Converting binary to BCD with basic gates

I was wondering if anyone knows any logic that can convert Binary to BCD with the basic gates, ie AND, OR, XOR, etc.

I know how to add BCD, convert to and from decimal, but I cannot figure out this seemingly simple task.

I would like to know so that I may incorporate it into my relay computer...

Thank You

Re: Binary to BCD

you should be able to do the transformation by first doing a mapping. Use the truth tables and the graphical method for each output bit.

Many years ago there were commercial ICs for this. Try looking through an old TTL data book. They will have diagrams of the logic inside the chips. 74185 is one example.

[added later] The 74185 turns out to really be a ROM. http://www.geocities.com/templarser/registers.html shows several circuits for conversion.

One stupid question, Why not do the math in binary and save the relays that to the conversion?

[added even later] http://electro-tech-online.com/viewtopic.php?t=14362&view=next is another discussion. It claims that the circuitry would be very large. Older logic circuits were serial and clocked to save on parts. You should look into this to save on relays. I think the early HP pocket calculators were also serial.

Binary to BCD

If the number is less than or equal to 9 then obviously binary is equivalent to BCD.
If the number is 10 and above then add 6 to it.
And the last 4 digits of this number's binary form will give your BCD representation's unit's digit.
For example say you want BCD of 12.
Add 6 to it and you get 18.
18 has binar representation given by 10010.
Taking the last 4 digits we get 0010 which is the binary representation of 2.

For BCD representation of numbers from 20 to 29 add 12 to the number and perform the same operation. Similarly you can work with other numbers.

Re: Binary to BCD

Older logic circuits were serial and clocked to save on parts. You should look into this to save on relays.

Click to expand...

The problem, however, is that the savings on relays of doing it serially is that I would have to make a shift register to convert to serial. Everything is parallel. Another problem is clocking. My clock frequency is ~ 1hz (clock is a car turn signal flasher). This means that converting large numbers may take minutes!
However, a relay shift register is basically two relays per bit. The adder I can do with four per bit. This means that it is not as comlex as one would think. Also, I can reduce large blocks of logic gates down to a few relays. For instance:
The XOR normally consists of an AND gate, an OR gate, and a NAND gate. I can do one with two relays.

Therefor, I think that all shall be ok, and I thank you for the help.