCD usually implies a full byte for each digit (often including a sign). 12. For example. To illustrate this in a simple manner. but it has active-low outputs..g.INTRODUCTION Encoders are the opposite of decoders. you get HHLL. error or overflow). The 74LS147 provides the same basic function as the circuit shown in Fig. Special bit patterns are sometimes used for a sign or for other indications (e. If you do not like negative true logic. This means that instead of getting an LLHH output when “3” is selected. and the other (the 74LS147) is expressed in negative true logic.e. 12.) The truth table in Fig.40 shows a 74LS147 decimal-to-BCD (10-line-to-4-line) priority encoder IC.39 explains the rest. one is expressed in positive true logic. you can slap inverters on the outputs of the 74LS147 to get positive true logic. In computing and electronic systems. Figure 12. They are used to generate a coded output from a single active numeric input. binary-coded decimal (BCD) is a class of binary encodings of decimal numbers where each decimal digit is represented by a fixed number of bits. negative true logic is useful when the device that you wish to drive uses active-low inputs. In this circuit. In byte-oriented systems (i. the switch corresponding to that decimal is closed. although other sizes (such as six bits) have been used historically. To generate a BCD output that is equivalent to a single selected decimal input. The choice to use positive or negative true logic really depends on what you are planning to drive. as in the previous encoder. usually four or eight. let’s take a look at the simple decimal-to-BCD encoder circuit shown below.39. The two outputs represent the same thing (“3”). whereas packed BCD . (The switch acts as an active-low input. normally all lines are held high by the pull-up resistors connected to +5 V. most modern computers). Although BCD is not as widely used as in the past. BCD's main virtue is a more accurate representation and rounding of decimal quantities as well as an ease of conversion into human-readable representations. BCD's principal drawbacks are a small increase in the complexity of the circuits needed to implement basic arithmetics and a slightly less dense storage. BCD was used in many early decimal computers. decimal fixed-point andfloating-point formats are still important and continue to be used in financial. and industrial computing. The precise 4-bit encoding may vary however. seeExcess-3 for instance. . As compared to binary positional systems.typically encodes two decimal digits within a single byte by taking advantage of the fact that four bits are enough to represent the range 0 to 9. for technical reasons. commercial. where subtle conversion and rounding errors that are inherent to floating point binary representations cannot be tolerated. DECIMAL to BCD ENCODER DIGRAM . DECIMAL to BCD ENCODER TRUTH TABLE . 74LS147 decimal-to-4-bit BCD Priority Encoder IC . 5.Another important difference between the two encoders is the term priority that is used with the 74LS147 and not used with the encoder in Fig. The circuit shown in Fig. again. if two or more inputs are applied at the same time. As with the 74LS147. it will only select the larger-order digit. The active-high BCD code is applied to inputs A through E. The transformed BCD is then fed into a 7447 seven-segment LED display decoder/driver IC. only the higher order number is selected. if 3. With the non priority encoder. 12. The term priority is applied to the 74LS147 because this encoder is designed so that if two or more inputs are selected at the same time. 12. that if two or more inputs are selected at the same time. For example. .42 shows a 74148 octal-to-binary priory encoder IC. the 74148 comes with a priority feature. complement and ten’s complement. The truth table in Fig. only the 8 (negative true BCD LHHH or 0111) will be output. 12. The G input is an active-low enable input. the output will be unpredictable. The 74LS147 encodes a keypad’s input into BCD (negative logic).41 provides a simple illustration of how an encoder and a decoder can be used together to drive an LED display via a 0-to-9 keypad. It is used to transform a specified single octal input into a binary 3-bit output code.40 demonstrates this look at the “don’t care” or “X” entries. Figure 12. and 8 are selected at the same time. A set of inverters then converts the negative true BCD into positive true BCD. meaning.39. Early models of the PlayStation 3 store the date and time in BCD.Application The BIOS in many personal computers stores the date and time in BCD because the MC6818 real-time clock chip used in the original IBM PC AT motherboard provided the time encoded in BCD. . [4] The Atari 8-bit family of computers used BCD to implement floating-point algorithms. This form is easily converted into ASCII for display. The MOS 6502 processor used has a BCD mode that affects the addition and subtraction instructions. rendering most functionalities inoperable. The last two digits of the year stored as BCDwere misinterpreted as 16 causing a paradox in the unit's date. This led to a worldwide outage of the console on 1 March 2010. gray code) Text (ASCII) Sound (.wav. .tiff) .Conclusions Bits are bits • Modern digital devices represent everything as collections of bits • A computer is one such digital device You can encode anything with sufficient 1’s and 0’s Binary codes (BCD. .jpg.mp3..) Pictures (.gif.. . . htm.ecu. http://publib.org/deliver/etsi_etr/001_099/060/02_60/etr_060e02p. http://speleotrove.References 1. ^ http://www.ibm.com/decimal/.com/iseries/v5r2/ic2924/books/c0925083170. ^ "ILE RPG Reference".au/units/ens1242/lectures/ens_Notes_08. .edu.etsi. http://www.1) in telecommunication application protocols". 2.pdf.boulder. ^ IBM BM 1401/1440/1460/1410/7010 Character Code Chart in BCD Order 4.se. 3. ^ "Signalling Protocols and Switching (SPS) Guidelines for using Abstract Syntax Notation One (ASN. ^ "General Decimal Arithmetic".pdf 5.