JPH0895901A - Bus switching method and bus switching device - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a bus switching method and device capable of connecting input / output control devices having a predetermined number of identifiers or more by a SCSI bus protocol. [Structure] Input / output control devices 21 to 2n, input / output devices 31 to 31
37 is individually connected to the bus switching device 10 by the bus cables 20 and 30. The port controller 60 of the bus switching apparatus 10 receives the port circuits 41 to 4 from the signals 70 and 71.
monitor the bus acquisition / device selection operation of the device connected to n, 51 to 57, and operate the bus signal with signals 80 and 81,
Control the connection and disconnection of bus signals with signals 90 and 91,
The internal bus 100 is monitored for changes in the state of the bus.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a computer system in which a plurality of computers and input / output devices are bus-connected, and more particularly to a bus switching system for sharing a plurality of input / output devices (external storage devices).

[0002]

2. Description of the Related Art In a computer system, an input / output control device (hereinafter referred to as "initiator") and an input / output device (hereinafter referred to as "initiator")
There is a small computer system interface (hereinafter referred to as SCSI) defined in ANSI X3.131-1986 as one of bus protocols for communicating information between the target and the target).

SCSI consists of four phases: bus-free, bus acquisition (arbitration), device selection (selection) / reselection (reselection), and information transfer. As shown in FIG. 7, a plurality of devices are connected to the same bus cable. It is configured by daisy chain connection with terminating resistors at both ends.

An identifier (hereinafter referred to as an ID) is set for each initiator and target connected on the bus, and a selection source and a selection destination are designated for the ID to perform arbitration and selection / reselection. SCSI I
D is limited to 8 from 0 to 7, and the bus use priority is I
D = 7 is the highest and ID = 0 is the lowest. Arbitration is determined by the priority of ID. The selection / reselection is completed when the ID indicating the selection source and the ID indicating the selection destination are simultaneously output on the bus, and the device corresponding to the selection destination ID responds to the selection source.

Each target receives a command from the initiator and transfers information according to the command. When it takes a long time to prepare a response, the bus is released (disconnect) promptly in order to improve the bus usage efficiency, the bus is arbitrated when it is ready, the initiator is reselected, and the information transfer is restarted. During disconnection, commands from initiators with other IDs other than the own ID are sequentially received and stored, and information is transferred according to the commands.

Since the number of IDs is limited in the SCSI bus protocol, the number of connectable initiators is limited. Further, if a device with a high priority starts arbitration continuously, another device cannot use the bus.

One of the methods for solving these problems is a bus switching device disclosed in Japanese Patent Laid-Open No. 63-150753 (cited example). According to this, by connecting the initiators with the same ID individually, and monopolizing the bus until one initiator finishes transferring information from the target, while not responding to the arbitration and selection of other initiators, , Allows sharing of targets from multiple initiators.

[0008]

However, when the bus connection is made by the method of the cited example, since a plurality of initiators having the same ID are connected, the target cannot accept and store a command from another initiator at the time of disconnection. There was a problem.

It is a first object of the present invention to provide a bus switching method and device which enables transmission between devices which is larger than the total number of identifiers used.

A second object of the present invention is to provide an information transmission system which operates according to the SCSI bus protocol,
It is an object of the present invention to provide a bus switching method and apparatus capable of connecting a larger number of initiators than the total number of SCSI IDs.

A third object of the present invention is to provide a bus switching method and device with high bus usage efficiency.

A fourth object of the present invention is to provide a bus switching method and apparatus which eliminates bias of bus occupation to a specific device in SCSI and equalizes opportunities of each device.

The other objects of the present invention will become clear through the following description.

[0014]

The first object of the present invention is to:
Dynamically connect a bus to a communication partner to transmit information between multiple devices (initiators) connected to the port on the initiator side and one or more devices (target) connected to the port on the target side At this time, in the bus switching method of controlling the bus connection according to the identifier (ID) of the device indicating the transmission source and the transmission destination, a physical ID common to each device and a fixed target logical ID are assigned to each target side port in advance. In case of the above, the initiator logical ID is dynamically given to the initiator side port according to the selection operation of the command including the target logical ID from the initiator, and the predetermined ID conversion is performed between these physical ID and logical ID. This is achieved by selecting the transmission destination target and connecting the transmission source device and the transmission destination device by a bus.

