Abstract: A mechanism for controlling the powering-on and powering-off of control units in a data processing system having a plurality of channels, a plurality of control units, and a communications network of links for linking the channels to the control units. Each control unit includes a power-control table for recording power-control allegiance of the control unit to the channels. Where a control unit receives a power-on command from a channel, it records the identity of the channel in its power-control table. When a channel orders a control unit to power-off, the control unit checks to see if it owes power-control allegiance to the ordering channel. If it does, the control unit deletes the identity from its power-control table. The control unit will not power-off unless its power-control table is empty, indicating that it does not owe allegiance to any other channel.

Abstract: A computer I/O system which includes a dynamic switch having a plurality of ports and a plurality of link-level facilities, with each link-level facility being attached to an individual one of the ports. Whenever the state of a port or its attached link-level facility is changed in such a way that the attached link-level facility's ability to communicate with another link-level facility might be affected, this fact is recognized by a dynamic switch control unit which causes a state change notification (SCN) frame to be transmitted to all other link-level facilities which might be affected. The link-level facilities receiving such a state change notification then determine if any of the associations or potential associations with the link-level facility which caused the state change notification are affected.

Abstract: Retrying a command from a CCW in a data processing I/O system having a channel connected to a control unit in which the channel detects an error condition and requests the control unit to retry the current command of an I/O operation. The control unit then determines if the requested retry can be accomplished, and, if it can, sends a response back to the channel requesting the channel to initiate a retry. After receiving the response, the channel checks to determine if conditions are still present which allow a retry, and if they are, executes the current command to perform the retry. If the control unit determines that a retry cannot be accomplished, the control unit has the option to perform a unit check procedure. If the control unit determines that neither a retry nor a unit check can be accomplished, the control unit has the option to perform a selective reset function.

Abstract: A link-level facility for managing the transmission of frames and continuous sequences of control characters over a link. The link-level facility includes a state machine which has a first state for allowing transmission of a frame over the link where a link-level facility at the other end of the link indicates that the link has a frame-transmitting status, and a second state for preventing the transmission of frames over the link when the link-level facility at the other end of the link has indicated that the link has a non-frame-transmitting status. The link-level facilitates signal the status of the link by means of continuous sequences of special characters intermediate frames transmitted over the link such that synchronism is maintained. The first state includes an inactive state and a working state, and the second state includes a link-failure state, and connection-recovery state and an off-line state.

Abstract: A dynamic switch and its protocol for establishing dynamic connections in a link by the use of frames, each frame having an identification of the source of the frame, an identification of the destination of the frame for the requested connection, and link controls to maintain, initiate or terminate a connection between the source and the destination. The frames are bounded by a start of frame delimiter and an end of frame delimiter which may also act as a connect link control and a disconnect link control, respectively, and the connections are made through the dynamic switch having dynamic-switch ports. The state of a dynamic-switch port is changed dependent on its present state, the dynamic connection requested, and the direction and type of frames passing through the dynamic-switch port.

Abstract: A data processing system includes a plurality of host systems and peripheral subsystems, particularly data storage subsystems. Each of the data storage subsystems includes a plurality of control units attaching a plurality of data storage devices such as direct access storage devices (DASD) for storing data on behalf of the various host systems. Each of the control units have a separate storage path for accessing the peripheral data storage devices using dynamic pathing. The storage paths can be clustered into power clusters. Maintenance personnel acting through maintenance panels on either the control units or the peripheral data storage devices activate the subsystem to request reconfiguration of the subsystem from all of the host systems connected top the subsystem. The host systems can honor the request or reject it based upon diverse criteria. Upon each of the host systems approving the reconfiguration, the subsystem 13 is reconfigured for maintenance purposes.

Abstract: Secondary storage subsystems exchange messages and data with host data processing systems and also forward messages between host systems. Host systems thereby communicate with each other in addition to having access to data in subsystem storage. Access to subsystem storage is initiated by a "request" sent from a host to the subsystem. Each request is a message containing an array of one or more commands, each command specifying a transfer of data or a control function to be performed by the subsystem. A subsystem may process more than one request at a time. It also may process the commands in a request in an arbitrary sequence suited to the availability of subsystem resources and data links to host systems. After all commands in a request have been processed the subsystem transmits an associated "completion" message to the host system which originated the request. The completion message indicates the status of completion or abnormal termination of each command in the associated request.

Abstract: A central processor through an asynchronous service processor, selectively resets an input/output channel designated in an instruction called Clear Channel that is executed by the central processor. As part of the execution of this instruction, the service processor also communicates a reset signal to the peripheral equipment associated with that channel in case the designated channel is malfunctioning and cannot relay a reset signal normally to the peripheral equipment associated with the designated channel. The reset signal for the peripheral equipment is supplied through a connection between the service processor and particular lines in I/O interface cables of that channel. Programming routines that use the Clear Channel instruction are designed to preserve the integrity of data held by peripheral equipment that is associted with two channels when the peripheral equipment has exclusive affiliations with the designated channel.