From the Fairchild Fast Applications Handbook, 1987, page 5-38:

Converting an 8 Bit BCD number into a 7 Bit binary number:

The BCD inputs are weighted like this:

10=2+8

20=4+16

40=8+32

80=16+64

Since 2,4,8,16,32,64 are pretty straight binary numbers,

it's easy to see why one BCD input signal connects to two

adder inputs in the schematic above... at least for me.

For 8 Bit binary to BCD conversion, you better use EPROMs,

sorry.

Found some neat schematics in:

TTL Kochbuch, TI Germany, 1980,

ISBN 3-88078-002-1.

Once there was another concept, called Excess 3.

Basically it looks like adding 3 to the BCD code.

Point is, that the book claims, that using this sort

of code simplifies decimal adder circuits.

If you happen to have 4 Bit binary adders,

converting BCD to Excess 3 and back is simple:

Converting BCD to Excess 3 by using logic gates:

Unfortunately, I had no example for converting

Excess 3 back to BCD by using logic gates.

Here we go:

Schematic for converting Excess 3 to BCD:

An alternative version for generating the B2 signal:

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(c) Dieter Mueller 2012