ACCESS.bus (or A.b) is a peripheral-interconnect computer bus developed by Philips in the early 1990's. 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 in use earlier it never became as popular, largely due to less industry backing.
A.b is based on the same signalling system as Philips' I2C bus used for motherboard prototyping, extending it "out of the box". A.b defines the physical cabling and connectors, as well as adding two additional pins for +5V and ground to supply power. A.b supports only the 100kbps standard mode and 10kbps low-speed mode, and does not support the faster I2C modes. It also supports only 125 devices instead of 1024.
The idea was to define a single standard that could be used for low and medium speed devices both inside and outside the computer. A single I2C/A.b controller chip would be used inside the machine, connected to internal devices like the clock and battery power monitor. A physical connection (a plug) on the computer would then allow additional devices to be plugged into the bus using the A.b cabling and simplified controllers in the devices. This way many devices could be controlled through 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 control and text devices (modems and other low-speed inputs). Depending on how much intelligence the device needed, the interface in the device could leave almost all of the work to the driver, which allowed A.b to scale effectively to price points for low-cost devices like mice.
Although A.b mice and keyboards have been available (in limited fashion) for some time, the only serious attempt to use the system was by the VESA group. They needed a standardized medium-speed bus for communicating device abilities between monitors and computers, and selected A.b. A number of monitors with A.b connectors started appearing in the mid-90's, but this was at about the same time USB was being created and the vast majority of computer companies ignored A.b and instead waited for USB.
Compared to USB, A.b has several advantages. One is that any device on the bus can be a master or a slave, and defines a protocol for selecting which one you should be under any particular circumstance. This allows devices to be plugged together with A.b without any computer, for instance a digital camera could be plugged directly into a printer. 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 considerable additional support, which is often simply ignored. 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 is much slower than USB. Had IEEE 1394 (FireWire) been widely available at the time a computer with A.b and FireWire would have been an attractive solution for all speed ranges. As it was, USB fit nearly into the niche between the two, and with USB soon being included in the standard motherboard control chips from Intel, A.b was pushed out on the low-end.
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