JP2004252580A - Connecting status detecting system equipped with absent device notifying means - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To quickly detect any absent device without waiting for time-out, and to easily and surely detect the generation of any failure. <P>SOLUTION: This system is provided with a plurality of devices 11, 12, 21, 22 and 23 connected to a computer main body 40 and a controllers 10 and 20 connected to the computer main body 40 for monitoring the connecting status of each device according to response signals transmitted from the respective devices 11, 12 or 21, 22 and 23. This system is also provided with absence notifying devices 31 and 32 unconnected to the computer main body 40 for setting the identification information of any absent device for specifying one or more connectable devices, and for transmitting a response signal including the identification information to the controller 10 or 20 to notify the absence of the device. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention provides, for example, a computer main body and a plurality of external storage devices connected to the computer main body, a plurality of computers and peripheral devices connected to each other via a network such as a LAN, and the like. The present invention relates to a connection status detection system that monitors and detects a connection status of a device.
In particular, the present invention sets identification information for one or more devices that are not connected to the system and may be connected, and outputs and transmits the identification information as an absence notification. By providing a means, it is possible to detect a device not connected to the system at high speed without waiting for a time-out for the presence or absence of a response signal from each device, and if there is an abnormality in the connected device, the abnormality can be easily and The present invention relates to a connection state detection system that can perform reliable, smooth, quick and efficient operation and realize a highly reliable system.
[0002]
[Prior art]
Generally, a plurality of devices in a system such as a computer main body and external storage devices such as a hard disk and a CR-ROM drive connected to the computer main body, and a plurality of computers and peripheral devices connected to each other on a network such as a LAN Are connected under the control of the controller serving as monitoring means, and are accessed by an identifier that uniquely identifies the device.
For example, in the case of an external storage device such as a computer main body and a hard disk or a CR-ROM drive, software operating on the computer main body accesses various devices by designating a controller and an identifier.
When a non-existing device, that is, a device that is not connected to the system is accessed, there is no response from the device, and when there is no response even after an appropriate waiting time has elapsed, a process for determining that the device does not exist Is done.
[0003]
Specifically, in the case of a computer device as shown in FIG. 4, a plurality of devices connected to the computer main body are connected with unique identification numbers under various controllers.
In the example shown in the figure, software operating on the computer main body 140 has a function of accessing the devices 111 and 112 connected thereto via the controller 110.
Similarly, the devices 121, 122, and 123 connected thereto are accessed via the controller 120.
[0004]
Each device 111, 112, 121, 122, 123 has a device identification number, and the software accesses each device by designating the identification number.
As shown in FIG. 4, each controller 110, 120 can be connected to a plurality of devices. That is, the software accesses the respective devices by designating the identification numbers of the respective controllers 110 and 120 and the respective devices 111, 112, 121, 122, and 123.
[0005]
The operation of the conventional system as described above is shown in FIG.
As shown in the figure, the controller 110 designates an identification number of a device and sequentially accesses a plurality of devices existing on the system.
First, a response signal can be immediately obtained for the identification number # 1 of the device 111 and the identification number # 2 of the device 112 existing in the system (steps B1 to B4), and the presence of the device is detected.
On the other hand, for the responses of the identification numbers # 3 and # 4 of the devices that do not exist, it is detected that the devices do not exist after waiting for the elapse of a predetermined timeout time (steps B5 and B6). The timeout time is set to be sufficiently longer than the normal access time, and detection of a device that does not exist is determined after waiting for the timeout time, so that a longer processing time is required than when a response signal is received.
[0006]
The method of specifying various identification numbers and accessing various devices on the system is used in many scenes (see, for example, Patent Documents 1-4).
For example, a SCSI bus designed to connect an external storage device such as a hard disk or a CD-ROM device has a SCSI controller in the computer body, and accesses the external storage device by specifying it with an identification number called a SCSI ID.
In the IP protocol, which is a kind of network protocol, another computer connected to a network card is designated and accessed by a 32-bit identification number called an IP address.
In general, more identification numbers are prepared than the number of devices that are actually connected in consideration of future expansion, and no response is made when a device that does not exist is accessed. For this reason, an appropriate timeout time is set, and if there is no response until the timeout time elapses, it is considered that no device exists.
