Loop mode transmission system with bus mode backup and a method of maintaining continuity thereof

Abstract: A transmission system formed by a master station (23) and a plurality of satellite stations (24 - 26) which are connected by a transmission line (31 - 34) into a loop, wherein the data and command signals from the master station (23) are passed sequentially from station to station around the loop. A failure in a satellite station (24 - 26) that prevents further loop transmission is detected by watchdog timers in one or more successive stations (23 - 26). As a result, the respective stations detecting such a failure switch (35 - 38) themselves into a bus mode of operation, the master station (23) switches itself and the remaining stations into the bus mode of operation. The switching back to the loop mode of operation can be performed under control of the master station (23) automatically, upon detection that the failure has been corrected. The switching also can be performed under external command.

Description: The invention relates to a transmission system for transmitting data and control signals comprising: a master station operative for transmitting periodic control signals and for periodic transmitting and receiving data signals; a plurality of satellite stations operative for periodic receiving and transmitting control and data signals; transmission means connecting the master station and the satellite stations sequentially in a loop, each of the satellite stations being connected for receiving the control and data signals and for transmitting the signals to the following station in the loop; and detection means provided in each satellite stations for detecting an abnormality in the periodicity of received signals. The invention also relates to a method of maintaining the continuity of a transmission system having a master station and a plurality of satellite stations connected sequentially in a loop by transmission means. Conventionally, a transmission network may be organized to operate in a loop mode for transmitting information in serial data format between the master and one or more satellite stations, see e.g. "Sequence Control Engineering", published by the Institute of Electric Engineers, Tokyo, on November 20, 1988 and Patent Abstracts of Japan, vol. 6, no. 226 (E-141) 11 November 1982 & JP-A-57 129 048 (Hitachi) 10 August 1982. Fig. 4 illustrates the configuration of a conventional loop mode transmission system comprising a plurality of stations 1 through 6. In the figure, a single master station 1 serves to monitor the system's operation and control the polling of other satellite stations that also are connected in the loop. The master station 1 is connected directly to only two adjacent slave stations 2 and 6 by transmission links 7 and 12, respectively. The remaining satellite stations 3 through 5 are connected only to adjacent slave stations by transmission lines 8 - 11 and communicate with the master station through other slave stations in the loop. Each of satellite stations 2 through 6 has transmission equipment, including a computer and input/output devices, for receiving/transmitting data from/to the master station 1. In an operation of the system in Fig. 4, data for one or more satellite stations in the network is addressably transmitted from the master station 1 via the transmission line 7 to satellite station 2. Station 2 selects the data addressed to it and passes on the balance of the data to satellite station 3 via the transmission line 8. Station 2 may also insert into the serial stream data addressed to the master station. In a similar fashion, each of stations 3 - 6 remove their addressed data and pass the remainder, including data addressed to the master station, to other stations through respective transmission circuits 9 - 11. The last station 6 transmits the assembled data stream via the line 12 to the master station. Thus data is transmitted in the transmission system in a loop mode. In a loop transmission system, both data and control signals that are transmitted from the master station are regenerated, amplified and relayed to the next satellite station in the loop. Consequently, the transmission distance can be made long. Moreover, the transmission speed can be raised to a high rate. On the other hand, if an abnormality in any of the satellite stations occurs, amounting to a station breakdown, the entire system will be shut down. Fig. 5 illustrates the configuration of a conventional, addressable data transmission system that is organized to operate in a bus mode. In the figure, the master station 13, which controls the data transmission, is connected to a plurality of satellite stations 14 - 17 that are operative to receive/transmit data from/to the master station by means of input/output devices and to process data by a resident computer. A transmission trunk line 18 connects each of the satellite stations in parallel with the master station. Lines 19 - 22 are the branch lines that respectively connect the stations 14 - 17 with the trunk line. Each of the stations inputs only the transmitted data that is addressed to it. Next, the function of the bus mode of data transmission is described in Fig. 5. As shown in the figure, data transmission from the master station 13 is transmitted through the transmission trunk line 18; the transmission is received by the satellite stations 14 - 17 through the branch lines 19 - 22 and input in the respective satellite stations. In a transmission system operative in a bus mode, the transmission trunk line 18 is not interrupted by any of the satellite stations connected to it. Hence, the impedance of the satellite stations is set higher than that of the transmission trunk line. Thus, if any abnormality should occur within any of the satellite stations, its breakdown will not affect the other stations. The master station can continue to transmit data and communicate with the satellite stations other than the one affected by the breakdown. However, unlike the loop mode, because this mode does not employ the regenerative relay method, its transmission distance and transmission speed is limited. A transmission system and a method of maintaining the continuity of a transmission