# Is possible for information in a logical circuit to travel in the opposite direction as the current? [closed]

Is possible for information in a logical circuit to travel in the opposite direction as the current?

I'm asking about theoretical possibilities, not the current status. For example, if circuits used alternating current (AC), could they perform logical processing during both the positive and negative phases of the same cycle?

• It is actually very common for current and information to move in different directions. PCIe for example actually encodes ones and zeros in the direction that current flows, so half of the time each line in a pair will have data moving opposite the current. Commented Jun 16 at 2:43
• Welcome to SE/EE! Please take the tour to learn how this site works, and read "How to Ask". Then come back and edit your question to clarify. What current direction do you mean? The technical current flows from positive to negative, but the physical current in the other direction. Do you have a specific scenario in mind? Commented Jun 16 at 5:36
• @the busybee As I understand, alternate current flows in both directions, and it doesn't matter what sign you assign to each direction. I always assumed that in CPU's the current flows in the same direction as the calculations, and I wonder if it could go the other way, or use alternate current. I don't know enough to understand if my question has a problem, how to write it better, or why you are implying that it needs clarification. Commented Jun 16 at 8:27

Is possible for information in a logical circuit to travel in the opposite direction as the current?

Yes.

(You assume silently that information is encoded in current. This is just one of many possible options, and not that common.)

Let's start with a simple definition of "information." The smallest possible information uses the Boolean value with its set of false and true. The size of the set is 2.

To encode this information with electrical representation, we can use whatever we see fit. For example:

• 2 different voltages. It does not matter whether this is DC or AC, and in case of DC the polarity.
• 2 different currents. Again, AC/+DC/-DC does not matter.
• 2 different frequencies. It does not matter whether the representation is voltage or current, and if it is AC or has a DC offset.
• 2 different pulse widths.
• ... [There are more.]

The important points are: One part generates the signal. Other parts read the signal.

The information always flows from the generator to the reader. The direction of the electrical current does not matter.

I'm asking about theoretical possibilities, not the current status. For example, if circuits used alternating current (AC), could they perform logical processing during both the positive and negative phases of the same cycle?

Yes, they can.

Assume an encoding of the Boolean value as "normal" polarity (for example, first the positive phase, second the negative phase) for true, and "inverse" polarity for false. Then a logical processor can use both phases.

Or let's look at the function of the lights in the hall, which have multiple toggle switches. The mains power is AC, and the switches are built and wired in a way that the lights are on only under certain conditions. The logical processing (exclusive-or in this case) is done independently from the polarity or phase.

(In your comment) I always assumed that in CPU's the current flows in the same direction as the calculations, and I wonder if it could go the other way, or use alternate current.

By far the most CPUs use voltages to represent Boolean values. These days nearly all CPUs are realized with MOSFETs that have an input capacity.

For the upper voltage level, the generator of the signal needs to charge that capacitor and current flows technically to the signal reader (you call this "in the direction of the information") for a short time.

For the lower voltage level, the generator of the signal needs to discharge that capacitor and current flows technically to the signal generator (you call this "against the direction of the information") for a short time.

Which voltage to use for what Boolean value is not fixed, and both variants are common.

Certainly. An example is the 1-wire interface with a power supply line that also functions as a data line. If a device wants to transmit a 1, it pulls the line down for 1-15μs. If it wants to transmit a 0, it pulls the line down for 60μs.

Yes.

All it takes to match your definition is to send information by sinking current. So any open-drain output driving a LED or relay or some other load or sending data by sinking/pulling current instead of sourcing/pushing current.

Includes output such as dry contacts and pushbuttons.

And many circuits already use AC signal to synchronize what they are doing. For example microcontrollers use a clock signal. Some circuits perform actions on both edges of the clock. For example DDR memory buses and chips use both edges of the clock.