[0007]
[Patent Document 1]
Japanese Patent Laid-Open No. 01-137846 (second page, FIG. 2)
[Patent Document 2]
JP 06-282346 A (page 3-4, FIG. 1)
[Patent Document 3]
Japanese Patent Laid-Open No. 10-023046 (page 2-3, FIG. 1)
[Patent Document 4]
JP 11-212901 A (page 2-3, FIG. 1)
[0008]
[Problems to be solved by the invention]
Thus, in the conventional method of detecting the presence / absence of a device connected to the system based on the presence / absence of a response signal, if the presence / absence of a device is confirmed only at the time of starting the system, the timeout time may be a problem. There was almost no.
However, when it is necessary to detect the presence or absence of a device even while the system is operating, the influence of the timeout time on system operation cannot be ignored.
For example, in a large-scale system that operates continuously, devices may be added or deleted during operation. In such a system, software for managing a large number of devices is used. For this reason, when a user or the like requests it, it is necessary to search all devices on the system, and the waiting time when accessing non-existing devices and devices becomes a problem.
[0009]
In recent years, the number of devices connected to computers has tended to increase due to technological advancement and management centralization, and recently, computers have been searched for devices instead of manual settings for efficient management. In many cases, the method is used. In order for management software that performs such an automatic search to display a list of connected devices, it is often necessary to search for devices and search all device numbers. Further, since the apparatus can be inserted / removed during operation in order to improve the operation rate, the search for all connected devices is necessary not only when the system is started but also when maintenance is performed.
In such a case, even if the timeout time of each device can be set short, the total waiting time in a large-scale system connecting a large number of devices will increase by several times the number of devices. Especially in a large-scale computer system, the timeout waiting time may become a serious problem.
[0010]
In addition to the problem of waiting for timeout, the conventional system also has a problem that it is difficult to easily and accurately detect when there is no device and when there is an abnormality in the existing device.
In a conventional system, when a failure such as a failure occurs in an apparatus existing on the system, no response is made as in the case where no apparatus exists. For this reason, when receiving a response of no response, it is necessary to distinguish whether a failure has occurred or whether the device does not exist.For example, the software keeps the state of each device and changes from the previous state In this case, it is detected that the device has changed to a failure state, and the user is notified.
However, in this method, if a failure occurs while the system is operating, the device status can be detected by software to detect the failure and notify the user. If it is, it must be determined that it is a nonexistent device that does not exist, or it must be detected by setting the device to be connected by software, and the occurrence of a failure should be detected easily and reliably. It was difficult.
[0011]
The present invention has been proposed in order to solve the problems of the conventional techniques as described above, and is not connected to the system, and may be connected to one or more devices that may be connected. By setting the identification information, and providing the absence device notification means for outputting and transmitting the identification information as an absence notification, it is possible to detect a device not connected to the system at high speed without waiting for a response signal from each device until timeout. And it aims at provision of the connection state detection system which can detect the abnormality easily and reliably when abnormality exists in a connection apparatus.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, a connection state detection system according to claim 1 of the present invention provides a connection state of each device by one or more devices connected to the system and a response signal transmitted from each device. Monitoring means for monitoring, setting identification information of an absent device that is not connected to the system and that identifies one or more connectable devices, and includes the identification information for the monitoring means An absence device notifying means for notifying the absence of the device by transmitting a response signal is provided.
[0013]
According to the connection state detection system of the present invention having such a configuration, the absence device notification means for setting the identification information of the device that is not connected to the system is provided, thereby registering a device that is known in advance as not existing. In addition, an error or absence information can be returned as a positive response signal for access to a non-existing device.
As a result, access to the absent device does not become a non-response, and a response indicating that the device is absent is immediately notified, so there is no need to wait for a response signal from the absent device until timeout, and an access response to the absent device Time can be shortened.
In the conventional system that does not include the absent device notifying unit, a response to a non-existing device is a non-response. Therefore, it is necessary to set an appropriate timeout time and determine that no device exists when there is no response until the timeout time. For this reason, detection of a non-existing device is determined after waiting for a timeout time, and a longer processing time is required than when a response signal is received. In the present invention, by providing the absent device notification means, there is no need to wait until the response signal from the absent device times out, and the absent device can be detected immediately, and high-speed and efficient system operation is possible.
