Rockwell Automation Ethernet Design Considerations Reference Manual - Manual (Page 71)

Rockwell Automation Publication ENET-RM002C-EN-P - May 2013 71
EtherNet/IP Protocol Chapter 4
Messaging The EtherNet/IP network supports both time-critical (implicit) and non time-
critical (explicit) message transfer services of CIP. Exchange of time-critical
messages is based on the producer/consumer model where a transmitting device
produces data on the network and many receiving devices can consume this data
simultaneously.
Implicit Messages
Implicit messages are time critical in nature. This includes I/O and produced/
consumed tags. Implicit refers to information (source address, data type, and
destination address) that is implied in the message, but not contained in the
message. Examples of implicit applications include the following:
• Real-time I/O data
• Functional safety data
• Motion control data
Implicit messages use the User Datagram Protocol (UDP) and can be unicast or
multicast. Implicit messages transport data via transport class 0/1 (Class 1):
• The data source/destination is an application object (assembly object).
• There is no protocol in the message data—it is all I/O data.
• Data transfer is more efficient because the meaning of the data is known
ahead of time.
• Transfer is initiated on a time basis (cyclic trigger) or requested packet
interval (RPI).
• There is a connection timing mechanism to alert the application if the
other side has stopped communicating.
• Messaging is always connected—there are is unconnected implicit
messaging.
An implicit message times out in
controller_multiplier x RPI. The multiplier is
selected by the controller firmware so that the timeout is greater than or equal to
100 ms. The minimum multiplier is 4.
These are examples:
• RPI = 2 ms; controller multiplier = 64. The timeout is 128 ms.
• RPI = 10 ms; controller multiplier = 16. The timeout is 160 ms.