Building a digital comparator

[digitalrain]

Hello, I'm pretty new to electronics, and I need to construct a 2-bit digital comparator using TTL devices. I've really thought about this problem, but for some reason I think I'm just missing something. I assume that it would rely heavily on XOR gates, but I need some help.
Thanks

 

[ericgibbs]

Hello, I'm pretty new to electronics, and I need to construct a 2-bit digital comparator using TTL devices. I've really thought about this problem, but for some reason I think I'm just missing something. I assume that it would rely heavily on XOR gates, but I need some help.
Thanks

Hi,
Do you mean a 2 BYTE digital comparator?, if Yes, look at the HEF4585 magnitude comparator ic.

 

[digitalrain]

As simple as it should be, I actually mean a 2-bit comparator. This is a pretty early project in a course of mine.
I've looked other places online, and this seems to be so basic and obsolete that it's not really covered anywhere.

 

[Pommie]

If you just want to check for the two 2 bit numbers being the same then 2 XOR gates feeding a NOR gate will work.

Mike.

 

[digitalrain]

I'm sorry for being so vague; I'll try to state more clearly what my problem is.

I have four inputs; A0 and A1, and B0 and B1. Each input is connected to a switch so that they are either HI or LO. I need to build a device that will output HI (light an LED) only when A is higher than B. So, the truth table will look like:

A0A1\B0B1\Q
----------------
0 0 | 0 0 | 0
x 1 | 0 0 |1
1 x | 0 0 |1
0 0 | 1 x |0

and so on.

 

[Roff]

There are a couple of simple ways to break this down. If A>B, then either
A=1 and B=0
or
A=2 and B=0 or 1
or
A=3 and B=0 or 1 or 2

Another way to look at it: either
A1=1 and B1=0
or
(A1 XOR B1)=0 (MSBs are equal) and A0=1, B0=0.

Can you work out the logic from there?

 

[Zordon]

Use a LS7485 comparator it will give you an output of A=B A>B A<B
its 2 bit
i just had this question on a test building a 8 bit comparator, all you need is an EXNOR gate for a 2 bit

 

[digitalrain]

I think where I'm having problems is translating the logic of truth tables into circuit diagrams. I've mentally gotten to complete the truth table and logic that follows, but I can't seem to imagine what gates would be required. As far as EXNOR goes, I don't believe we have those in our lab, and I'm pretty sure we wouldn't be advised to use them. (Unless by EXNOR you're just referring to a combination of XOR and NOR)

 

[Pommie]

Another way to look at it: either
A1=1 and B1=0
or
(A1 XOR B1)=0 (MSBs are equal) and A0=1, B1=0.

Can you work out the logic from there?

I assume that is a typo and you meant B0.

Mike.

 

[Roff]

I assume that is a typo and you meant B0.

Mike.

Yes. I just caught that myself, and fixed it about 5 minutes ago.