A second object of the present invention is that when the maximum number of available identifiers (IDs) is n, one of them is used as a physical ID common to each device and the remaining n-1 identifiers are used. A part or all of the above is fixedly attached to the port on the target side, and the n-1 identifiers are assigned to the logic I on the initiator side.
It is dynamically assigned as D according to the selection operation of the initiator, a predetermined ID conversion is performed between these physical ID and logical ID to select a destination target, and a bus connection is made between the source and destination devices. It is achieved by

A third object of the present invention is that, when a disconnection of data transmission is detected after the bus connection is made, the bus is maintained while the registration of the physical ID and the logical ID correspondence is held. This is achieved by breaking the connection.

A fourth object of the present invention is that the selection operation is
This is achieved by monitoring in a predetermined order from the next port where the bus connection was executed last time.

[0018]

According to the present invention, a bus switching device is connected between the initiator side where the ID is not fixed and the target side where the ID is fixed, and the logical ID is dynamically assigned to the initiator performing the selection operation. Assign to. The correspondence relationship including the ID and the port number is managed, and the target is selected from the ID converted from the correspondence relationship to enable the bus connection.

According to this, since the ID assigned to the initiator is a dynamically changeable logical ID, information can be transmitted between devices whose number is equal to or larger than the number of IDs available in the system, especially in SCSI operation. The system configuration can be expanded freely.

[0020]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 is a functional block diagram of an information processing system according to an embodiment of the present invention. The information processing system includes an initiator 2 of an input / output control device that uses a computer.
0 (21 to 2n) and the target 30 (3 of the input / output device)
1 to 37) are connected via the bus switching device 10.

The initiator 20 and the target 30 are S
Information is transferred by the CSI protocol. That is, it consists of four phases: bus acquisition (arbitration), device selection (selection on the initiator side) / reselection (reselection on the target side), information transfer, and bus-free. The initiator 20 issues a command to the target 30, and the target 30 receives the command for each ID and sequentially executes the command processing.

The bus switching device 10 is a device for switching the buses so that they are connected on the same bus when the initiator 20 and the target 30 communicate with each other. Initiator 2
0 of each port circuit 40 (41) of the initiator side port of the bus switching device 10 via the bus cable 200.
~ 4n). Similarly, each of the targets 30 is connected to the port circuit 50 (51 to 5n) of the target side port via the bus cable 300. The internal bus 100 is configured so that it can be connected to all the bus cables 200 and 300 via the port circuits 40 and 50, and thereby constitutes a SCSI bus.

The port controller 60 includes a CPU 61 and RA.
M62, ROM 63 and I / O port 64 from each signal line
˜66. The CPU 61 controls each port circuit 4 according to the control program stored in the ROM 63.
I / O ports 64 to 66 with 0, 50 and internal bus 100
Also, the RAM 62 which manages the ID of each port is accessed to perform bus state monitoring and operation, and port switching control. That is, the bus of each port circuit is monitored to detect the operation signals 70 and 71 of arbitration and selection / reselection of the initiator 20 and the target 30, and the signals 80 and 81 for operating the bus or the signal 90 for opening and closing the bus 90. , 91 to the respective port circuits 40, 50.

FIG. 2 is a block diagram of the port circuit on the initiator side. SCSI signals are BSY, SEL, AT
N, RST, ACK, I / O, C / D, MSG, REQ
Data signals DB0 to DB7 corresponding to the respective signals of ID0 to ID7
And a DBP which is a parity signal. The configuration of the port circuit and each signal are the same as those of the conventional SCSI, and the details are omitted.

The port control device 60 reads the ID of the device that has started arbitration from the port selected by the monitoring signal 70 and stores it. At the same time, the bus operation signal 80 is output to the port circuit 40 to perform command ID conversion and suppression of arbitration by other ports. I
The conversion of D is performed by operating DB0 to 7, and the arbitration is suppressed by operating DB7 (133) and BSY (112). In addition, the bus opening / closing signal 90 is output, and the gate 11
1, 121-2, 131-2, 140, 150 on /
Turn off to connect / disconnect each SCSI signal of the port circuit 40 to / from the internal bus 100.

RST is a reset operation that clears an incomplete command in each device and releases it from the bus (bus free). According to the standard of the SCSI signal, both the initiator and the target can reset, but in the present embodiment, the reset is output only from the initiator to the target.

