# Can logic level be based on polarity?

#### TtOoWwAa

##### Member
Hi,

Can you have a logic level circuit that has one voltage, only, but two states, depending on the polarity of the current instead of if the voltage is low or high.

I ask this out of curiousity because diodes help prevent the backflow of electrons,
so therefore I assume that you can deliberatley cause the current to backflow if you wanted to use it to represent two states (I/0) as opposed to having circuitry that changes the voltage itself.

I know that wikipedia says the following: " Logic levels are usually represented by the voltage difference between the signal and ground, although other standards exist. "
Maybe it's just talking about active high or active low representation.

I have no idea if this would even be useful or not but I don't think what i'm talking about is the same thing as relay logic.

PS, I've been away from ETO a while but I'm back.

Regards, Tt

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#### crutschow

##### Well-Known Member
Can you have a logic level circuit that has one voltage, only, but two states, depending on the polarity of the current instead of if the voltage is low or high.
Sure, but it just means the voltage is changing two times the logic voltage instead of just equal to.
For example, a 0 to +5V logic change is a 5V difference, whereas a +5V to -5V change is a 10V difference.

So there is no advantage to do this. and the circuitry to do it would be more complex.

#### TtOoWwAa

##### Member
Sure, but it just means the voltage is changing two times the logic voltage instead of just equal to.
For example, a 0 to +5V logic change is a 5V difference, whereas a +5V to -5V change is a 10V difference.

So there is no advantage to do this. and the circuitry to do it would be more complex.
Ohhh, I thought the polarity and voltage didnt effect each other so was assuming you could have say +5v in either polarity. It makes perfect sense now!.

So the backflow of electrons means it is being made into a negative voltage? (ignoring all the antics about electron v conventional current etc)
I assume you can use a diode as a saftey machanism to prevent curent from accidentally going positive too?

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#### Pommie

##### Well-Known Member
Google differential pair. Is that what you mean?

Mike.

#### crutschow

##### Well-Known Member
I assume you can use a diode as a saftey machanism to prevent curent from accidentally going positive too?
You use a diode anywhere you want current to flow in only one direction.
They allow (conventional) current to flow from anode to cathode (direction of the diode arrow on a schematic) and block (not just help prevent) current flow in the opposite direction (see typical diode plot below).
(Note that the forward and reverse voltage display is not to the same scale).

They are used to rectify AC current to DC, block the accidental reverse connection of power to a circuit, suppress the inductive transient of an inductor, etc.

#### TtOoWwAa

##### Member
Google differential pair. Is that what you mean?

Mike.
Maybe, I'll do some more googling

#### Diver300

##### Well-Known Member
The RS232 standard for communication uses logic 0 of +3 to +15 V, and logic 1 of -3 to -15 V. It is not differential. There is only one data wire for each direction.

The standard was set in 1960. It was common for communication between computers and modems. The 9 pin D connector on the back of PCs is RS232.

It has largely been superseded, with USB doing most of what it was used for, but it has been used very extensively.

#### Pommie

##### Well-Known Member
I'm so used to RS232 being TTL levels now that I'd forgotten it was originally ±15V. Com ports seem to be making a comeback due to Arduino boards.

Mike.
P.S. To the OP, RS485 is the differential version of RS232.

#### TtOoWwAa

##### Member
So, here's something I don't understand:

Apparently you shouldn't use direct current to drive an LCD display, so therefore you have to use a negeative voltage hooked up to VCC and a negative voltage hooked up to GND which means that the average voltage should always be 0v as long as GND and VCC are the same voltage but just respective of + and -.

But then someone used 0 and 5v meaning DC with an average voltage of 2.5v:

Is he doing it wrong or did I miss something?

Regards, Tt

#### gophert

##### Well-Known Member
He is not doing it wrong cause tat is an LCD MODULE. The module has a driver chip that creates the plus/minus voltages for you. The connections he made are just for power and data to the industry standard LCD character display driver chip.