Pass multiple 24V slow-changing binary signals over 3 wires using some multiplexer-demultiplexer

Question

I need a device which looks very simple. I've expected to just be able to buy it for few dollars, but I was not able to find one.

I need to replace multiple 24V wires that can only be on and off with only 3 wires.

The idea is very simple. Imagine there are two devices (e.g. thermostat and A/C system) connected with, say, N=8 24V wires. One wire is always +24V and the rest are possible return wires that can be connected to the hot wire using relays in the first device. The second device uses the signal wires to turn on some appliances (e.g. heater, fan, cooler etc).

I need to reduce the number of wires to 3 (or maybe even 2) while making sure everything works as before. The three wires are (+24V), Signal, [(-24V)].

On the sender end there will be a circuit that detects the binary voltages of the N=7 incoming wires, encode that information and send it over the signal wire.

On the receiver end, the controller receives the signal, decodes the bits and turns the outgoing relays on/off as needed.

The scheme looks very simple.

Are there common small cheap devices that do this?

Update: I'm not asking for a retail product. I'm asking about possible names for this kind of circuit pair, types of chips that implement this, protocols etc. I've never said anything about buying a product.

Is there an established standard for slowly reliably sending a small fixed number of bits (e.g. 8) over a single signal wire in one direction?

Is there an simple off-the-shelf chip that would decode that signal and transform it in a format understood by relay arrays?

Is there a simple off-the-shelf chip that allows converting 24V AC wire voltage "bits" into low-voltage DC signal[s] understood by the encoding chip?

Is there an established standard for controlling relay array using only a fixed number of wires (2-4)?

Answer 1

That's usually called a SERDES (serializer/deserializer) because it encodes parallel data into serial, then decodes it back into parallel on the other end of the wire. I can't quote a chip that will do what you want.

If you want to use only one data wire, then the information has to be encoded so it can be properly decoded at the other end. Just sending bits one after the other wouldn't work, because the receiver would not know which one is the first bit. There has to be some synchronization embedded in the protocol to know where the first bit starts, how long it takes, maybe error checking, etc.

One example is the oldskool serial UART.

A simple solution would be to use a cheap microcontroller on the sending side that samples its inputs and outputs the bit values on the UART, for example as an integer followed by a carriage return.

Another microcontroller on the other end would receive that, and set its output pins according to the bits.

You will need some level shifting on the input, and appropriate switches on the output, but that depends on what you need (isolation, current and voltage to switch, etc)...

Answer 2

What is needed is a multiplexer like this: https://www.phoenixcontact.com/online/portal/us?uri=pxc-oc-itemdetail:pid=2861205&library=usen&tab=1 A RS485 system can also be used. Here is a series of chips: https://www.arrow.com/en/products/search?cat=&q=74hc4051

Better yet, check out this site:https://www.fast-stat.com/