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Rockwell Automation Publication 1734-UM013C-EN-P - August 2010
Chapter 8
Considerations When Replacing POINT Guard I/O Modules
The following, simplified example is of CompactBlock Guard I/O on a
DeviceNet network. Your products may differ, but the function is the same.
EXAMPLE
The DeviceNet network supports 64 node numbers, so if you have 100
devices on multiple DeviceNet networks, there are at least 36
duplicate node numbers being used. Even though the duplicate nodes
are on separate DeviceNet networks, this must still be considered in a
safety system.
In this example, the DNB scanner #1 is connected to node 5. The DNB
scanner #2 is connected to another node 5. If the cables get
inadvertently crossed, the scanners may be communicating with the
incorrect node 5.
Crossed Cable Example
This crossed-cable scenario is unacceptable for a safety system. The
SNN guarantees unique identification of every safety device. In this
next example, all of the devices connected to DNB scanner #1 have an
SNN of 100. All devices connected to DNB scanner #2 have an SNN
of 101. If the cables get inadvertently crossed, the node connected to
DNB scanner #1 changes from 100/5 to 101/5. The node connected to
DNB scanner #2 changes from 101/5 to 100/5. Therefore, the safety
connections are
not
made if the cables get crossed.
Connections Not Made Example
DNB #1
DNB #2
DNB #1
DNB #2
5
5
5
5
DNB #1
DNB #2
DNB #1
DNB #2
5
SNN 100
5
SNN 101
5
SNN 100
5
SNN 101
Connections
not
made.