ACCESS.bus (or A.b) is a peripheral-interconnect computer bus developed by Philips in the early 1990s. 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 than USB, it never became popular, largely due to considerably less corporate backing in the industry.
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 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.
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 bus for communicating device abilities between monitors and computers, and selected I²C because it required only two pins; by re-using existing "reserved" pins in the standard VGA they could implement a complete A.b bus (including power). 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. The bus remained the standard way for a monitor to communicate setup information to the host graphics card.
Compared to USB, A.b has several advantages. One is that any device on the bus can 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 the dual-role devices in order to emulate a host and provide similar functionality. 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 (also known as FireWire) been widely available at the time, a computer with both A.b and FireWire would have been an attractive solution for all speed ranges. As it was, USB fit neatly into the niche between the two. 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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