Allen-Bradley GuardPLC 1753, Safety Reference Manual

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58 Publication 1753-RM002C-EN-P - September 2008
Chapter 5 GuardPLC DeviceNet Safety Scanner
Safety Network Number
The Safety Network Number (SNN) is a unique number that identifies
the safety network sub-net. The SNN in conjunction with the target’s
node address, enables a target to determine with high integrity
whether or not safety connection requests it receives have reached the
correct destination.
Each end node within a DeviceNet safety control system must have a
unique node identifier. The unique node reference for a DeviceNet
safety node is a combination of a SNN and the node address of the
node. It is used to precisely identify the intended target device during
configuration and I/O connection establishment.
Any device that originates a safety connection to another safety device
must be configured with the SNN of the target device.
The assignment of a time-based SNN is the default when adding a
DeviceNet safety scanner or new DeviceNet safety I/O modules.
IMPORTANT
The automatic time-based SNN is generally adequate for most
applications. If you assign SNNs manually, take care to ensure
that system expansion does not result in duplication of SNN
and node address combinations.
ATTENTION
If a safety project is copied to another project intended for a
different hardware installation and that installation may reside
within the same routable safety system, the SNN must be
changed to ensure that the SNN is not repeated.