34 Rockwell Automation Publication 1756-RM093J-EN-P - April 2018
Chapter 4 CIP Safety and the Safety Network Number
Unique Node Reference
The CIP Safety protocol is an end-node to end-node safety protocol. The CIP
Safety protocol allows the routing of CIP Safety messages to and from CIP Safety
devices through non-certified bridges, switches, and routers.
To prevent errors in non-certified bridges, switches, or routers from becoming
dangerous, each end node within a routable CIP Safety control system must have
a unique node reference. The unique node reference is a combination of a safety
network number (SNN) and the node address of the node.
Safety Network Number
The safety network number (SNN) is assigned by software or by the user. Each
CIP Safety network that contains Safety I/O nodes must have at least one unique
SNN. Each ControlBus™ chassis that contains one or more safety devices must
have at least one unique SNN. Safety network numbers assigned to each safety
network or network subnet must be unique.
Figure 9 - CIP Safety Example with More Than One SNN
Each CIP Safety device must be configured with an SNN. Any device that
originates a safety connection to another safety device must be configured with
the SNN of the target device. If the CIP Safety system is in the start-up process
prior to the functional safety testing of the system, the originating device may be
used to set the unique node reference into the device.
The SNN used by the system is a 6-byte hexadecimal number. The SNN can be
set and viewed in one of two formats: time-based or manual. When the time-
More than one SNN can be assigned to a CIP Safety subnet or a ControlBus
chassis that contains more than one safety device. However, for simplicity,
we recommend that each CIP Safety subnet have one, and only one, unique
SNN. This is also the case for each ControlBus chassis.
Router/
Firewall
SNN_1
SNN_3 SNN_5
SNN_2
SNN_4
SNN_6
SNN_7
1756-L62S
1756-LSP
1756-DNB
1756-ENBT
1768-PB3
1768-L43S
1769-ECR
Switch
Switch
CIP Safety I/O
SmartGuard
1756-IB16
1756-DNB
1756-ENBT
1756-OB16
1768-ENBT
1768-ENBT
CIP Safety I/O
CIP Safety I/O
CIP Safety I/O
CIP Safety I/O
CIP Safety I/O
CIP Safety I/O
CIP Safety I/O
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