[0014]
Further, by providing such an absent device notifying unit, it is possible to assume that all devices are connected by the absent device notifying unit, and it is possible to easily and reliably detect a device abnormality. In other words, it is possible to determine that there is some abnormality with respect to a device that does not respond to the monitoring unit despite the absence device information being set.
In a conventional system, if a failure occurs during system operation, the user can be notified of the failure detection by holding the device status in software. However, if there is no response at system startup, it is not a failure. It is assumed that the device is not to be configured, or the device to be connected must be set by software and detected. In the present invention, by using the absence device notification means, it is possible to immediately detect and determine that a device that has not responded is “device abnormality”, and to easily and quickly and reliably detect a failure of the connected device. And a highly reliable system can be realized.
As described above, in the present invention, by providing the absent device notification means, it is possible to maintain the same state as when all devices on the system are connected, thereby realizing efficient system management. .
[0015]
In the connection state detection system according to the second aspect, the absent device notifying unit arbitrarily sets identification information of the absent device via a predetermined input unit.
On the other hand, in the connection state detection system according to claim 3, the absent device notifying means searches the system and automatically sets identification information of the absent device.
[0016]
According to the connection state detection system of the present invention having such a configuration, the setting of the device identification information to the absent device notification means is manually performed by the user through various user interfaces such as a switch, a button, and a Web browser, for example. In addition, the absence device notifying means itself can automatically set by searching for other devices on the system, and a method in which manual and automatic settings are integrated can also be adopted.
As described above, the absent device notification unit according to the present invention can be a flexible and highly expandable unit and can be easily applied to an existing system.
[0017]
In addition, since the absence device notifying unit searches for connection devices and devices on the system, settings can be automated and more efficient system operation can be realized.
Furthermore, the absence device notification means automatically detects the connection device by searching, so that the absence device notification means can monitor the access status to the system. For example, the connection of a new device or device to the system is also possible. It is possible to provide a highly efficient and reliable system.
[0018]
According to a fourth aspect of the present invention, the absence device notification means is provided independently of each device and the monitoring means.
On the other hand, in the connection state detection system according to the fifth aspect, the absent device notification unit is integrated with at least a part of each device or the monitoring unit.
[0019]
According to the connection state detection system of the present invention having such a configuration, the absent device notifying unit is configured so that each device and the monitoring unit can be separated from each other as long as the absent device on the system can be detected. It may be configured as a part of the monitoring means, and a system excellent in flexibility and expandability can be realized.
When the absent device notification means is configured separately from each device and monitoring means, it can be easily and quickly added to and expanded from an existing system.
Further, by providing the absence device notification means, for example, as a part of the monitoring means, the entire system can be reduced in size and high-speed and multifunction absence notification processing can be performed.
[0020]
Further, in the connection state detection system according to claim 6, when the connection reliability differs depending on the connection order of one or more devices to the system, the absent device notification means connects to the system in the order of high connection reliability. The configuration is as follows.
[0021]
According to the connection state detection system of the present invention having such a configuration, the absent device notification means can be connected to the system in an arbitrary connection order, and further reliable by connecting in the order of higher connection reliability. It becomes possible to realize an excellent system.
The connection reliability of the system means the operation rate of the target devices that constitute the system.
As a method of connecting one or more devices constituting a system, there are roughly two types, a bus type (star type) and a chain type. In general, in a bus type system, a connection cable and a new synthetic fiber may be shared, but a controller (monitoring means) and each device are connected one-on-one. Therefore, in a bus type system, even if a failure or failure occurs in a certain device, the other devices are not affected.
On the other hand, in a chain type system, the controller is connected to other devices via one device, so if the device located in the middle fails or the cable is disconnected, the operation of the subsequent devices The rate goes down.
[0022]
Therefore, in the present invention, when the connection reliability, that is, the operation rate is different depending on the connection order of devices as in a chain-type connected system, by connecting the absent device notification means in the order closer to the controller, The operating rate of the absent device notification means can be increased, thereby improving the reliability of failure detection.
Note that the connection order of the absent device notification means is not limited to the case where the connection reliability is high, and it is possible to connect in the order of low connection reliability. For example, the absence device notification means can be connected to the end of the system, and in this way, the influence of the failure or failure of the absence device notification means itself on the original device can be reduced.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a preferred embodiment of a connection state detection system including an absent device notification unit according to the present invention will be described with reference to FIGS.
