Editing ACCESS.bus (section)

==Technical standard==

A.b is a [[physical layer]] definition that describes the physical cabling and connectors used in the network. The higher layers, namely the signaling and protocol issues, are already defined to be the same as Philips' [[I²C|Inter-Integrated Circuit]] (I²C) bus.<ref name=standard>{{cite web|url=http://www.mcc-us.com/techsrc.htm |title=Technical Resources & Links |publisher=Mcc-us.com |access-date=2013-07-31}}</ref><ref>{{cite book |first=Brad |last=Hansen
|title=The Dictionary of Multimedia: Terms and Acronyms |publisher=Routledge |date=2014 |page=3 |url= https://books.google.com/books?id=lcO2AgAAQBAJ|isbn=9781135930585
}}</ref> Compared to I²C, A.b:

* adds two additional pins to provide power to the devices (+5{{nbsp}}V and GND)
* allows for only 125 devices out of I²C's 1024
* supports only the 100&nbsp;kbit/s "standard mode" and 10&nbsp;kbit/s "low-speed mode"

The idea was to define a single standard that could be used both inside and outside a computer. A single I²C/A.b controller chip would be used inside the machine, connected on the [[motherboard]] to internal devices like the clock and battery power monitor. An A.b connector on the outside would then allow additional devices to be plugged into the bus. This way all of the low- and medium-speed devices on the machine would be driven by a single controller and [[protocol stack]].<ref name=standard/>

A.b also defined a small set of standardized device classes. These included monitors, keyboards, "locators" (pointing devices like mice and joysticks), battery monitors, and "text devices" (modems, etc.). Depending on how much intelligence the device needed, the interface in the device could leave almost all of the work to the [[device driver|driver]]. This allows A.b to scale down to price points low enough for devices like mice.<ref name=standard/>

Compared to USB, A.b had several advantages. Any device on the bus could be a master or a slave, and a protocol is defined for selecting which one a device should use under any particular circumstance. This allows devices to be plugged together with A.b without a host computer. For instance, a digital camera could be plugged directly into a printer and become the master. Under (standard) USB the computer is always the master and the devices are always slaves.

In order to support the same sort of device-to-device connection, USB requires additional support in dual-role devices to emulate a host and provide similar functionality. This was only standardized years later as part of [[USB On-The-Go]] system. Another advantage of A.b is that devices can be strung together into a single daisy-chain&mdash;A.b can support, but does not require, the use of hubs. This can reduce cable-clutter significantly.<ref name=standard/>