ACCESS.bus
Encyclopedia
ACCESS.bus 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.
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:
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 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. 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. 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.
Computer bus
In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...
developed by Philips
Philips
Koninklijke Philips Electronics N.V. , more commonly known as Philips, is a multinational Dutch electronics company....
in the early 1990s. It is similar in purpose to USB
Universal Serial Bus
USB is an industry standard developed in the mid-1990s that defines the cables, connectors and protocols used in a bus for connection, communication and power supply between computers and electronic devices....
, 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.
A.b is a physical layer
Physical layer
The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
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
I²C is a multi-master serial single-ended computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, cellphone, or other electronic device. Since the mid 1990s, several competitors I²C ("i-squared cee" or "i-two cee"; 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
Motherboard
In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...
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
Protocol stack
The protocol stack is an implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them....
.
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
Device driver
In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....
. 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
VESA
VESA is an international standards body for computer graphics founded in 1989 by NEC Home Electronics and eight other video display adapter manufacturers.VESA's initial goal was to produce a standard for 800×600 SVGA resolution video displays...
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
NEC
, a Japanese multinational IT company, has its headquarters in Minato, Tokyo, Japan. NEC, part of the Sumitomo Group, provides information technology and network solutions to business enterprises, communications services providers and government....
, 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 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. 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. 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.