Rockwell Automation 1791DS-IBxxxx Guard I/O DeviceNet Safety Modules - Manual (Page 116)

116 Rockwell Automation Publication 1791DS-UM001J-EN-P - May 2013
Chapter 7 Considerations When Replacing Guard I/O Modules
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.
Figure 26 - Connections Not Made Example
Why You Need to Manually Set the SNN
The examples above showed how the SNN is used to guarantee
safety-connection integrity after the system is operational. But the SNN is also
used to guarantee integrity on the initial download to the Guard I/O module.
If a safety signature exists, then the Guard I/O module must have a proper SNN/
node number identification that matches the module within the project, before it
can receive its configuration. And to keep integrity, the setting of the module’s
SNN is required to be a manual action. This manual action is to use the ‘set’
function on an out-of-box Guard I/O module.
DNB #1 DNB #2 DNB #1 DNB #2
5
SNN 100 5
SNN 101 5
SNN 100 5
SNN 101
Connections not made.