When do I need to use an OR gate IC?

Question

I'm currently designing a logic circuit to compare two 4-bit numbers using an XOR gate IC (TI CD4070B).

I'll then need to combine the output of these four XOR gates to into a single line using OR gates. If this combined feed returns a 1, then I'll know the numbers aren't equal.

My question is: do I need to use a dedicated OR gate IC, or can I simply wire together the outputs of the four XOR gates? My main concern is if one of the XOR gates returns a 1, then voltage will be back-fed into the outputs of the other three XOR gates (potentially damaging the chip or spreading back to the inputs of the XOR gate?).

Answer 1

You should not wire outputs together, ever (excepting open drain types or special types designed for such use). Push-pull outputs will "fight" if any one is at a different output voltage to the others, draw excessive current, and will not necessarily provide valid or correct logic levels at the output.

In order to combine the 4 XOR gates you could use a 4-input OR gate (eg. CD4072) or a 4-input NOR gate eg. CD4002 (which would have an inverted output compared to the OR gate).

As implied by the lower number, NOR is a more common function in 4000-series CMOS than OR, but both are still easily available.

Answer 2

You could form a simple diode resistor 4 input or gate like this.

^{simulate this circuit – Schematic created using CircuitLab}

Note that there will be a 0.7V drop across the diodes when an input is in the high state. For a smaller 0.3V drop across the diodes use BAT85 diodes. Saves using another I.C.