Is any signal ever one way

Question

I am trying to confirm a suspicion/understanding about electrical signals, and especially One-way CONTROL signals i.e. specifically NON-Power type signals in electronics like,

• Trig/Echo pin in UltraSonic Sensor,
• Digital IO line on an application specific IC/Sensors feeding into a microcontroller via high speed buses.
• High Speed USB Camera connected via Serial Tx/Rx

which DO not EXPLICITY require a DEDICATED Ground (unlike power signal which requires a ground connection to function), but IMPLICITLY always return via Ground/PCB Ground Plane to the point of origin.

In short, what I am trying to confirm is that, no signal (control or power) is ever one way, and MUST find a way back to its origin to complete its journey. In practice, we observe this on PCBs with ground planes, as every signal will find its own path back, usually right under the trace, on the ground plane underneath.

Answer 1

A physicist would say that "you are right - every signal transferring method which supplies a voltage between 2 wires from one device to another along 2 wires uses 2 wires". That's common solution because 2 wires is a technically simple way to transfer energy which actually travels in the space as electromagnetic wave between and around the wires. The wires only guide the wave. The current in the wire is a necessary interaction between the wires and the fields. It gives to the wires the ability to guide waves, but the energy isn't packed to the electrons in metal, it is in the space outside the metal.

For very short distances a transformer is an useful way to put the fields to affect without a wire connection between 2 devices.

A little more complex transfer method to use electromagnetic waves which propagate without any wires between transmitting and receiving antennas.

Somehow intermediate way is to use transmission lines which have only 1 conductor like a waveguide (=tube which keeps the wave inside) or Goubau-line (= a single line without a pair which keeps the wave symmetrically around one wire)

Most of us here are practical electricians. We avoid thinking the electromagnetic fields. If the signal frequency is low enough and the dimensions of our system are small enough we can calculate what happens in our systems very accurately with circuit theory. It uses voltages and currents but doesn't care how those things are related with electromagnetic fields. That's handy because e.m fields are complex 3D vector fields, a current flows along a wire and a voltage is a number between 2 points - much simpler. Physicists have created the circuit theory for us who like to keep our thoughts simple and still get some work done.

Answer 2

no signal(control or power) is ever one way, and MUST find a way back to its origin to complete its journey.

Your understanding is correct. While omitted for simpler diagrams, a closed return path is required for current flow (not considering charge migration in electrostatics when 2 bodies come in contact).

For example, when you use a regular voltmeter you connect it between 2 points, and a current (albeit very small) must flow through the meter for it to measure voltage.

Answer 3

There are a few situations where there’s no return path - radio transmissions being one and the likes of touch sensors that rely on ‘body capacitance’ but in most electronic circuits a return path of some kind is required.