FIG. 1 is a block diagram showing a schematic configuration of a connection state detection system according to an embodiment of the present invention.
Referring to FIG. 1, an embodiment of the present invention includes a computer main body 40, controllers 10 and 20, devices 11, 12, 21, 22 and 23 connected thereto, and absence notification devices 31 and 32. Has been.
[0024]
The computer main body 40 is an information processing apparatus such as a personal computer or a server, and constitutes one system together with each connected apparatus.
The software operating on the computer main body 40 has a function of monitoring the connection state by accessing the devices 11 and 12 connected thereto via the controller 10, and constitutes the monitoring means according to the present invention. Yes. Similarly, the software operating on the computer main body 40 serves as monitoring means for accessing the devices 21, 22 and 23 connected thereto via the controller 20 and monitoring the connection state.
[0025]
The devices 11, 12 and the devices 21, 22, 23 are devices that can be connected to the computer main body 40 such as a hard disk or a CD-ROM device, and constitute devices connected to the system according to the present invention.
As shown in FIG. 1, in this embodiment, each of the devices 11, 12, 21, 22, 23 (and absence notification devices 31, 32) constituting the system is respectively connected to the corresponding controllers 10, 20, respectively. Although it is connected in a bus type, the connection method between the controller and each device may be a chain type instead of a bus type (star type).
The devices 11 and 12 have device identification numbers and respond when the identification number designated by the controller matches their own identification number.
Here, a plurality of controllers can be connected to the computer main body 40. In the example of FIG. 1, the devices 21, 22 and 23 are accessed by the controller 20. That is, the software accesses the devices by specifying the controller and device identification numbers.
[0026]
The absence notification devices 31 and 32 are set in the absence response device identification information for identifying the device that is not connected to the system and can be connected to the system, and a predetermined response including the identification information. This is a device that transmits signals to the controllers 10 and 20, and constitutes an absent device notification means according to the present invention.
Specifically, as shown in FIG. 1, the absence notification devices 31 and 32 are connected to the corresponding controllers 10 and 20 and have the same identification numbers as the devices connected to the controllers 10 and 20. However, unlike a general device, a plurality of identification numbers can be stored and set as an absent device list, and only absence information indicating that no device exists is returned to the controller.
For example, unconnected information is added to the device identification information such as the device name and device type and returned, or an error is returned for access. Generally, since no response is made when a device that does not exist is accessed, it is necessary to set an appropriate timeout and wait for a response. In this embodiment, the absence notification devices 31 and 32 are provided, so that the controllers 10 and 20 and software Will immediately see that it has accessed a device that does not exist.
[0027]
In FIG. 1, the absence notification devices 31 and 32 have a configuration independent of the controller and each device, but this is to implement the present invention in a manner that extends to an existing system. Therefore, a mode in which the functions of the absence notification devices 31 and 32 are integrated into the controllers 10 and 20 may be employed.
In addition, since the devices connected to the computer main body 40 are generally connectable in random order, if the reliability changes depending on the connection order, the absence notification devices 31 and 32 may be connected to places with high reliability. it can.
[0028]
When the system is configured by one or more devices, there are a chain type (not shown) as a connection method in addition to a bus type (star type) as in this embodiment. In the case of the bus type, the connection cable and the new synthetic fiber may be shared, but the controller and each device are connected one-on-one (see FIG. 1). Does not affect other devices.
However, in the case of the chain type, the controller is connected to another device via one device, so if the device located in the middle breaks down or the cable is disconnected, the operation rate of the devices after that decreases. Sisters. That is, in the case of a system connected in a chain type, the connection reliability, that is, the operation rate varies depending on the connection order of devices. Therefore, in the case of such a chain type connection, the absence notification devices 31 and 32 are connected in the order closer to the controllers 10 and 20, whereby the operating rate of the absence notification device can be increased.
[0029]
Here, the setting of the device identification information in the absence notification devices 31 and 32 is performed manually by a user or the like manually by various input interfaces represented by predetermined input means such as switches, buttons, and Web browsers. A method in which the absence notification device itself automatically sets by searching for another device and a method in which both are integrated can be adopted.
