reverse polarity digital logic

[walters]

reverse polarity chip

What can cause the output to in a digital chip to have the reverse polarity when its not suppost too?

Like its suppost to be +5 volts but its output is - 5 volts , what can cause this ? any examples please?

What should i test for that would cause this?

 

[kiprono]

Some digital circuits are designed to operate with negative digital logic such that -5V represents logic 1 (TRUE) and is equivalent to +5V in positive digital logic circuits.
**broken link removed**

 

[walters]

where specific voltage levels applied to the inputs of a digital function cause corresponding voltage responses on the outputs


Truth char using voltages:
A B Y

0 Volts 0V +VCC
0V +VCC +VCC
+VCC 0V +VCC
+VCC +VCC 0Volts


74HC86D chip " positive logic"

Pin 1 is +VCC 5 volts
Pin 2 is the input for the digital signal (toggling state) square waveform
Pin 3 is the output

The output is Positive polarity square waveforms

74HC86D chip "Negative Logic"

Pin 1 is "Ground"
Pin 2 is the input for the digital signal (toggling states) square waveform
Pin 3 is the output

The output is Negative polarity square waveforms


Questions:

1.) How and why did the output convert from Positive logic to Negative logic?

2.) How else can i convert Positive logic to Negative logic without using a inverter chip?

 

[cesare]

Are you sure you are talking about TTL logic ?

There are other logic families that have negative output with respect to GND.

 

[walters]

yes why can't TTL logic have negative logic output?

Added after 1 hours 59 minutes:

Since you don't have a -5 supply, the negative polarity must mean that the output is inverted

1.) Whats the difference between TTL inverted VS TTL polarity?



If you are actually seeing a negative output on a 74HCxx chip, then either you're using +5 as the measuring reference, or your o'scope is running with the INVERT switch active on that channel.

Yes im actually seeing a negative output on the 74HCxx chip , the circuit does have a +5 supply and a -5 supply

But the way the 74HCxx chip is reversing the polarity doesn't use the -5 supply at all, it just uses the ground and +5 volts on the inputs to reverse the polarity on the output

I'm guessing since the circuit uses a bipolar power supply +5 ,ground, -5

Then you can reverse the polarity on any digital logic chips output by changing the ground and +5 volts on the inputs to any logic chip ? so the output flips the polarity?