Abstract:
A system, method, and computer readable medium allow a cable break or other network link failure to be detected and reported. The method may include steps of periodically transmitting a packet from a root node in the network to a next node in the network; at each successive non-root node in the loop, manipulating the packet and forwarding the manipulated packet to a next successive node in the loop; when the packet is received back at the root node within a predetermined time period, reporting that the loop is intact; and when the packet is not received back at the root node within the predetermine time period, reporting that a network break has occurred. The network may conform to the STP or RSTP protocols, and may contain one or more rings possibly including dual rings. The PLC may operate as the root node, and the reporting may be done at an application level in the PLC.

Description:
BACKGROUND 
       [0001]    Industrial automation systems often include a Programmable Logic Controller (PLC) coupled to other devices, such as remote I/O devices, over one or more communication networks. In a factory or other industrial setting, such networks may include dozens of sensors that constantly monitor such things as temperatures, fluid levels, or robotic arm positions. The sensors may be coupled to I/O devices that report back to the PLC many times per second. The PLC may include rules for acting on the changing conditions, such as issuing commands to other devices over the network to perform various operations in the system. The PLC may also monitor and report on the status of various operating parameters and errors detected in the system. 
         [0002]    As shown in  FIG. 1 , a user console  101  is coupled to a programmable logic controller (PLC)  102 . Console  101  may comprise one or more processors and memory storing application software and/or a user interface that allows a user or a computer program to configure, alter, control, and monitor the operation of functions in PLC  102  and a network in which the PLC operates. PLC  102  may comprise any of various types of commercially available PLCs, such as the Modicon™ Quantum™ PLC available from Schneider Electric, Inc. 
         [0003]    PLC  102  may be coupled to one or more communication controllers  103  and  104  through a backplane interconnection or other means. Each communication controller handles communication with one or more remote I/O devices, such as devices  105 ,  106 , and  107 , over one or more networks  108  and  109 , for the purpose of monitoring and controlling various devices and sensors in the system. 
         [0004]    Industrial Ethernet is one type of network used in industrial automation systems to communicate among nodes. A network such as Industrial Ethernet may be configured into one or more rings that are configured and communicate according to Spanning Tree Protocol (STP) or Rapid Spanning Tree Protocol (RSTP). STP is a network protocol that prevents bridge loops (cycles) and the ensuing broadcast radiation (i.e., packets that are transmitted forever endlessly in the network). It also allows a network to include redundant links to provide automatic backup paths if a link fails, without the danger of creating loops that could lead to redundant broadcast messages. 
         [0005]    STP creates a spanning tree within a mesh network of devices and disables any links (blocks ports) between nodes that are not part of the spanning tree, leaving only a single active path between any two network nodes. One node is selected as a root node based on its bridge ID. Other nodes are configured to determine and use the shortest path to the root node. Upon detecting a link failure, the protocol computes and creates a new shortest-path tree structure (e.g., by activating blocked ports), and the network automatically reconfigures itself to work around the failed link or node. STP is standardized as IEEE 802.1D. 
         [0006]    RSTP is an improved version of STP, standardized as IEEE 802.1w, that is able to respond to changes more quickly than STP. Other variations and modifications of STP have also been proposed. 
         [0007]    It would be desirable to provide improved detection and reporting, particularly at an application level in a PLC, of a detected cable break or other network failure in a network such as an RSTP ring of devices. 
       SUMMARY 
       [0008]    Described herein are a system, method, and computer readable medium for detecting a cable break or other link failure in a network, such as an Industrial Ethernet network. The method may include steps of periodically transmitting a packet from a root node in the network to a next node in the network; at each successive non-root node in the loop, manipulating the packet and forwarding the manipulated packet to a next successive node in the loop; when the packet is received back at the root node within a predetermined time period, reporting that the loop is intact; and when the packet is not received back at the root node within the predetermine time period, reporting that a network break has occurred. The network may conform to the STP or RSTP protocols, and may contain one or more rings possibly including dual rings. In some variations, the PLC may operate as the root node, and the reporting may be done at an application level in the PLC. In some variations, the packet may be transmitted as a multi-cast packet in the network. The packet may be handled differently when it is sent over a blocked port. 
         [0009]    Also described herein is a system including a plurality of network nodes configured into a spanning tree topology. A root node in the system may periodically transmit a packet to a next node in the network. Each successive non-root node in the system may manipulate the packet and forward the manipulated packet to a next successive node. When the packet is received back at the root node within a predetermined time period, a report or other indicator may be generated to indicate that the network is intact. When the packet is not received back at the root node within a predetermined time period, a loop break message or indicator may be generated. The network may conform to the STP or RSTP protocols, and may contain one or more rings possibly including dual rings. In some variations, the packet may be transmitted as a multi-cast packet in the network. The packet may be handled differently when it is sent over a blocked port. 
