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ACCESS.bus, or A.b for short, is a peripheral-interconnect computer bus developed by Philips in the early 1990s based on their I²C system. It is similar in purpose to USB, in that it allows low-speed devices to be added or removed from a computer on the fly. While it was made available earlier than USB, it never became popular as USB gained in popularity.
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' Inter-Integrated Circuit (I²C) bus.
Compared to I²C, A.b:
- adds two additional pins to provide power to the devices (+5 V and GND)
- allows for only 125 devices out of I²C's 1024
- supports only the 100 kbit/s "standard mode" and 10 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.
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 driver. This allows A.b to scale down to price points low enough for devices like mice.
The only serious attempt to use the A.b system was by the VESA group. They needed a standardized bus for communicating device abilities and status between monitors and computers, and selected I²C because it required only two pins; by re-using existing reserved pins in the standard VGA cable they could implement a complete A.b bus (including power). The bus could then be offered as an external port simply by adding a socket on the monitor case. A number of monitors with A.b connectors started appearing in the mid-1990s, notably those by NEC, but this was at about the same time USB was being heavily promoted and few devices were available to plug into them, mostly mice and keyboards. The bus remained the standard way for a monitor to communicate setup information to the host graphics card.
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 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, and 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—A.b can support, but does not require, the use of hubs. This can reduce cable-clutter significantly.
On the downside, A.b was much slower than USB. As it was, USB fit neatly into the performance niche between A.b and Firewire. With USB soon included in the standard motherboard control chips from Intel, A.b was pushed out onto the low-end and quickly disappeared.
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