Electrical computers and digital data processing systems: input/ – Access arbitrating
Reexamination Certificate
1997-11-05
2001-02-06
Sheikh, Ayaz R. (Department: 2781)
Electrical computers and digital data processing systems: input/
Access arbitrating
C710S120000, C710S120000, C710S107000, C710S110000, C710S112000
Reexamination Certificate
active
06185651
ABSTRACT:
FIELD OF THE INVENTION
The invention generally relates to computer systems, and, more particularly, to input/output interfaces used to connect peripheral devices to a digital computer.
COPENDING APPLICATIONS
This application is the one of two patent applications filed on an even date herewith and commonly assigned, including U.S. patent application Ser. No. XX/XXX,XXX, entitled “SCSI Bus Extender Abitration Apparatus and Method.” The subject matter of the above-identified copending patent application is incorporated herein by reference.
BACKGROUND OF THE INVENTION
The well recognized Small Computer System Interface (“SCSI”) standards of the American National Standards Institute (ANSI X3.131-1986) specify the electrical, mechanical and logical characteristics of an input/output (I/O) bus interface for coupling physically small computers with other small computers and peripheral devices. Such peripheral devices may include, for example, disk drives, tape drives, printers, compact disk read-only memories, and scanners. The SCSI standards are available from the American National Standards Institute, Inc., New York, N.Y., U.S.A., and are incorporated herein by reference.
The SCSI standards generally specify an I/O bus, which commonly is referred to as a “SCSI bus.” Instead of requiring a separate bus controller, a SCSI bus utilizes a SCSI bus protocol, specified by the SCSI standards, that is implemented distributively within either central processing units, host adapters of computers on the bus, and/or controllers of peripheral devices on the bus.
SCSI bus protocol provides means for the transfer of information between a pair of devices attached to the bus. This information is usually associated with the execution of device input-output commands, such as read or write. To enable the transfer of information, the bus protocol includes an addressing mechanism for identifying the devices and establishing a pathway or connection between them for the movement of command data. The addressable entities that participate in a connection are:
The initiator: The bus device originating the connection.
The target: The bus device to which the connection is directed.
The logical unit: An I/O device attached to the target, which services the I/O command.
A bus device is physically connected to the SCSI bus and is referenced by specifying its bus I/D as described in the SCSI standard. A SCSI bus may contain up to 16 bus devices having bus I/Ds
0
through
15
. At least one bus device must be capable of performing initiator operations.
The I/O operation is executed by a logical unit attached to the target. The SCSI standard allows up to 32 logical units per target. Logical units are addressed by means of a logical unit number or LUN which is sent by special control messages during the MESSAGE IN or MESSAGE OUT phases described below. A connection is fully specified by a triplet consisting of the initiator bus ID, the target bus ID and the logical unit number of the I/O device attached to the target.
The connection between a pair of devices may have one of the following states:
ACTIVE: The connection is established over the bus such that data can be physically transferred between the source and destination.
LATENT: The connection exists but the shared physical resources needed for the transfer of data are free or in use by other connections.
NONEXISTENT: The connection does not exist.
According to the SCSI standard, a bus device alters connection state through the series of bus phases set forth below.
ARBITRATION: The process through which a bus device obtains exclusive access to the bus in order to create or reestablish a connection.
SELECTION: The process through which an initiator alerts a target device that a connection is to be formed.
MESSAGE OUT: The transfer of connection control information from the initiator to the target. During connection creation, the message data contains the identity of the logical unit, i.e. the logical unit number.
RESELECTION: The process through which the target alerts the initiator that a connection is to be reestablished.
MESSAGE IN: The transfer of connection control information from the target to the initiator. Used by the target when the connection is reactivated to send the logical unit number of the I/O device.
In the SCSI standard, the bus device creating a connection is called the initiator. The bus device to which the connection is directed is called the target. As described in the SCSI standard, a connection comes into existence through the sequence of ARBITRATION, SELECTION and MESSAGE OUT phases, referred to hereinafter as the SELECT-MESSAGE OUT operation.
After arbitrating for the bus, the initiator enters the SELECTION phase and notifies the intended target by asserting its bus I/D as specified in the SCSI standard. Upon receiving the expected target response, the initiator passes control of the bus to the target. The target then solicits the logical unit number by placing the bus in the MESSAGE OUT phase and fetching the appropriate control message from the initiator. At this point, the connection enters the ACTIVE state and data specifying the command to be processed is sent to the logical unit.
While processing a command, it is usually the case that the logical unit must perform some preliminary work, such as a seek to a particular disk sector and track, before actual data transfer can take place. Since the bus is not needed during this time, the target will release control so the bus can be used to service other connections. In this case, the target relinquishes the bus and places the connection in the LATENT state using the messages and signals specified in the SCSI standard.
When the physical transfer of data is required, the target places the latent connection in the ACTIVE state through the sequence of ARBITRATION, RESELECTION and MESSAGE IN phases specified in the SCSI standard, collectively referred to hereinafter as the RESELECT-MESSAGE IN operation.
To reactivate a connection, the target arbitrates for the bus then enters the RESELECTION phase. During this phase, the target notifies the initiator that a connection is to be reestablished by asserting the initiator's bus ID. Upon receiving the expected initiator response, the target completes reactivation by entering the MESSAGE IN phase and passing the logical unit number to the initiator. At this point, the transfer of command data can be continued. Upon completion of data transfer, the target terminates the connection by placing it in the NONEXISTENT state using the signals and messages described in the SCSI standard.
The technical benefits of SCSI-based I/O systems have led developers to build systems of ever increasing complexity and storage capacity. As the demand for storage capacity increases, the limitation on the number of addressable bus devices becomes a severe constraint on the size and storage capacity that can be achieved. For that reason, the technique for extending bus connectivity described in U.S. Pat. No. 5,274,783 (House et al.) was developed. This patent, the subject matter of which is incorporated herein, in its entirely, by reference, is assigned to Digital Equipment Company. It describes a means for increasing connectivity by using a bus bridge or extender to expand the number of bus devices that can be addressed.
A bus extender environment consists of the following elements:
i. Provisions for connection to a main SCSI bus.
ii. An auxiliary SCSI bus, to which as many as 15 targets may be attached.
ii. Logic for forming a connection between initiators attached to the main bus and targets attached to the auxiliary bus, in a manner that is transparent to the targets and initiators.
A main bus can have up to 15 extenders and each extender may have up to 15 targets attached. As a result, the number of bus devices that can be connected to an I/O system is increased from a maximum of 16 to a maximum of 241, 15 extenders each having 15 targets; plus one initiator on the main bus.
To the main bus initiator, the extender achieves transparency by presenting the auxiliar
Ham William
Lee Fee
Monia Charles
Compaq Computer Corp
Hogan & Hartson L.L.P.
Kubida William J.
Phan Raymond N
Sheikh Ayaz R.
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