reverse polarity digital logic

beenthere

Joined Apr 20, 2004
15,819
The 74XX86 is a quad two input XOR gate. Positive logic, no possibility of a negative output, assuming +5 to pin 14 and ground to pin 7. Are you certain of the negative voltage output? Does it measure -5 volts with respect to ground (or at least pin 7)?

No 74HCxx chip can handle 10 volts across it. The factory smoke comes out very quickly. Absolute maximum voltage across Vcc and ground is +6 volts.
 

Thread Starter

relicmarks

Joined Oct 13, 2006
355
NO , your getting lost

Look at TRUTH TABLES for AND, OR, NOT, NAND, NOR, EXOR and EXNOR gates

The supply voltage for VCC for these truth table are based on +5volts VCC


Now Try to make TRUTH TABLES for AND, OR, NOT, NAND, NOR, EXOR and EXNOR gates that are "negative polarity" but are still based on +5 volts VCC

example:

AND gates Truth Table is
(positive logic)

A input is ground
B input is ground
output is ground

A input is ground
B input is +5 volts
output is ground

A input is +5 volts
B input is ground
output is ground

A input is +5 volts
B input is +5 volts
output is +5 volts

--------------------------------------------------------------------------

AND gates Truth Table is
(Negative logic)

A input is
B input is
output is negative -5 volts

1.) How would you get -5 volts on the output of a AND GATE?

2.) The input voltages are +5 volts

Goal is trying to make the AND gate into a "polarity converter"
 

Thread Starter

relicmarks

Joined Oct 13, 2006
355
OR gates Truth Table is
(Negative logic)

A input is
B input is
output is negative -5 volts

1.) How would you get -5 volts on the output of a OR GATE?

2.) The input voltages are +5 volts

Goal is trying to make the OR gate into a "polarity converter"



NOT gates Truth Table is
(Negative logic)

A input is
B input is
output is negative -5 volts

1.) How would you get -5 volts on the output of a NOT GATE?

2.) The input voltages are +5 volts

Goal is trying to make the NOT gate into a "polarity converter"



NAND gates Truth Table is
(Negative logic)

A input is
B input is
output is negative -5 volts

1.) How would you get -5 volts on the output of a NAND GATE?

2.) The input voltages are +5 volts

Goal is trying to make the NAND gate into a "polarity converter"


EX-OR gates Truth Table is
(Negative logic)

A input is
B input is
output is negative -5 volts

1.) How would you get -5 volts on the output of a EX-OR GATE?

2.) The input voltages are +5 volts

Goal is trying to make the EX-OR gate into a "polarity converter"
 

Caveman

Joined Apr 15, 2008
471
1.) How would you get -5 volts on the output of a AND GATE?

2.) The input voltages are +5 volts

Goal is trying to make the AND gate into a "polarity converter"
You cannot get -5V directly out of an AND gate if it is powered by +5V and GND. You cannot drive it by +5V and -5V because the chip will blow. If you drive it with GND and -5V, and the +5V input will overdrive the input.

But. Let's say you have an inverter that is powered by GND and -5V. Then you do what is in the schematic here.
+5V in will output -5V.
0V in will output 0V.

EDIT:
Actually, I screwed the image up. Replace the bottom resistor with a 5k and the top with a 15k. Then, when the input is +5V, the inverter input will be -2.5V which is above the VIHmin for the inverter. When the input is 0V, the inverter input will be -3.75 which is below the VILmax for the inverter. So the above works.
 

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beenthere

Joined Apr 20, 2004
15,819
Is there some deeper purpose to wishing for something that is not possible? Get data sheets on any gate, TTL or CMOS. They use a positive Vcc & output 0 volts for low and +5 for high (or more if 4000 series CMOS).

They do not and can not have a negative voltage at the output as a result of any non-destructive input.
 

Thread Starter

relicmarks

Joined Oct 13, 2006
355
So you just put the pull up resistor to negative -5 volts on the input


If you put a pull up resistor with + 5 volts on pin#1 input to the 74HC86D chip
then put a pull up resistor with +5 volts on pin#2 input on the 74HC86D chip

When i put a pulse waveform on pin#2 the output(pin3) is a toggling +5 "Positive pulse"

When i put a GROUND to the pin#1 on the 74HC86D chip
then put a pull up resistor with + 5 volts on pin#2 on the 74HC86D

When i put a + 5 volt pulse waveform on pin#2 input the output ( pin#3) is a toggling negative -5 volts " negative pulse"

How and why did it convert and change polarities?
 

beenthere

Joined Apr 20, 2004
15,819
Why are you using a pull-down resistor on the output? It's a pull-down if it's tied to a negative voltage. It's not an open-drain chip.

What is the value of the resistor? If I had a current-limited positive voltage source and tied it to a negative voltage through a small value resistor, the output would certainly go negative.

The question remains - why is it important to you to do this?
 
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