system of the type specified above are known from the publication Patent Abstracts of Japan, vol. 6, No. 226 (E-141) of November 11, 1982 and the corresponding publication JP-A-57 129 048. The document discloses detection means, namely PLL circuits, at each satellite station. Also, the system only provides for terminating data transmission from the master station when a failure is detected at any one satellite station by its PLL circuit. However, no switching means are provided nor a bus transmission mode of the stations downstream in the loop transmission direction from the failed satellite station. The document Patent Abstracts of Japan, vol. 6, No. 160 (E-126) and the corresponding publication JP-A-57 080 847 discloses a loop data transmission system comprising a control synchronization station and a stand-by synchronization station, both connected with satellite stations to form a loop data transmission system. Data transmitted by the control station contain flags monitored by the stand-by station. In response to a fault in the control station, the system switches to the stand-by station by disconnecting the transmission from the control station. This document discloses switching means provided for disconnecting the various stations, however, there is no switching means to provide a parallel connection of the satellite stations for transmission in a bus mode. The document Patent Abstracts of Japan, vol. 5, No. 23 (E-45) (695) and the corresponding publication JP-A-55 150 636 discloses a data transmitting unit which forms a satellite station wherein the station itself performs trouble-shooting by a microprocessor. If the station is in normal condition, a switch is connected to a first side, and trouble-shooting is performed in the transmitting mode as well as the receiving code. In case that the electric power of the station is broken, if a fault is decided by a microprocessor and an additional circuit, the station is separated, and the switch is connected to a second side in order to bypass a receiving signal. When the separated statoin has been repaired it is provided that bypassing of the transmission line is stopped. For this purpose, a reset signal is transmitted from a transmission circuit, and the switch is reset by a reset permission signal from a central transmission control station. In this conventional device no detecting means are provided in the satellite station for detecting an abnormality in the periodicity of a received signal. Since the document deals only with one faulty substation, no details are disclosed as to the activation of switching means in any other satellite stations in response to a fault in this one satellite station. It is an object of the present invention to overcome the problems that occur in the conventional transmission system operating in a loop mode when an abnormality occurs in any single satellite station in the loop, such that the operation of the entire system will not have a breakdown. It is a further object of the present invention to identify a station in which the abnormality occurs. It is yet another object of the present invention to avoid any significant delay after a station breakdown before system operation can be restored. Finally, it is an object of the present invention to provide a network design that is well suited for long-distance, high-speed transmission. In particular, the present invention is designed to resolve the problems in the conventional loop mode and bus mode systems while being capable of transmitting data at a relatively hi-gh speed. The system is designed so that even if abnormalities occur in a number of satellite stations, the system will not breakdown and can continue to transmit data. The object underlying the present invention is solved by a transmission system of the type specified above which is characterized in that upon failure of a satellite station, each of the detection means are adapted, responsive to abnormality detection, to activate switching means being arranged to provide a parallel connection of that satellite station with the transmission means, thereby providing a bus transmission mode of those stations downstream in loop transmission direction from the failed satellite station; detection means are provided in the master station for detecting an abnormality in the periodicity of received signals; the detection means of the master station are adapted, responsive to abnormality detection, to activate switching means being arranged to provide parallel connection of the master station with the transmission means and to transmit a command signal to activate the switching means of satellite stations downstream in loop transmission direction from the master station, thereby providing a bus transmission mode of the master station and the downstream stations therefrom. According to a further development of the transmission system according to the invention the master station further includes means for detecting that the abnormality has been removed comprising means for transmitting a diagnostic message that requires a response from each satellite station. A further development of the transmission system according to the invention comprises means responsive to the detection of the removal of the abnormality for sending commands to the switching means to all stations to switch from the bus transmission mode to the loop transmission mode. In a specific embodiment of the transmission system according to the invention, the detection means comprise a watchdog timer for each station. According to a further development of the transmission system according to the invention, the switching means are switchable in response to a command from the master station. In a specific embodiment according to the transmission system of the invention, the switching means additionally are externally switchable. According to the invention, a method of maintaining the continuity of a transmission system having a master station and a plurality of satellite stations connected sequentially in a loop by transmission means comprises the following steps: monitoring the signals periodically transmitted via the transmission means; detecting an abnormality in the periodicity of the transmitted