In addition, as a method of searching for each device on the system, the absence notification devices 31 and 32 have a function of searching for devices in the same manner as the controllers 10 and 20, and actively search, and the signals output from the devices are monitored. Thus, there is a method of actively reflecting the existing device information, and any method can be adopted in the system of this embodiment.
The active search method is suitable for searching at system initialization.
The passive search method is effective when the system has a mechanism for outputting the ID of the device when the device is added while the system is operating. For example, when the transmission source is known like TCP / IP, it can be regarded as a new device when a new transmission source IP address is detected.
[0030]
In the example shown in FIG. 1, four controllers having IDs of # 1, # 2, # 3, and # 4 can be connected to the controllers 10 and 20, respectively. There is no limit on the maximum number of devices.
When notifying all the combinations that can be connected, an N-bit storage area is required to manage the presence / absence of a bitmap. Further, when M device identification numbers are stored and absence management is performed, an area of a bit size of M × device identification number is required. If the number of identification numbers is relatively small as in SCSI, the former method may be used, and if there is a possibility of supporting a wide range of addresses such as IP addresses, the latter method may be employed.
[0031]
Next, the absence device detection operation in the present system configured as described above will be described.
FIG. 2 shows an operation when the software acquires information of devices connected under the control via the controller 10.
As shown in the figure, first, the controller 10 issues a search command for a device whose identification number is # 1 (step A1). In response to this, the device 11 returns its own device information (step A2).
When the search result is obtained, the controller 10 issues a search command for the device whose identification number is # 2 (step A3), and the device 12 responds to this (step A4).
Further, when a search is performed for device numbers # 3 and # 4 where the controller 10 does not exist (steps A5 and A7), the absence notification device 31 responds (steps A6 and A8).
[0032]
As a result, in this system, similarly to the devices 11 and 12 (identification numbers # 1 and # 2) connected to the system, devices that are not connected (identification numbers # 3 and # 4) are also stored in the controller 10. A response signal will be sent and returned immediately.
On the other hand, when the absence notification device does not exist or when the device number is not set in the absence notification device, the controller 10 responds with identification numbers # 1 and # 2 as shown in FIG. Although it can be obtained immediately (steps B1 to B4), the responses of the identification numbers # 3 and # 4 must wait for the timeout time (steps B5 and B6). Since the timeout time is set to be sufficiently longer than the normal access time, a longer processing time is required as compared with the case of this system shown in FIG. For this reason, the conventional system cannot realize a high-speed system operation like the present system.
[0033]
Next, an example of the device search operation when the absence notification device 31 is configured as an automatic search means will be described.
FIG. 3 is a flowchart showing the operation of the absence notification device 31 having an automatic detection function.
As shown in the figure, when the absence notification device 31 receives a signal such as data or a command from another device (step 301), it confirms whether or not the transmission source of the signal is a device included in the absent device list. (Step 302).
When the transmission source is included in the absent device list, the transmission source is regarded as a new device connected, and the device can be deleted from the absent device list (step 303).
Further, when a response signal or the like is transmitted from the absence notification device 31, it is confirmed whether or not the transmission destination of the signal is included in the absent device list (step 304), and the transmission destination is included in the absent device list. Can respond to a predetermined absence notification such as an error or absence information (step 305).
[0034]
The absence notification devices 31 and 32 of the present embodiment have a function of searching for a connected device on the system according to designation by a user or the like, and automatically adding and setting a device that has timed out to the absent device list. However, it is preferable that such a function is used in a stable state before the system is operated and is not used during the operation of the system. This is because when a response signal to a device that is in operation of the system times out, it is important to detect the device as a failure rather than being absent and to notify the user or the like. In this case, the management software can present a predetermined warning to the user side, assuming that the device that has timed out is abnormal.
[0035]
As described above, according to the connection state detection system including the absence notification device according to the present embodiment, it is possible to immediately respond to access to the absence device by including the absence notification devices 31 and 32. Thus, it becomes possible to detect a device that does not exist in the system at high speed.
In a conventional system that does not include the absence notification devices 31 and 32, a response to a non-existing device is a non-response. Therefore, it is necessary to set an appropriate timeout time and determine that no device exists when there is no response until the timeout time. It was. For this reason, detection of a non-existing device is determined after waiting for a timeout time, and a longer processing time is required than when a response signal is received.
