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I'm new in electronics. I'm trying to make a BCD calculator, but I got stuck in BCD subtraction. I got stuck in: how can I connect multiple bcd subtrator units? I'm trying to make a 3 digit calculator, but this can only subtract 1 digit.

Can anyone help me? Thank you in advance.

PS: I've posted the circuit diagram below.

The design I'm using

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  • \$\begingroup\$ You need a borrow input for more than one digit, same as you need a carry input for a BCD adder of more than one digit. \$\endgroup\$ – Spehro Pefhany Apr 27 '17 at 17:21
  • \$\begingroup\$ First of all thank you for the answer, but how can I implement a borrow input? \$\endgroup\$ – Matteo Guglielmetti Apr 27 '17 at 17:30
  • \$\begingroup\$ Good question. That would be more of an answer than a comment and I'll leave the fame and glory to someone else. \$\endgroup\$ – Spehro Pefhany Apr 27 '17 at 17:34
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After a lot of research I built a circuit diagram which should work.

PS: Every Input wire (a0,b0,a1,b1,a2,b2) stands for 1 decimal digit (4 wires)

schematic

simulate this circuit – Schematic created using CircuitLab

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