signals at the master station and the satellite stations; and in response to an abnormality detection, switching the loop mode connection to a bus mode connection by activating switching means at every station to connect each station in parallel to the transmission means. A further development of the method according to the invention comprises monitoring the abnormality while in the bus mode and, when the abnormality has been removed, automatically switching the bus mode to a loop mode connection. A further development of the method according to the invention comprises the step of externally switching between the bus mode and the loop mode. In a still further development of the method according to the invention the switching step comprises commanding satellite stations to switch from a loop mode and a bus mode by the master station. In a still further development of the method according to the invention the switching step comprises a satellite station commanding itself to switch from a loop mode and a bus mode. A still further development of the method according to the invention comprises testing for the continued presence of a failure at a satellite station while in the bus mode, and automatically switching to the loop mode when the failure has been removed. In a specific embodiment of the method according to the invention, the testing step is performed by the master station. Fig. 1 shows an example of the application of the present invention to a transmission system. Fig. 2 illustrates the method of switching the transmission mode of Fig. 1. Fig. 3 illustrates a configuration of circuitry that makes possible the switching of the transmission modes. Fig. 4 illustrates a conventional loop transmission system. Fig. 5 illustrates the conventional bus transmission system. DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of the first and second embodiments of the invention will be described with respect to Figs. 1 and 2; the third aspect is described with respect to Fig. 3. Items in the figures that bear the same symbols are identical or functionally equivalent. Fig. 1 illustrates a transmission system representing the first embodiment of the invention, the configuration of the loop transmission mode. Fig. 2 illustrates both the first and second features of the invention, and the condition under which the loop transmission mode of Fig. 1 is switched to the bus transmission mode. Specifically, Fig. 1 shows how,under normal conditions,the master station 23 and each of the plurality of the satellite stations 24, 25 and 26 function normally and transmit data in the loop transmission mode. The master station 23 transmits to each of the satellite stations various control and data signals. From the internal circuitry 27 of master station 23, data is output by transmitter 39 to transmission line 31 and is received by the receiving device 44 of satellite station 24. The internal circuit 28 of the satellite station 24 selects and removes from the serial stream the data addressed to it and transmits the remainder of the date via transmitting device 40 and through the transmission line 32 to the next satellite station 25. Likewise satellite stations 25 and 26 respectively receive, remove and transmit the data in the loop circuit. Data from each slave that is intended for the master station also may be added to the serial data stream as it proceeds to the master station. If an abnormality occurs in a satellite station and its operation breaks down, the integrity of the communication is maintained, as illustrated in Fig. 2. For example, when the operation of satellite station 25 breaks down as a result of an abnormality in its receiver, transmitter or processor, the station will not be able to transmit data to satellite station 26. Since the normal operation of the data transmission system in a loop mode ordinarily results in data being transmitted around the loop periodically, the absence of periodic data can indicate a failure somewhere in the loop. A conventional "watch dog timer", which is operative to trigger an alarm unless reset by the occurrence of data on the loop at a frequency comparable to the normal data period, can be used. Thus, when data periodically transmitted to satellite station 26 cease to be transmitted, an abnormality in the transmission is assumed and the timer will output a signal that turns ON the bus mode relay 38, as shown in Fig. 2. If there were additional satellite stations downstream of the failure, their watch dog timers also would trigger their corresponding bus mode relays. The master station 23 also detects the transmission abnormality, because it does not receive the data stream on a periodic basis, and turns ON its own bus mode relay 35. Moreover, the master station 23 transmits a command to subsequent satellite stations in the direction of transmission around the loop, such as station 24, informing them of the occurrence of the transmission abnormality. As a result bus mode relay 36 of satellite station 24 also is turned ON. As to station 25 where the abnormality occurred, if the failure is in the receiver, the stations's own watchdog timer will actuate the relay 37. If the failure is in the processor or the transmitter, the command from the master that is forwarded by slave 24 will actuate the relay 37. Once all of the bus mode relays 35 - 38 have been activated, the master station 23 and the satellite station 24 to 26 are connected in parallel with the transmission cable. The transmission may include a diagnostic message that requires a response by each satellite station and the master station starts transmitting data in the bus mode. When the master station 23 attempts to communicate with satellite station 25, however, due to its breakdown, communication is impossible. Consequently, satellite station 25 will be identified by the master station as the one which is abnormal. Fig. 2 describes the application of the second aspect of the invention. In the figure, the operation of satellite station 25 was abnormal; thus, data transmission was switched to the bus mode. All of the bus mode relays 35 - 38 are turned ON. The master station 23 continues to periodically transmit signals to all stations, including satellite station 