A computing system may use an interface to connect to one or more peripheral devices, such as data storage devices, printers, and scanners. The interface typically includes a data communication bus that attaches and allows orderly communication among the devices and the computing system. A system may include one or more communication buses. In many systems a logic chip, known as a bus controller, monitors and manages data transmission between the computing system and the peripheral devices by prioritizing the order and the manner of device control and access to the communication buses. Control rules, also known as communication protocols, are imposed to promote the communication of information between computing systems and peripheral devices. For example, Small Computer System Interface or SCSI (pronounced “scuzzy”) is an interface, widely used in computing systems, such as desktop and mainframe computers, that enables connection of multiple peripheral devices to a computing system.
In a desktop computer SCSI enables peripheral devices, such as scanners, CDs, DVDs, and Zip drives, as well as hard drives to be added to one SCSI cable chain. In network servers SCSI connects multiple hard drives in a fault-tolerant cluster configuration in which failure of one drive can be remedied by replacement from the SCSI bus without loss of data while the system remains operational. A fault-tolerant communication system detects faults, such as power interruption or removal or insertion of peripherals, allowing reset of appropriate system components to retransmit any lost data.
A SCSI communication bus follows the SCSI communication protocol, generally implemented using a 50 conductor flat ribbon or round bundle cable of characteristic impedance of 100 Ohm. SCSI communication bus includes a bus controller on a single expansion board that plugs into the host computing system. The expansion board is called a Bus Controller Card (BCC), SCSI host adapter, or SCSI controller card.
In many systems, a capability to detect attachment of a cable or connector is useful. For example, a system capable of detecting whether a device is attached at the end of a transmission line is useful to supply proper termination impedance to the line. In a specific example, a commonly used parallel input/output (PIO) system for computers, the SCSI protocol interface, requires termination at each end, and only at each end, in a chain of devices. Despite some standardization, many proprietary variations, proposed extensions, and improvements exist that make uncertain the actual configuration of a system. SCSI signal lines may be single ended or differential, either low voltage differential or high voltage differential. Furthermore, a variety of termination alternative exist such as passive termination internal to a device, typically socketed or jumpered for removability, or active termination internal to a device. Other termination alternatives include manually switchable or automatically switchable internal termination, either active or passive, or external termination requiring an additional external connector with termination circuitry plugged into the extra external connector.
The multiple connector and termination schemes have led to confusion and the possibility of excessive termination within a device chain. Specifically, a user typically cannot determine from external examination whether a particular device has an internal termination and whether any internal termination is socketed, jumpered, or switched, either active or passive. If a terminator is missing, or a terminator is enabled when improper, the SCSI bus may not function reliably.
Plug and Play SCSI standard attempts to simplify connector and termination configurations by specifying one standard connector for external devices and specifying that termination for external devices are external to the devices. Specifically, active external termination is required with terminator power supplied by a designated line in the SCSI bus. Each external device must have two visible external connectors. When external devices are chained, only one connector can remain open and the open connector must receive the one external active termination circuit. This simplification still requires manual intervention, requires a separate part with additional cost, and creates a risk of performance loss if the part is lost. A customer must purchase a separate terminator plug, including active circuitry and a connector, and properly install the terminator plug on the one open external device connector.