The target side port circuit 50 has the same basic configuration as the initiator side port circuit 40.
Y, ATN, RST, ACK, I / O, C / D, MS
The gate directions of G and REQ are reversed. Port control device 6
0 receives the monitoring signal 71 of the arbitration and reselection operation from the port circuit 50, and outputs the bus operation signal 81 and the bus opening / closing signal 91.

The bus switching device 10 is constructed in this way,
The number of ports on the initiator side is n, the number of ports on the target side is 7, and consecutive port numbers (# 41 to 4)
n, # 51 to 57) are added. There are eight connectable IDs in SCSI. In the present embodiment, one of them (ID = 0) is assigned as a physical ID of each initiator 20 or target, and the remaining seven IDs are assigned as logical IDs and the assignment is dynamically changed according to the selection operation. According to this, a large number of initiators can be connected even in the case of a system complying with the SCSI protocol. Less than,
The operation will be described.

3 and 4 are conceptual diagrams for explaining the ID conversion processing of the bus switching device. As a premise, the initiator 2
1 to 2n and targets 31 to 37 all have physical ID =
It is assumed that 0 is added and the bus switching device 10 is notified.

FIG. 3 shows a process of selecting a target from the initiator. The initiator 22 connected to the port number # 42 outputs a command of selection source physical ID = 0 and selection destination target logical ID = 7 to make a selection request (). When the bus switching device 10 detects this selection request (), it assigns a free ID (ID = 1 in the figure) from among the seven initiator logical IDs (), and the initiator port corresponds to this initiator ID = 1. The number # 42 and the target logical ID = 7 are registered in the ID management table 620 of the RAM 62 (,).

On the other hand, on the target side, the correspondence between the seven target logical IDs and the target port numbers # 51 to 57 is fixed in advance. The bus control device 10 converts the ID of the above command into the selection source initiator logical ID = 1 and the selection destination target physical ID = 0, and the target logical I
It outputs (,) to the target port number # 57 corresponding to D = 7, receives the response from the target 37, connects the bus, and enables communication.

FIG. 4 shows a process of reselecting the selected initiator from the selected target side. The target 37 outputs a command of selection source physical ID = 0 and selection destination initiator logical ID = 1 to request reselection (). When the bus switching device 10 detects the reselection request (), it refers to the ID management table 620 and reads the correspondence between the target logical ID = 7 and the initiator logical ID = 1 (,), and the corresponding initiator port number #
42 is searched (), and the ID of the above command is selected source target logical ID = 7, selection destination initiator physical ID = 0
It is converted to () and output to the initiator port number 42 (), and upon receiving a response from the initiator 22, the bus is connected and communication is enabled. Note that other registrations shown in the ID management table in the figure are due to the selection operation performed by another target during the period from the selection shown in FIG. 3 to the reselection.

Next, the operation of this embodiment will be described in detail.
FIG. 5 is a flowchart showing the operation of the bus switching device. The port control device 60 periodically monitors the “selection operation” output from the initiator 20 and the target 30 to the ports 40 and 50 via the I / O 64 and 66 (s101). This "selection operation" is detected by arbitration in which the BSY signal and the data signal of the port circuit are turned on.

The selection operation is monitored from the port next to the previously used port so that the priority of bus acquisition is not fixed to a specific device. Of course, it is also possible to give each device a predetermined priority. Further, the target side may be monitored first, and when there is no free initiator logical ID in the ID management table 620, unnecessary monitoring of the initiator side may be omitted.

When arbitration is detected in a certain port circuit, the BSY and DB7 of all the other ports are turned on to suppress the arbitration of other devices and control the conflict at the time of bus switching.

Now, when the selection operation is detected, it is judged whether the operation is the initiator side or the target side (s10).
2). If it is the operation from the initiator side, the transition from arbitration to selection is confirmed (this transition is not essential to this embodiment, and it is simply described as “selection operation” for arbitration or selection). The assigned initiator logical ID is registered in the ID management table in association with the initiator port number and the selected target logical ID (s10).
3). Note that when an initiator (= port number) to which a logical ID has already been assigned has selected another target, another initiator logical ID can be assigned.

