Unique Node Reference; Safety Network Numbers (Snn) - Allen-Bradley GuardLogix 5580 Original Instructions Manual

Unique Node Reference

Safety Network Numbers
(SNN)
CIP Safety and Safety Network Numbers
Topic
Unique Node Reference
Safety Network Numbers (SNN)
Routable CIP Safety System
Considerations for Assigning SNNs
How SNNs Get to Safety Devices
SNN Formats
SNNs for Out-of-box Devices
CIP Safety control systems are composed of CIP Safety devices that are
interconnected via communication networks. These networks consist of
devices (switches, bridges, adapters, and so on) that may not be SIL 2 or SIL 3
certified. Therefore, the CIP Safety devices must be inherently protected from
network delivery errors.
The CIP Safety protocol is an end-node to end-node safety protocol. This
configuration allows the routing of CIP Safety messages to and from CIP
Safety devices through non-certified bridges, switches, and routers.
A key element of the CIP Safety protocol is the concept of a Unique Node
Reference (also called Unique Node ID or UNID). Every CIP Safety device
must have a UNID value assigned to each CIP Safety-capable port.
IMPORTANT It is your responsibility to make sure that all UNIDs are truly unique within
the scope of all devices that could possibly communicate with each other.
Communications within a control system travel over subnets that are
interconnected with bridging or routing components. Examples of subnets:
• The backplane of a chassis
• A bank of I/O modules
• An Ethernet subnet within a LAN
Rather than creating a UNID directly for each CIP Safety device (which could
be prone to error in a large system), each subnet is assigned a unique Safety
Network Number (SNN), and the UNID is created from the SNN + the
Node Address.
Rockwell Automation Publication 1756-RM012B-EN-P - April 2018
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