76 Rockwell Automation Publication 1769-UM011H-EN-P - April 2012
Chapter 5 Manage Controller Communication
The number of available connections limits the total number of tags that can be
produced or consumed. If the controller uses all of its connections for I/O and
communication devices, no connections are left for produced and consumed tags.
Send and Receive Messages
Messages transfer data to other devices, such as controllers or operator interfaces.
Messages use unscheduled connections to send or receive data. Connected
messages can leave the connection open (cache) or close the connection when the
message is done transmitting.
Connected messages are unscheduled connections on both ControlNet and
EtherNet/IP networks.
Each message uses one connection, regardless of how many devices are in the
message path. You can program the target of a MSG instruction to optimize
message transfer time.
Table 12 - Message Transmission
Message Type Communication
Method
Connected Message Can the message be
cached?
CIP data table read or write NA Yes Yes
PLC-2, PLC-3, PLC-5, or SLC
(all types)
CIP No No
CIP with Source ID No No
DH+ Yes Yes
CIP generic NA Optional
(1)
(1) You can connect CIP generic messages. However, for most applications we recommend you leave CIP generic messages unconnected.
Yes
(2)
(2) Consider caching only if the target module requires a connection.
Block-transfer read or write NA NA Yes
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