Here’s the basics for those of you that wanted a real quick overview of the CIP Ring Technology:
WHAT ARE THE DEVICE REQUIREMENTS?
Any device with two ports can be part of the ring. A device must support the OSI Layer 2 Ring Protocol and implement the EtherNet/IP application objects associated with the protocol.
WHAT IS ITS PURPOSE?
The CIP Ring protocol is designed to provide a fault tolerant CIP network. With the CIP Ring Technology a node failure or physical network cable failure is quickly detected and messages are routed around the failure.
HOW DOES IT WORK?
A Ring Supervisor controls operation of the ring. It blocks messages from continuously traveling around the ring and monitors the integrity of the ring. Any device in the ring can be the ring supervisor.
HOW IS THIS DIFFERENT THAN AN ETHERNET/IP DEVICE WITH TWO PORTS?
An EtherNet/IP device with two ports can be implemented in a linear topology like a conveyor line. But without implementation of the Ring Protocol the device can not participate in a CIP Ring. Until a Ring Supervisor initiates Ring operation all devices act like normal two port EtherNet/IP devices arranged in a linear fashion.
WHY CAN’T I JUST LOOP AN ETHERNET/IP NETWORK AND CREATE A RING?
Closing a standard EtherNet/IP network and making a ring would lead to continuous network traffic. Every message would continuously travel around the ring until a monitoring device noticed that its lifetime counter had expired. Traffic on the ring would escalate and messages would only be added as other messages expired. Without the Ring Supervisor it just wouldn’t work.
ARE THERE CLASSES OF DEVICES?
There are three devices types. The first is a Ring Supervisor. Ring Supervisors manage the ring. Ring Supervisors must be able to send special control frames (Beacon Frames) out every 400 microseconds. The second device type is a Ring Node with Beacon capability. These nodes lack the capability to act as Supervisors but can process a Beacon Frame every 400 microseconds. The last device type is a Ring Node with “Announce” capability. These nodes lack the ability to be a Supervisor or process the Beacon Frame in the 400 microsecond time period. These nodes simply forward messages.
WHAT ARE RING SUPERVISORS?
Ring Supervisors initiate and manage ring operation by sending Beacon Frames into the Ring. Ring Supervisors are the only device in the ring that does not transfer messages from one port to the other. They prevent flooding of the ring with already processed traffic. Ring Supervisors also do the fault detection by timing the reception of the Beacon messages previously issued. If Beacon nodes issued on one port fail to arrive on the other port a Ring Supervisor classifies the Ring as being in a Fault state and begins transferring traffic across its switch to facilitate connections between the nodes on the ring.
CAN THERE BE MULTIPLE RING SUPERVISORS?
No, only one device can be the Ring Supervisor at a time. On startup a protocol is used for multiple devices to work together to select one device as the Ring Supervisor. All other nodes with Ring Supervisor capability continuously monitor the ring. If no Beacon nodes are detected in the timeout period they will restart the process of selecting another Ring supervisor.
I’ll explain the messages types and EtherNet/IP object requirements in the next installment.