         [0010]    Also described herein is a computer-readable medium, such as a memory device, including executable instructions that, when executed, perform functions relating to the method and system described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    A more complete understanding of the present disclosure and the potential advantages of various aspects described herein may be acquired by referring to the following description in consideration of the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
           [0012]      FIG. 1  is a block diagram of an industrial automation system including a PLC and various network elements. 
           [0013]      FIG. 2  is block diagram of a system including one or more networks configured to operate according to various principles described herein. 
           [0014]      FIG. 3  is a flowchart showing various steps or functions that may be performed according to various principles described herein. 
           [0015]      FIG. 4  shows one possible frame format for a loop check message. 
           [0016]      FIG. 5  shows a device that may be used to implement various functions described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 2  shows a system including one or more networks configured to operate according to various principles described herein. As shown in  FIG. 2 , a first (primary) ring of networked elements  201  includes a root node  205 , which may comprise a PLC or other networked device. The ring also includes a first dual-ring switch  206 , which couples the primary ring to a second (secondary) ring of networked elements  202  (left side of  FIG. 2 ). The dual-ring switch permits packets originating from a network element on one ring (e.g., ring  201 ) to be transmitted to a network element on another ring (e.g., ring  202 ). The primary ring also includes dual-ring switch  207 , which couples primary ring  201  to secondary ring  202 , as well as dual-ring switches  208  and  209 , which couple primary ring  201  to a third secondary ring  203 , and dual-ring switch  210 , which couples primary ring  201  to a fourth secondary ring  204 . 
         [0018]    In certain embodiments, primary ring  201  is configured and managed according to STP or RSTP, allowing ring  201  to automatically detect and repair network links as is well known. For convenience, the dual-ring switches  206  through  210  are labeled with both sides of the rings to which they couple (e.g., dual-ring switch  206  is labeled with both RSTP2, corresponding to ring  202 , and RSTP1, corresponding to ring  201 .) 
         [0019]    Also as shown in  FIG. 2 , secondary ring  202  includes three dual-port switches  211 ,  212 , and  213 . Although not explicitly shown, each switch may be coupled to one or more devices (e.g., sensors or controllers) for the purpose of performing one or more functions in an industrial automation system using packets that are transmitted among the networked devices shown in  FIG. 2 . Similarly, secondary ring  204  includes dual-port switches  217 ,  218  and  219 , and secondary ring  203  includes dual-port switches  214 ,  215 , and  216 . 
         [0020]    In accordance with STP or RSTP protocols (or one of their variants), certain ports on some of the switches shown in  FIG. 2  are blocked (set to inactive) in order to prevent cycles in the rings. In  FIG. 2 , blocked ports are shown by hash marks  220  (on dual-port switch  212 ), hash marks  221  (on dual-port switch  215 ), hash marks  222  (on dual-port switch  219 ), and hash marks  223  (on dual-ring switch  207 ). 
         [0021]    According to certain embodiments, root node  205  periodically transmits a loop check message around ring  201  to determine whether the loop integrity is intact. If the packet traverses the ring and returns to root node  205  within a predetermined time period, the loop is deemed to be intact, and an indicator may be set to indicate that condition. If the packet does not return to the root node  205  within the predetermined time period, root node  205  may deem the loop to be broken, and an indicator may be set to indicate that condition. 
         [0022]    The root node may comprise a PLC, and the indicator may comprise a bit or other data storage unit in a memory of the PLC. The indicator may also be monitored and reported at an application layer in the PLC, such that a human-readable error message, display screen, or other type of indication (e.g., an email or pager message) is provided. 
         [0023]      FIG. 3  shows various steps and functions that may be performed to carry out a process to diagnose and report a cable break or other network defect. These steps may be implemented in software, hardware, firmware, or a combination of these. Software instructions may be embodied in tangible readable media, such as a memory, and executed by one or more processors or other computing devices. Such instructions may be stored in any of various devices, such as those shown in  FIG. 2 . One or more processors and memories may be used to perform any or all of the steps and functions shown in  FIG. 3 . 
         [0024]    Beginning in step  301 , the root node (e.g., node  205 , which may be a PLC) sends a loop check packet to a neighboring node in the primary ring and sets a timeout timer for receipt of the loop check packet back at the root node. The timeout timer may be set for any value that is reasonable for the size and configuration of the network, or it may be set to a default value. For example, depending on the number of nodes the packet must traverse, an assumption can be made about the longest time it would take to traverse the ring and return to the root node. Alternatively, a default value can be chosen that is larger than the longest time for traversal in the largest possible ring. 
