How does A-Bus actually work?


Active Member
I know A-Bus sends the audio and amp power down the cat5, but does anyone know the gritty details that go on "behind the scenes"?

From what I've gathered, it's entirely analog, and intentionally designed to be as "low-tech" as possible to keep the cost down. I'm guessing, but with 4 pairs of wires in a cat5 cable, it looks like:

* 1 pair for right channel (hopefully, sent as a balanced signal... but disturbingly, I saw a message posted somewhere that said it's NOT balanced).

* 1 pair for left channel

* 1 pair for amp power

* 1 pair for infrared relaying

HOWEVER, it's not inconceivable that it might use two pairs of wires to carry the amp power and modulate the IR data over one of them. It would make it a lot more expensive to add IR capabilities (going against one of the designer's main goals), but would have doubled its potential amp power (a big consumer selling point).

This is purely a guess, but extrapolating from what I've read about the way the different keypads and hubs work together:

* Keypad power control is entirely a function of the hub. When a keypad is "off", the hub doesn't send it any power.

* The power/sel button is "dumb", in the sense that a power/sel "button press" is a purely mechanical event that either shorts out a normally-open pair of wires in the cat5 cable, or maybe interrupts a normally-closed circuit between the keypad and hub. Taking a stab in the dark, I'd guess that pressing power/sel shorts out the pair of wires used to relay IR, since IR events are fairly infrequent anyway, and it would be easy for the logic at the hub to differentiate between long bursts of voltage (indicating button presses) and modulated data. It's not inconceivable that a future, more sophisticated keypad made by someone might have additional buttons that, when pressed at an "on" keypad, would emulate buttons on an imaginary remote control and simply inject fake IR commands into the IR relay wires... but at the moment, nothing of the sort takes place.

The hub interprets a button press in different ways, depending upon whether the keypad is currently "on" or "off", and whether it's a multi-source hub:

* if the hub sees a button press at an "off" keypad, it begins sending power to the keypad.

* if a non-multisource hub sees a button press at an "on" keypad, it stops sending power to the keypad.

* if a multisource hub sees a button press at an "on" keypad, it cycles to the next available source.

* if a multisource hub sees a LONG button press an "on" keypad, it stops sending power to the keypad and turns it off.

* on SOME hubs, if the hub sees a LONG button press at an "off" keypad, it stops sending power to ALL of the keypads (providing a convenient way to turn off all the keypads from any keypad, like if you're leaving or going to bed).

Does this look more or less correct? Or am I *way* off?
Hi miamicanes,

Welcome to CocoonTech! I personally am not too familiar with the A-BUS innter workings, but a few people here are, so hopefully they can chime in.