Using DC wires to transfer signal

[loop123]

Please mention any circuit or Equpment where it uses the main 2 wire dc lines to transfer signal. How is this done? What are the pro and con?  What does the signal look like?

[boB]

PV/Solar can and does use carrier current (signals over power wires)  for various things such as rapid shutdown and detecting DC arcs, etc.

The pros are that it doesn't take an extra set of wires.  The cons are that inverter or charge controller noise can interfere with the signals.

Another con is that the control signals can interfere with the DC arc fault detection.

[themadhippy]

phantom powered microphone,although really the DC's riding on the signal lines

[Jwillis]

You can put an AC signal over DC. This is done on many circuits. Take for example the Vacuum tube amplifier. The analog signal ,which is essentially AC, rides on a high voltage DC power line.  That AC signal can ride a DC negative and/or positive. This can be done at any voltage with any type of signal whether analog or digital. The signal looks just the same as it would on it's own dedicated line. It just has a DC bias. So if the DC is for example is +5V, the signal rides that +5V the same way it would at 0V.
Pros are that you can have both signal and power on a single line. The cons are that the signal and power need to be "put together" at the source and then "taken apart" at the destination. 

[Andy Chee]

Another good example of DC + signal, is a TV masthead amplifier.  A DC injector near the power point, which sends DC into the coax up to the amplifier on the antenna mast.

In a similar vein, Ethernet over Powerline adapters are capable of transmitting data signals on top of mains AC wiring.

[MarkL]

You'll have to be more specific about your requirements, but here's a couple...

M-Bus is a 2-wire system that provides DC power to an end device and and also communicates over the same two wires.  It's arranged as a bus with one master and multiple slave devices.  The master supplies power.

Data from the master to a slave is done by varying the voltage a little bit, and responses from the slave is sent by varying the current.

  https://m-bus.com/documentation-wired/04-physical-layer

It supports 300 to 9600 bps.


The Dallas/Maxim 1-wire is another protocol that operates over two wires that can also provide power (they call it "parasite power") and communicates bidirectionally:

  https://en.wikipedia.org/wiki/1-Wire

Typically the end device is one or more chips, such as temperature sensors, arranged on a bus.  The chips power down between interrogations unless they have their own local power.  Each chip has its own unique address.  The protocol supports (approx) 14.4bps or 100kbps and has device discovery built into the protocol.

[pqass]

The 4–20 mA current loop is an old yet reliable way to provide sensor power with return of an analog signal.  Or, it can transmit analog position information to a remote actuator (like a valve).

For example, a 24V DC source supplies a miles-long wire pair to a remote temperature sensor.  The sensor will change its current draw based on the local temperature.  The other end receives the temperature information by measuring the voltage drop across a 250R resistor which corresponds to 1-5V (4ma-20mA) into an ADC.

The advantages provided are: (a) use of only a single wire pair, (b) the ability to power the remote end, (c) immunity to noise or voltage drop since the information is in the form of a current, and (d) fault indication if current draw is outside the 4-20 mA bounds.

Additionally, HART protocol is a backward-compatible enhancement to 4-20 mA instrumentation that allows two-way serial communication with smart, microprocessor-based field devices.

[CaptDon]

We sensed 4-20ma. across a 100 ohm resistor giving .4 to 2.5 volts which worked well with 8 bit A to D converters giving a decimal conversion of 40 to 250 which fit perfect into a max count of 256. This number also directly related to the sense voltage and current just by moving the decimal point. As for D.C. signaling look at the old Motorola remote located 2-way radio systems run via telco 'D.C. Pair'. There were 4 D.C. signal levels +20ma, +40ma, -20ma  and -40ma. This was used to select 1 of 2 receive channels and 1 of 2 transmit channels and voice audio rode on the same D.C. pair. We also had an 'equalized pair' for broadcast quality audio which in reality I think used 2 copper pairs with some sort of frequency shifting scheme by the telco. Distance from studio to remote mountain top transmitter site was around 6 drive-able copper pair miles and about 2.5 miles line of sight. All of this was later replaced with 900MHz digital audio / data linkage. B.T.W., of the 25 copper pairs going up the mountain at least half were destroyed by lightning / EMP and were either shorted and couldn't be blown open or were extremely unbalanced to ground. It was a constant problem in the summer and also during 'thunder snow'.

[Jason Henry]

In some simple two-wire DC circuits, signals are transferred using techniques like voltage modulation. While cost-effective, these setups may face challenges in signal integrity over long distances. The signal typically appears as variations in voltage levels.

[Picuino]

UART signal + ground line.
At standby it delivers 5V.
When operating to send information, it delivers 5V and 0V pulses.
In a protocol with parity bit and stop bit, the signal will be at least 10% of the time at high level (5V), from which the power supply must be extracted.

Sounds bizarre, but it works. Although this should really be called "Using signal wires to transfer DC current".

[thermistor-guy]

Please mention any circuit or Equpment where it uses the main 2 wire dc lines to transfer signal. How is this done? What are the pro and con?  What does the signal look like?

Old-fashioned telephones, powered by DC via twisted pair wiring from a telephone exchange.