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bcd2_7.htm
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<HTML><HEAD>
<TITLE>Excess 3, BCD, binary</TITLE>
</HEAD>
<BODY>
<P><CENTER><H2>Excess 3, BCD, binary</H2></CENTER>
<HR>
From the Fairchild Fast Applications Handbook, 1987, page 5-38:<BR>
Converting an 8 Bit BCD number into a 7 Bit binary number:
<P><IMG SRC="bcd2_bcd2bin.png">
<P>The BCD inputs are weighted like this:<BR>
10=2+8<BR>
20=4+16<BR>
40=8+32<BR>
80=16+64
<P>Since 2,4,8,16,32,64 are pretty straight binary numbers,<BR>
it's easy to see why one BCD input signal connects to two<BR>
adder inputs in the schematic above... at least for me.
<P>For 8 Bit binary to BCD conversion, you better use EPROMs,<BR>
sorry.
<HR>
<P>Found some neat schematics in:<BR>
TTL Kochbuch, TI Germany, 1980,<BR>
ISBN 3-88078-002-1.
<P>Once there was another concept, called Excess 3.<BR>
<P><IMG SRC="bcd2_x3t.png">
<P>Basically it looks like adding 3 to the BCD code.<BR>
Point is, that the book claims, that using this sort<BR>
of code simplifies decimal adder circuits.
<P>If you happen to have 4 Bit binary adders,<BR>
converting BCD to Excess 3 and back is simple:
<P><IMG SRC="bcd2_xbc.png">
<HR>
<P>We could draw the Excess 3 to BCD conversion a bit different:
<P><IMG SRC="bcd2_xcb.png">
<HR>
<P>Converting BCD to Excess 3 by using logic gates:
<P><IMG SRC="bcd2_b2x.png">
<HR>
<P>Unfortunately, I had no example for converting<BR>
Excess 3 back to BCD by using logic gates.
<P>Here we go:
<P><IMG SRC="bcd2_xbt.png">
<P>Schematic for converting Excess 3 to BCD:
<P><IMG SRC="bcd2_x2b.png">
<P>An alternative version for generating the B2 signal:
<P><IMG SRC="bcd2_x2ba.png">
<HR>
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<P>(c) Dieter Mueller 2012
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