         [0025]    In some variations, the loop check packet is sent using a multicast destination MAC address. In one variation, the destination MAC address of the loop check packet is fixed at 01:80:63:07:00:0A (Hex). All nodes on the ring may register to receive this packet. 
         [0026]    In some embodiments, the packet is not transmitted to the secondary rings (e.g., through the dual-ring switches) but instead only traverses the primary ring. For example, root node  205  transmits the loop check packet only to the RSTP1 port of dual-ring switch  206 , not to the RSTP2 port of dual-ring switch  206 . In some other embodiments, separate loop checks may also be conducted on secondary rings. One possible format for a loop check message is shown in  FIG. 4 , which is explained in more detail below. 
         [0027]    Each node in the ring receives the packet and may manipulate the packet in various ways. In step  302 , the neighboring node (e.g., dual-ring switch  210  in  FIG. 2 ) receives the packet. In step  303 , if the root node is the one receiving the packet (i.e., the packet has traversed the ring and returned to the root node), then in step  304  the packet is discarded and, in step  313 , the ring integrity is reported to be intact, assuming that the timeout time had not expired by the time the packet was received (step  314 ). 
         [0028]    The ring integrity reporting (whether success or failure) may be performed in any of various ways. In some embodiments, an indicator light (green for success, red for failure) may be provided. In other embodiments, an error message may be generated at an application level in a networked device, such as a PLC. The PLC may take further action based on the report, such as transmitting an email, a pager alert, an automated phone call, or other electronic notification techniques. The error may also be logged in a database and/or reported to other applications in the PLC or other devices. 
         [0029]    In step  305 , a check is made as to whether a “no-forward flag” has been set in the loop check packet. If the no-forward flag is set, a check is made in step  306  to determine whether the packet was received over a blocked port. 
         [0030]    If in step  306  the loop check packet was received over a blocked port, then in step  307  the packet is discarded. This ensures that a message is only sent over a blocked port once, and prevents loops or cycles. 
         [0031]    In step  308 , a check is made to determine whether the HostMAC field in the loop check packet is the same as the HostMAC value of the node that received the packet. In an embodiment, use of the HostMAC field may determine if more than one root node exists. If there is a match, then in step  309  the packet is discarded. In an embodiment, the HostMAC address may be set by the originator of the loop check frame. 
         [0032]    In step  310 , a check is made to determine whether the packet is being sent on a blocked port. If the packet is being sent on a blocked port, then in step  311 , the no-forward flag is set. This prevents the packet from being further forwarded a second time across a blocked port. 
         [0033]    After steps  310  and  311 , in step  312  the loop check packet is sent to the next node in the same ring, and the process returns to step  302 . 
         [0034]    If the timeout timer expires before the loop check packet is received back at the root node, then in step  314  an error report is generated. As explained above, the reporting may occur at an application level in a PLC, and may be communicated further via email or other means to a human or to an application program for further handling. 
         [0035]    In step  315 , the process may be repeated (i.e., returning to step  301 ) on an automated basis, such as one a two-minute interval. 
         [0036]      FIG. 4  shows one possible format for a loop check frame. The values shown in  FIG. 4  are in Hex. The invention is of course not limited to the particular values shown in  FIG. 4 . 
         [0037]      FIG. 5  shows an apparatus that can be used to implement devices such as those shown in  FIG. 2 . Device  501 , such as root node  205  or dual-ring switch  206 , may include one or more processors  503  and one or more memories  504  having stored therein instructions that perform the functions described above. The device may also include one or more I/O circuits  502  and  504  to communicate with other devices on the network through ports. References to a processor and memory are also intended to encompass various types of processing structures including, but not limited to, application-specific integrated circuits (ASICs) and field programmable gate arrays (FPGAs). 
         [0038]    The functions and steps described above may be implemented by hardware and/or by software stored in tangible computer-readable media (e.g., a memory) and executed by various computing devices or apparatus, such as a server computer including one or more processors programmed with software. 
         [0039]    The divisions between functional blocks in the figures are merely illustrative, and the physical division of computing devices and other equipment may be different from the functional division. Moreover, some or all of the functional blocks may be combined or further subdivided functionally and/or physically. For example, devices  102  and  103  could be combined into a single device, and even the functions of console  101  could be combined into a single device, such as an industrial PC. 
         [0040]    Unless otherwise explicitly stated, steps of method claims (and corresponding functional elements) herein should not be limited to being performed in the order in which they are recited. 
         [0041]    References to “spanning tree protocol” refer to and include not only standardized STP and RSTP, but other variations and modifications of such protocols. “Spanning tree protocol” should be understood to include a protocol that establishes and repairs a loop-free topology in a network of devices, such as Ethernet switches.