Computer systems are usually made up of several components that cooperate with each other to achieve a desired result. Typically, the components of a computer system require electrical power in order to perform their prescribed tasks. Each of the various components has its own electrical power requirements and they are not necessarily the same. Some components require electrical power with different electrical characteristics than the power supplied to other components. For example, some components may require higher or lower voltage than other components, some components may require greater or lesser current than other components, or some components may utilize alternating current (AC) and some direct current (DC). In certain instances, it is advantageous for some of the components to have a ground reference that is isolated from the ground reference of other components.
Implicit in a scheme of components with varying electrical power requirements, such as isolated ground references, is the ability to transmit electrical power to the components in an appropriate and compatible manner. If electrical power is not transmitted correctly to the components of the computer system the components will not function accurately. Developing conventions that set forth the physical and operational characteristics of computer systems is one way to ensure that electrical power is transmitted in an appropriate and compatible manner. Typically, conventions are specified in standards promulgated by industry organizations.
An organization that specifies standards for the computer industry is the Institute of Electrical and Electronics Engineers (IEEE). One standard the IEEE has promulgated is the IEEE 1394, which primarily specifies conventions for high speed serial system communications. IEEE 1394 communication is utilized in conjunction with computer systems, audio and video products, printer and scanner products for imaging, hard disks (especially arrays) and digital video camera recorders and displays. As part of specifying conventions for high speed communications, IEEE 1394 also specifies some electrical power conventions regarding the various components involved in the high speed communications. In particular, IEEE 1394 addresses instances where some computer systems are organized in a system of layers. It is preferable to have some of the components of the computer system associated with a physical layer tied to a reference ground that is isolated from other components.
Normally it is desirable to have a link layer and physical layer of the IEEE standard that are capable of operating in an isolated mode in which the ground references of the two layers are isolated. IEEE 1394 specifies that the link layer and physical layer of a computer system be capable of operating in an isolated mode. In addition IEEE 1394 specifies that the link layer be tied to a system ground in isolation mode. In most computer systems the system ground is a chassis ground that is usually tied to earth ground. Thus, in isolation mode the physical layer ground is isolated from the chassis ground.
In the prior art, isolated ground references are achieved by utilizing at least two different electrical power supplies with ground references that are isolated from one another. One power supply feeds electrical power to some of the components and another separate power supply provides electrical power to other components that require ground references that are isolated from ground references of the first power supply. Although this prior art mechanism provides a separate power supply to the physical layer circuit, it is extremely costly because two power supplies are used. The addition of the second power supply also adds undue complexity to the overall system and increases the likelihood of system failure because multiple power systems are required.