
Modeling a Ring Network Topology in BlockSim
When designing a control system, a major consideration is the topology of the computer network that will be used to communicate between the humanmachine interface (HMI) and the controlled devices. There are many topologies in use today, such as pointtopoint, bus, star, ring, mesh, and hybrid networks [1]. In order to select the best topology for the control system, it is necessary to understand the benefits and risks associated with each type of network topology. By modeling proposed network configurations in BlockSim, the analyst can predict the availability of each topology before the control system design is finalized.
This twopart article addresses how to use BlockSim 10 to create a reliability block diagram (RBD) model of a ring network with bidirectional communication paths that will provide information on both the system availability and the availability of particular communication paths. The first part discusses how to create the systemlevel RBD model starting from the ring network schematic. The subsequent article will demonstrate how to augment the model to incorporate blocks that represent the state of communication between the HMI and each connected device.
A simplified version of a ring network will be studied. The network consists of the components listed in Table 1. A schematic is shown in Figure 1.
Item  Location(s) in Ring Network 
Programmable Logic Controller  PLC 
Input/Output Board  I/O Board 1, I/O Board 2 
Ethernet Switch  ENS 1, ENS 2, ENS 3, ENS 4, ENS 5 
Human Machine Interfaces  HMI 1, HMI 2 
Ethernet Cable  C 12, C 15, C 23, C 30, C 34, C 40, C 45, C 50, C 101, C 102, C 201, C 202 
Figure 1  Simplified Ring Network Schematic
The first step to create an RBD from the schematic is to determine the success criteria. The system is considered operational when either HMI 1 or HMI 2 can communicate with either of the I/O boards or the PLC. Therefore, if at least one of the following scenarios is true, then the system is operational:
 HMI 1 communicates with I/O Board 1
 HMI 1 communicates with I/O Board 2
 HMI 1 communicates with PLC
 HMI 2 communicates with I/O Board 1
 HMI 2 communicates with I/O Board 2
 HMI 2 communicates with PLC
The second step is to determine what items must operate for each success criterion. For success criterion 1, I/O Board 1, C 50, ENS 5 and HMI 1 must operate along with either of the following paths:
 C 15 and ENS 1 and [C 101 or (C 12 and ENS 2 and C102)]
 C 45 and ENS 4 and C 34 and ENS 3 and C 23 and ENS 2 and [C 102 or (C 12 and ENS 1 and C 101)]
For success criterion 2, I/O Board 2, C 30, ENS 3 and HMI 1 must operate along with either of the following paths:
 C 23 and ENS 2 and [C 102 or (C 12 and ENS 1 and C 101)]
 C 34 and ENS 4 and C 45 and ENS 5 and C 15 and ENS 1 and [C 101 or (C 12 and ENS 2 and C 102)]
For success criterion 3, PLC, C40, ENS 4 and HMI 1 must operate along with either of the following paths:
 C34 and ENS 3 and C 23 and ENS 2 and [C 102 or (C 12, ENS 1, and C 101)]
 C45 and ENS 5 and C 15 and ENS 1 and [C 101 or (C 12, ENS 2, and C 102)]
For success criterion 4, I/O Board 1, C 50, ENS 5 and HMI 2 must operate along with either of the following paths:
 C15 and ENS 1 and [C 201 or (C 12 and ENS 2 and C 202)]
 C45 and ENS 4 and C 34 and ENS 3 and C 23 and ENS 2 and [C 202 or (C 12 and ENS 1 and C 201)]
For success criterion 5, I/O Board 2, C 30, ENS 3 and HMI 2 must operate along with either of the following paths:
 C 23 and ENS 2 and [C 202 or (C 12 and ENS 1 and C 201)]
 C 34 and ENS 4 and C 45 and ENS 5 and C 15 and ENS 1 and [C 201 or (C 12 and ENS 2 and C 202)]
For success criterion 6, PLC, C40, ENS 4 and HMI 2 must operate along with either of the following paths:
 C34 and ENS 3 and C 23 and ENS 2 and [C 202 or (C 12 and ENS 1 and C 201)]
 C45 and ENS 5 and C 15 and ENS 1 and [C 201 or (C 12 and ENS 2 and C 202)]
The third step is to create the RBD. Although a diagram could be constructed directly from the preceding logical statements, the number of blocks in the diagram can be reduced by combining the statements prior to creating the diagram. One way to do this is described next.
By reviewing the logical statements above, it can be seen that for any device to communicate with either HMI 1 or HMI 2, one of the following sets of components must be operational:
 Clockwise Path: C 15 and ENS 1 and [(C 101 and HMI 1) or (C 12 and ENS 2 and C 102 and HMI 1) or (C 201 and HMI 2) or (C 12 and ENS 2 and C 202 and HMI 2)]
 Counterclockwise Path: C23 and ENS 2 and [(C 102 and HMI 1) or (C 12 and ENS 1 and C 101 and HMI 1) or (C 202 and HMI 2) or (C 12 and ENS 1 and C 201 and HMI 2)]
Now, one logical statement can be written for each device in terms of the Clockwise Path and Counterclockwise Path above. Therefore, for communication between I/O Board 1 and either HMI 1 or HMI 2:
I/O Board 1 and C 50 and ENS 5 and [Clockwise Path or (C 45 and ENS 4 and C 34 ENS 3 and Counterclockwise Path]
For communication between I/O Board 2 and either HMI 1 or HMI 2:
I/O Board 2 and C 30 and ENS 3 and [(C 34 and ENS 4 and C 45 and ENS 5 and Clockwise Path) or Counterclockwise Path]
For communication between the PLC and either HMI 1 or HMI 2:
PLC and C 40 and ENS 40 and [(C 45 and ENS 5 and Clockwise Path) or (C 34 and ENS 3 and Counterclockwise Path)]
Using these statements, the RBD is constructed as shown in Figure 2. Note that because the communication is bidirectional, it is necessary to use mirror blocks to construct the RBD. All the blocks in a given mirror group in the RBD represent one component in the schematic. For more information on using mirror blocks and creating mirror groups, please see http://help.synthesisplatform.net/blocksim_reno10/index.htm#mirroring_using_blocks_in_multiple_locations.htm.
Figure  2 Simplified Ring Network RBD
Conclusion
This article presented the procedure for building a reliability block diagram to represent a ring network. First, the success criteria for the system were developed. Then, these criteria were used to create logic statements that represent the combinations of components that are necessary for the system to be operational. Finally, the logical statements were combined and used to create the systemlevel RBD.
References
1. https://en.wikipedia.org/wiki/Network_topology accessed October 5, 2016.