25; however, because communication is impeded by the abnormality in satellite station 25, the fact that it has not yet been restored from its abnormal condition is regularly detected. When the cause of abnormality of satellite station 25 is remedied and the station's operation is restored, it will respond to data transmitted from the master station 23. The master station 23 will detect the restoration of satellite station 25 when it receives answering data signal from satellite station 25. When the master station 23 detects the remedy of the abnormal station, it transmits data signals that turns OFF all the relays 36 - 38 of satellite stations 24 - 26. Also, the master station 23 turns OFF relay 35. When master station 23 turns OFF relays 36, 37 and 38 it may take into consideration the delay time required for the transmission line to be reconnected in the loop transmission mode before recommencing transmitting data signals in the loop mode. Thus, as described above, under normal conditions, data transmission is implemented in the loop transmission mode, which is the high-speed mode of transmission. However, when the operation of any of the plurality of satellite stations is interrupted by abnormal breakdown, data transmission is maintained uninterrupted by a switch to the bus transmission mode, although its transmission speed is slow. Moreover, since the master station can detect the abnormal breakdown of any station, the affected station can be identified so that corrective action can be taken and the system can be restored quickly to normality. Also, when data is transmitted in the bus mode and service of the abnormal station is restored to normality, the system automatically reverts to the normal high-speed mode of data transmission; thus, the system's reversion to normal high-speed mode of transmission is readily achieved. Fig. 3 illustrates the third aspect of the invention. Here, the circuitry of a satellite station, which may be a communication device of many types in a variety of data transmission systems, is illustrated. In the Figure, 47 and 48 generally represent the communication devices, 49 - 51 the transmission lines connecting each of the communication devices, 52 and 53 the external switches for switching between the loop and bus transmission modes, 54 and 55 the internal circuitry of the communication devices, 56 and 57 the data receiving devices, 58 and 59 the transmitting devices, and 60 and 61 the pull-up resistors. In operation, when the external switches 52 and 53 are OFF, the transmission lines 49 - 51, which connect each of the communication devices 47 and 48, form a loop. When the input of the input port of the internal circuitry 54 and 55 is raised to the "H" level, the loop mode of the communication devices 47 and 48 of the transmission system is detected; and data transmission is implemented in the loop mode. Next, when the external switches 52 and 53 are turned ON, the transmission lines 49 - 51 that connect each of the communication devices 47 and 48 will connect them in paral.lel. Then the input of the input ports of the internal circuitry 54 and 55 will be lowered to the "L" level. As a result, the communication devices 47 and 48 will be detected as being in the bus mode of transmission; and data transmission will be implemented in the bus mode. As described above, in this transmission system, the communication devices' specifications are designed to optionally operate in either the loop or data mode of transmission. Because the system is designed to switch between the two modes of transmission, the user's system configuration design will be more economical than if he were using either of the two, and had to purchase an alternative mode. In connection with the first aspect of the system, under normal conditions, a number of satellite stations receive data and control signals from the master station and communicate with it in the loop mode. However, when an operational abnormality occurs in any of the satellite stations, the system is designed so that the transmission mode is switched to the bus mode. In other words, under normal conditions communication between the master and satellite stations is performed at high speed. However, when an abnormality occurs in any of the satellite stations, communication between the master and satellite stations can still be maintained uninterrupted under this dual-mode system. In connection with the second aspect of the invention, the system is designed so that when the master station detects that the abnormal satellite station's operation has been restored to normal, the communication mode between the master and satellite stations is switched automatically from the bus to the loop mode. As a result, the transmission system is automatically restored to the normal high-speed mode of communication. In the third aspect of the invention, the master and the plurality of satellite stations are equipped with an externally established means for switching between either the loop or bus mode of transmission. Thus, this system offers an option of readily and effectively switching the transmission mode effectively when necessary. As shown in Fig. 1 and 2 of the drawings, all of the stations 23 to 26 have a similar structure and are connected through transmission lines 31 to 34. In addition to the above explanations, it should be clear that the transmission system comprises the following components: 27 to 30 are internal circuitries, 35 to 38 are bus mode relays, 39 to 42 are transmitters, and 43 to 46 are receiving devices. The function of these components in the respective stations is the same as described above by way of example. The entire disclosure of each and every foreign patent application from which the benefit of foreign priority has been claimed in the present application is incorporated herein by reference, as if fully set forth. Although this invention has been described in at least one preferred form with a certain degree embodiment particularity, it is to be understood that the present disclosure of the preferred embodiment has been made only by way of example and that numerous changes in the details and arrangement of components may be made without departing from the scope of the invention.

