Safety Network Number (SNN)
Communication within a control system travels over subnets that are interconnected with
bridging or routing components. The following are examples of subnets.
- The backplane of a chassis
- A bank of I/O modules
- An Ethernet subnet within a LAN
Rather than creating a Unique Node Reference (UNID) directly for each safety device, which
can be prone to error in a large system, each subnet has a unique SNN and the UNID is created
from the SNN + the node address.
SNN Assignment
When you create controller projects,
FactoryTalk®
Design Studio™
generates an SNN
value automatically whenever it recognizes a new subnet that contains devices:- Each safety-capable port on the controller is assigned an SNN. The safety controllers have up to three safety network numbers: a separate SNN for each Ethernet port, and one SNN for the backplane.
- If a bridge or adapter device is in the I/O tree and a child safety device is added, the subnet that the bridge or adapter creates is assigned an SNN.
For typical users, the automatic assignment of a time-based SNN is sufficient. However,
manual assignment of the SNN is required if the following is true:
- One or more controller ports are on a safety subnet that already has an established SNN.
- A safety project is copied to another hardware installation within the same routable safety system.
We recommend that you change each SNN to the SNN already established for that subnet, if
one exists. That way, devices that are created later in the project are automatically
assigned the correct SNN.
Each safety network must have a unique safety network number. Confirm that a unique SNN is
assigned to each safety network that contains safety devices.
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