# Bcd subtractor units connections

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. • 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. – Spehro Pefhany Apr 27 '17 at 17:21
• First of all thank you for the answer, but how can I implement a borrow input? – Matteo Guglielmetti Apr 27 '17 at 17:30
• Good question. That would be more of an answer than a comment and I'll leave the fame and glory to someone else. – Spehro Pefhany Apr 27 '17 at 17:34

## 1 Answer

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) simulate this circuit – Schematic created using CircuitLab