Positive logic Or Negative Logic

[danny davis]

When looking at a schematic and seeing different gates, how do you know if the designer is using the gate as a positive logic gate or a negative logic gate?

Example: You can use a NOR gate either way
a positive logic NOR gate or a negative logic NAND gate.

What other examples do you know please?

 

[chuckey]

The only place that I have come across negative logic is in the " Teledac", a diode programmed computer, built in the 1960s for telemetry use. The negative logic lines were tagged with a little triangle. Its difficult enough remembering the truth table of positive logic without a second set to remember, just putting a "NOT" bar over the signal descriptions I think is enough.
Frank

 

[danny davis]

What circuits or stages uses negative logic?

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Thanks for you info

What circuits or stages uses negative logic? that is simple common circuits or you know of?

What would a designer use an Inverted Clock signal?

Why would a designer want to invert logical signals? I see inverted schmitt triggers a lot in logic circuits

Example is:
You have to invert the logic state when to light an LED , since the pull up resistor is tied to +5 volts right?

When you have to invert the logic state or signal mostly?

When I'm probing around on a logic circuit, I don't understand why a designer would want to invert the logic state or logic signal or Trigger signal or Clock signal

 

[danny davis]

Positive Logic means what combination of the inputs cause the output to be HIGH?
Negative Logic means what combination of the inputs cause the output to be LOW?

 

[chuckey]

Suppose you have a lift control circuit, the switches are:- gate closed and button pressed. So to actuate the motor, both of these conditions must be true. So if the switches provide a +5 V, or a '1' in positive logic you use a AND gate, with the output being a 1. If the switches actually provide a 0V or a '0' in positive logic, then it may be easier to call these signals 1 and use negative logic with an OR gate to give a 0V or a '1' in negative logic terms to actuate the motor.
If you have lots of inputs which are 0 V in their good states it might be worth using negative logic, I prefer to use a load of invertors and use positive logic.
Frank

 

[danny davis]

I prefer to use a load of invertors and use positive logic

Yes I see a lot of schmitt trigger invertor in logic circuits, it must keep everything in positive logic

What other invertors do u use besides schmitt triggers?

 

[chuckey]

Schmidt triggers are a specialized form of inverters. they are normally used for changing a slowly changing signal into one with fast edges, i.e. they will turn a sine wave into a square wave.
Frank

 

[danny davis]

Schmidt triggers are a specialized form of inverters. they are normally used for changing a slowly changing signal into one with fast edges, i.e. they will turn a sine wave into a square wave.

Yes True

Is this the only inverter you use or is there other inverters?

 

[godfreyl]

There are other inverters e.g. the 7404 TTL chip and the 4049 CMOS chip.