Then, the signal 81 is output to output the command I
The target port number (physical ID = 0) corresponding to the original selected target logical ID of the command after converting D
To the bus (s104). After confirming the response from the target, signals 90 and 91 are output to connect the bus between the selection source initiator and the selection destination target (s105). That is, the port circuit 4 on the initiator side
At 0, the gates of the data signals DB and DBP corresponding to the initiator physical ID (= 0) and the target logical ID are turned on, and at the target side port circuit 50, the data signal DB corresponding to the initiator logical ID and the target physical ID.
The gates of DBP and DBP turn on, and the internal bus 100
Connected to

On the other hand, if the selection operation is reselection from the target side in step s102, the ID management table is referred to search for the initiator port number corresponding to the combination of the target logical ID and the selected destination initiator logical ID. (S106), the signal 80 is output, the ID of the command is converted and output to the bus of the relevant port number (s107), and after confirming the response from the corresponding initiator,
The signals 90 and 91 are output to connect the bus between the selection source target and the selection destination initiator (s108).

The port controller 60 monitors the BSY signal and the I / O, C / M, and MSG signals read from the internal bus 100, and transmits information that makes the I / O, C / M, and MSG signals all true. During the phase, the data signal (DB
The status is monitored from 0 to 7) (s109). If the status is normal, disconnect the bus connection (s11
0), the initiator logical ID registered in the ID management table is deleted (s111).

When the status is the check condition or the check condition with flag (NO in s109), even after disconnecting the bus (s112),
The initiator logical ID is saved without being released. This makes it possible to transfer the failure information of the target to the initiator of the transmission source.

After steps s105 and s108, the port controller 60 monitors the BSY signal to determine the disconnection of the data signal (s113), and temporarily releases the bus as soon as BSY is turned off (disconnect). (S114). Also in this case, the initiator logical ID is saved without being released. Then, when the target side is ready, reselection is performed. As a result, when it takes a long time for the target to start the input operation, the bus of another device can be used and the bus usage efficiency is improved.

As described above, according to the bus switching device of this embodiment, both the initiator and the target see
Since the total number of IDs is 8, it is possible to connect a large number of devices that exceed the constraint of SCSI ID = 8 while guaranteeing the operation according to the SCSI standard. Therefore, SC
It is possible to flexibly change and expand the configuration of the information input / output system having a limited connectable ID such as SI.

Further, since the port that used the bus last time is stored and the target side is sequentially monitored from the next port, the priority order of the device that acquires the bus is fixed like normal SCSI. It is not easy to use.

Further, at the time of disconnecting the data signal, the bus is disconnected while maintaining the ID correspondence, and the reconnection can be performed by the reselection from the target side, so that the bus is not occupied and the bus is used efficiently. Can be improved.

Although this embodiment is suitable for a system that operates in accordance with the SCSI protocol, it can be widely applied to control the transmission of information between a number of devices that are connected beyond the limit of the ID in the system. is there.

Next, another embodiment of the present invention will be described. FIG. 6 is a conceptual diagram illustrating the configuration and selection operation of the bus control device.

In this embodiment, an ID management table is provided for each target port number. Similar to the above-described embodiment, the target logical ID fixedly corresponds to the target port 50, and the initiator logical ID is dynamically assigned to the requested initiator port 40.

Now, when the initiator # 22 selects the target logical ID7 (), the ID management table 6 of the target port # 57 corresponding to the target logical ID7.
Initiator logical ID managed by 27 (ID in the figure
= 1) is assigned and registered together with the port number (). Further, when the initiator # 23 selects the target logical ID 6 (), the initiator logical ID (ID = 1 in the figure) managed by the ID management table 626 of the target port # 56 is assigned ().

As a result, when selections are made in parallel, the same initiator logical ID is assigned to a plurality of initiator ports, but since these ports are not connected to the bus at the same time, no conflict occurs.

According to this, the logical IDs of the initiators can be apparently increased to (total initiator logical ID number) × (target port number), and efficient processing can be performed when the selection and reselection operations are complicated. Further, even when a plurality of commands from the initiator are issued in parallel, it is possible to handle the command having only the apparent logical ID.

[0052]

According to the present invention, even when the number of usable IDs is limited, information can be transmitted between devices having the number of IDs or more through bus switching using logical IDs. There is an effect that the configuration can be freely expanded by applying it to a system that operates in SCSI.

Furthermore, the fixed priority of SCSI can be solved, and the bus can be used efficiently by temporarily freeing the bus during disconnection.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of a bus switching device of the present invention.

FIG. 2 is a logic diagram showing details of a port circuit of the bus switching device.

FIG. 3 is a conceptual diagram illustrating ID correspondence and conversion in an initiator selection operation.

FIG. 4 is a conceptual diagram illustrating ID correspondence and conversion in a target reselection operation.

FIG. 5 is a flowchart showing the operation of the embodiment of the present invention.