Claims: A transmission system for transmitting data and control signals comprising: a master station (23) operative for transmitting periodic control signals and for periodic transmitting and receiving data signals; a plurality of satellite stations (24 - 26) operative for periodic receiving and transmitting control and data signals; transmission means (31 - 34) connecting the master station (23) and the satellite stations (24 - 26) sequentially in a loop, each of the satellite stations (24 - 26) being connected for receiving the control and data signals and for transmitting the signals to the following station in the loop; detection means (28, 29, 30) provided in each satellite station (24 - 26) for detecting an abnormality in the periodicity of received signals; characterized in that upon failure of a satellite station, each of said detection means (28, 29, 30) are adapted, responsive to abnormality detection, to activate switching means (36 - 38) being arranged to provide a parallel connection of that satellite station (24 - 26) with the transmission means (31 - 34), thereby providing a bus transmission mode of those stations downstream in loop transmission direction from the failed satellite station; detection means are provided in the master station (23) for detecting an abnormality in the periodicity of received signals; the detection means of the master station are adapted, responsive to abnormality detection, to activate switching means (35) being arranged to provide parallel connection of the master station with the transmission means (31 - 34) and to transmit a command signal to activate the switching means (36 - 38) of satellite stations (24 - 26) downstream in loop transmission direction from the master station (23), thereby providing a bus transmission mode of the master station (23) and said downstream stations (24 - 26) therefrom. The system according to Claim 1, wherein the master station (23) further includes means (27) for detecting that the abnormality has been removed comprising means for transmitting a diagnostic message that requires a response from each satellite station (24 - 26). The system according to Claim 2, further comprising means (23, 27, 35 - 38) responsive to the detection of the removal of the abnormality for sending commands to the switching means (35 - 38) to all stations (23 - 26) to switch from the bus transmission mode to the loop transmission mode. The system of any of Claims 1, 2 or 3, wherein the detection means (28, 29, 30) comprise a watchdog timer for each station (23 - 26). The system of any of Claims 1 to 4, wherein the switching means (35 - 38) are switchable in response to a command from the master station (23). The system of any of the Claims 1 to 5, wherein the switching means (52, 53) additionally are externally switchable. A method of maintaining the continuity of a transmission system having a master station (23) and a plurality of satellite stations (24 - 26) connected sequentially in a loop by transmission means (31 - 34) comprising: a) monitoring the signals periodically transmitted via the transmission means (31 - 34); b) detecting an abnormality in the periodicity of the transmitted signals at the master station and the satellite stations; and c) in response to an abnormality detection, switching the loop mode connection to a bus mode connection by activating switching means (35 - 38) at every station (23 - 26) to connect each station in parallel to the transmission means (31 - 34). The method of Claim 7, further comprising monitoring the abnormality while in the bus mode and, when the abnormality has been removed, automatically switching the bus mode to a loop mode connection. The method of Claim 7 or 8, further comprising externally switching between the bus mode and the loop mode. The method of any of Claims 7 to 9, wherein the switching step comprises commanding satellite stations (24 - 26) to switch from a loop mode and a bus mode by the master station (23). The method of any of Claims 7 to 10, wherein the switching step comprises a satellite station (24 - 26) commanding itself to switch from a loop mode and a bus mode. The method of any of Claims 7 to 11, further comprising testing for the continued presence of a failure at a satellite station (24 - 26) while in the bus mode and automatically switching to the loop mode when the failure has been removed. The method of Claim 12, wherein the testing step is performed by the master station (23).

Inventors: HATTORI NORIYUKI, TAKAHASHI NOBORU, HATTORI, NORIYUKI, TAKAHASHI, NOBORU, Hattori, Noriyuki, c/o MITSUBISHI El. Eng. Co Ltd., Takahashi, Noboru, c/o MITSUBISHI DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: H04L12
IPC codes: H04L12