In the present embodiment, by providing the absence notification devices 31 and 32, there is no need to wait until the response signal from the absence device times out, and the absence device can be detected immediately, and high-speed and efficient system operation can be performed. .
[0036]
Moreover, according to the connection state detection system of this embodiment, since it can be considered that all the apparatuses are connected by the absence notification apparatuses 31 and 32, the apparatus abnormality can be detected easily and reliably. .
That is, it can be considered that there is some abnormality for a device that does not respond even though the absence notification devices 31 and 32 are set.
In a conventional system, if a failure occurs during system operation, the user can be notified of the failure detection by holding the device status in software. However, if there is no response at system startup, it is not a failure. It is assumed that the device is not to be configured, or the device to be connected must be set by software and detected.
In the present embodiment, by using the absence notification devices 31 and 32, it is possible to immediately detect “device abnormality” for a non-responding device, and easily and quickly detect a failure or the like of the connected device. A highly reliable system can be realized.
[0037]
The connection state detection system of the present invention has been described with reference to the preferred embodiment. However, the absence notification device and the connection state detection system according to the present invention are not limited to the above-described embodiment, and the present invention is not limited thereto. It goes without saying that various modifications can be made within the range described above.
For example, in the above embodiment, as a system to which the absent device notification means of the present invention is applied, a case of one computer device and an external storage device such as a hard disk and a CR-ROM drive connected to this computer has been shown. Of course, the subject of the invention is not limited to a single computer device. That is, the present invention has any configuration as long as a plurality of devices are connected to the system, each device is connected under the controller serving as a monitoring unit, and is accessed by an identifier that uniquely identifies the device. In addition to the case of the single computer device described above, the present invention can also be applied to a plurality of computer devices and peripheral devices connected to each other on a network such as a LAN.
As described above, the system connection method includes a bus type (star type) and a chain type, but it goes without saying that the present invention can be applied to any connection method.
[0038]
【The invention's effect】
As described above, according to the connection state detection system of the present invention, identification information about one or more devices that are not connected to the system and that may be connected is set, and the identification information is set. The absence device notification means for outputting and transmitting the message as an absence notification makes it possible to detect a device not connected to the system at high speed without waiting for a timeout.
In addition, when there is an abnormality in the connection device, the abnormality can be detected easily and reliably.
Thereby, in this invention, smooth, quick and efficient operation is attained, and a highly reliable system is realizable.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an overall configuration of a connection state detection system including an absence notification device according to a coincidence embodiment of the present invention.
FIG. 2 is a flowchart showing an operation in the connection state detection system provided with the absence notification device according to the embodiment of the present invention shown in FIG. 1;
FIG. 3 is a flowchart showing an operation in the absence notification device according to the embodiment of the present invention.
FIG. 4 is a block diagram showing an overall configuration of a conventional connection state detection system.
5 is a flowchart showing an operation in the conventional connection state detection system shown in FIG.
[Explanation of symbols]
10 Controller
11-12 Connecting device
20 controller
21-23 Connection device
31 Absence notification device
32 Absence notification device
40 Computer body

Claims (6)

One or more devices connected to the system;
Monitoring means for monitoring the connection state of each device by a response signal transmitted from each device, and
By setting identification information of an absent device that identifies one or more devices that are not connected to the system and that can be connected, and sending a response signal containing the identification information to the monitoring means, A connection state detection system comprising absence device notification means for notifying absence. The connection state detection system according to claim 1, wherein the absent device notifying unit arbitrarily sets identification information of the absent device via a predetermined input unit. The connection state detection system according to claim 1 or 2, wherein the absent device notification means searches the system and automatically sets identification information of the absent device. 4. The connection state detection system according to claim 1, wherein the absent device notification unit is provided independently of each device and the monitoring unit. 5. The connection state detection system according to claim 1, 2, 3, or 4, wherein the absent device notification means is provided integrally with at least one part of each device or monitoring means. When the connection reliability differs depending on the connection order of one or more devices to the system,
6. The connection state detection system according to claim 1, wherein the absent device notification means is connected to the system in order of high connection reliability.

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