FIG. 6 is a conceptual diagram illustrating another embodiment of the present invention and illustrating the configuration and selection operation of the bus control device.

FIG. 7 is a block diagram of a conventional SCSI.

[Explanation of symbols]

10 ... Bus switching device, 20 (21-2n) ... Initiator (input / output control device), 30 (31-37) ... Target (input / output device), 40 (41-4n) ... Initiator side port circuit, 50 (51) ... 57) ... Target side port circuit, 60 ... Port control device, 70 ... Arbitration / selection monitoring signal, 71 ... Arbitration / reselection monitoring signal, 80 (81) ... Bus operation signal, 90 (91) ... Bus opening / closing signal, 100 ... Internal bus, 200 ... Initiator side bus cable, 300 ... Target side bus cable, 620 ... ID management table.

Claims (18)

[Claims] 1. A bus is dynamically connected to a communication partner between a plurality of devices (initiators) connected to a port on the initiator side and one or a plurality of devices (targets) connected to a port on the target side. In a bus switching method for controlling bus connection according to an identifier (ID) of a device indicating a transmission source and a transmission destination when transmitting information by using a physical ID common to each device and a fixed port for each target side port in advance. If the target logical ID is given,
The initiator logical ID is dynamically given to the port on the initiator side according to the selection operation of the command including the target logical ID from the initiator, and a predetermined ID conversion is performed between these physical ID and logical ID to make the destination target. The bus switching method is characterized in that a bus connection is made between the transmission source device and the transmission destination device. 2. A communication partner is dynamically connected to a bus between a plurality of devices (initiators) connected to a port on the initiator side and one or a plurality of devices (target) connected to a port on the target side. In the bus switching method in which the bus connection is changed according to the identifiers (IDs) given to the source and destination devices of information when transmitting information, the maximum number of identifiers (IDs) that can be used is In the case of n, one of them is used as a physical ID common to each device, and a part or all of the remaining n-1 identifiers are fixedly assigned to the target side port, and the n-1 Is dynamically assigned as an initiator-side logical ID in accordance with the initiator's selection operation, and a predetermined target is converted between these physical IDs and logical IDs to select a destination target, Bus switching method characterized by between Xinyuan and destination device for bus connection. 3. The bus switching method according to claim 1, wherein the selection operation is to monitor from a port next to a bus connection that was executed last time in a predetermined order. 4. The predetermined ID conversion according to claim 1, 2 or 3, wherein the physical ID of a transmission source and the target logical ID of a transmission destination, which are included in a command of an initiator selection operation, are dynamically assigned. And a logical ID and the physical ID are respectively converted. 5. The predetermined ID conversion according to claim 4, wherein the predetermined physical ID conversion is a physical ID of a transmission source and an initiator logical ID of a transmission destination which are included in a command for a target reselection operation. ID
A bus switching method, characterized in that the target logical ID and the physical ID are respectively converted. 6. The logical ID assigned to the initiator side according to claim 1, 2, 3, 4, or 5, wherein the initiator-side port number and the target-side logical ID are applicable.
Corresponding ID is registered when responding from the target side
A bus switching method characterized by being referred to for conversion. 7. The bus switching method according to claim 6, wherein the registration is erased after information transmission between the transmission source and the transmission destination is normally completed and the bus is disconnected. 8. The bus switching method according to claim 6, wherein the registration is held when the transmission source and the transmission destination do not normally complete the information transmission. 9. The bus connection according to claim 6, 7 or 8, wherein when the disconnection of data transmission is detected after the bus connection is made, the bus connection is disconnected while holding the registration. How to change the bus. 10. The method according to claim 1, 2, 3, 4, 5, 6, 7,
8 or 9, wherein the bus switching method is performed according to a SCSI protocol. 11. Each port on the initiator side for connecting a plurality of devices (initiators) and each port on the target side for connecting one or a plurality of devices (targets),
In a bus switching device that has a bus connecting between ports of a communication partner and switches the bus connection according to an identifier (ID) of a device indicating a transmission source and a transmission destination, a port terminal for connecting a bus for each device and a device A selection operation detection unit that detects a selection operation from a signal, a switching unit that connects / disconnects a port terminal and an internal bus, a port circuit provided for each port, a physical ID common to each device, each target side A fixed target logical ID is given to the port in advance, and an initiator logical ID is dynamically given to the corresponding port according to the selection operation of each initiator, and an ID management function for registering the correspondence relationship between these IDs, and the ID An ID conversion function for converting the IDs included in the response commands of the transmission source and the transmission destination into a response according to the correspondence relationship between the transmission source and the transmission destination; Bus switching device characterized by and a port circuit control means having a bus control function of outputting a connection signal / disconnection signal to the switching portion of the port circuit of the selected destination D converter. 12. An initiator-side port for connecting a plurality of devices (initiators), and a target-side port for connecting one or a plurality of devices (targets),
