Creating an OR Gate with Two TTL Output Signals

[pghislain]

Hi there,

I have a simple question.
OR gate between 2 output TTL signals can be done with diodes and simply join the signals toguether... ?

Output TTL signals have a pull up resistor included:

For one output:
---------------
Output level 1) 5 V / -4 ma
Output level 0) 0.7 V / -16ma


What happens if we join 2 wires:

TTL Output 1 ----|<------|
TTL Output 2 ----|<------|---------- TTL Input


1 + 1 = 5 V / -8 ma - okay
0 + 0 = 0,7 V / -32 ma - ... limit for the input TTL device

0 + 1 = 5 V / -4mA - the level 0, O.7 V can't provide current

I know, I can do that with TTL components.

Best Pierre.

 

[NoTime]

What happens if we join 2 wires:

It's called wired OR.
It works better if you use Open Collector (OC) devices.
The 74XX series had several OC devices.
It's not a recommended practice for non oc devices and tends to cause problems.
Don't try this with CMOS devices.

 

[oswaler]

Hello Pierre,

Thank you for your question. Yes, it is possible to create an OR gate with two TTL output signals by using diodes to combine the signals together. However, it is important to note that TTL output signals have a specific voltage and current range that they can handle. When combining two signals, the resulting output signal must still fall within this range in order to properly function.

In your example, it appears that the combined output signal for 0 + 0 (0.7 V / -32 ma) would exceed the current limit for the input TTL device. This could potentially cause the input device to malfunction or even damage it. It is important to carefully consider the specifications and limitations of the components being used when designing circuits.

Additionally, it is possible to create an OR gate using TTL components, which would ensure that the resulting output signal falls within the appropriate range. This may be a safer and more reliable option for creating an OR gate with TTL output signals.

I hope this helps answer your question. Best of luck with your project!

Sincerely,