In a bus switching device that includes a bus that connects ports for transmitting information and that switches a bus connection according to an identifier (ID) of a device that indicates a transmission source and a transmission destination, the maximum number of identifiers (ID) that can be given to the device is In the case of n, one of them is used as a physical ID common to each device, the remaining n-1 identifiers are fixedly assigned to the target side port, and each initiator is selected as a logical ID on the initiator side. ID management means that dynamically assigns the IDs according to the operation and registers the correspondence relationship between these IDs, and an ID that converts the IDs included in the response commands of the sender and the destination according to the ID correspondences so that the IDs can be responded. A bus comprising: a conversion unit; and a bus switching unit that connects / disconnects a port corresponding to a transmission source and a transmission destination determined from the correspondence relationship of the IDs via an internal bus. Changing apparatus. 13. The bus switching device according to claim 12, wherein the number of ports on the initiator side is n or more, and information transmission between devices exceeding the maximum number of IDs is possible. 14. The bus switching device according to claim 12, wherein the selection operation, connection / disconnection, and information transmission are performed according to a SCSI protocol. 15. A plurality of input / output control devices (initiators) and a plurality of input / output devices (targets) having different identifiers are connected to the same bus, and after the input / output control device acquires the bus, input / output is performed by the identifiers. I / O operation is performed by selecting a device and performing a I / O operation, temporarily releasing the bus when the I / O operation is disconnected, and then reselecting the I / O control device from the I / O device. In an information transmission system that transfers data according to a protocol for restarting the communication, between the plurality of input / output control devices and the plurality of input / output devices, one of the input / output control device or the input / output device that started the bus acquisition operation A bus switching device having a function of selecting a stand and connecting a bus between the input / output control device or the input / output device and the input / output device or the input / output control device that is the information transmission destination. Continuing, the bus switching device, for the selected input / output control device, an identifier preset in the input / output control device, and an identifier different from the preset identifier in the input / output device as a logical identifier. Means for temporarily assigning, means for storing the correspondence between the assigned logical identifier and the input / output control device, and converting the identifier preset in the input / output control device to the logical identifier when selecting the input / output device. Means for transmitting to the input / output device, and when the input / output device reselects the input / output controller, the identifier of the input / output controller to be reselected read from the bus is treated as a logical identifier, The means for storing the correspondence between the logical identifier and the input / output control device is used to find an identifier preset in the input / output control device, and the input / output control of the reselection destination is performed. And means for transmitting the location, to determine for the completion of the operation to monitor the input and output operations from the bus, the bus switching device, characterized in that it comprises a means for releasing the logical identifier. 16. The bus switching device according to claim 15, wherein when all usable logical identifiers are assigned to the input / output control device due to input / output multiplex operation or the like, at least one input / output operation is terminated. The method further comprises means for suppressing the bus acquisition operation of the I / O controller until the one or more logical identifiers are released and monitoring the operation of the I / O device reselecting the I / O controller. Bus switching device. 17. The bus switching device according to claim 15, further comprising a plurality of ports for connecting the input / output control device, the logical identifier assigned when the input / output control device starts a selection operation, and the input / output control. Means for storing device correspondence for each connection port of the input / output control device, and an input / output control device read from the bus when the input / output device reselects the input / output control device Is treated as a logical identifier, and a means for storing the correspondence between the logical identifier and the input / output control device is used to identify an identifier preset in the input / output control device and a port to which the input / output control device is connected. A bus switching device, which is provided with means for transmitting it to an input / output control device of a reselection destination. 18. The bus switching device according to claim 15, wherein an input / output control is provided in which the number of ports for connecting the input / output devices is less than the number of usable identifiers, and the input / output device connected to the port is selected. Means for temporarily assigning, as a logical identifier, an identifier different from the one set for the input / output device in addition to the identifier preset for the device, and means for storing the correspondence between the assigned logical identifier and the input / output control device A bus switching device comprising: