User manual

Publication 1768-UM001C-EN-P - November 2007
Manage Controller Communication 47
Sending and Receiving
Messages
Messages transfer data to other devices, such as other controllers or operator
interfaces. Some messages use connections to send or receive data. These
connected messages can leave the connection open (cache) or close the
connection when the message is done transmitting. Each message uses one
connection, regardless of how many devices are in the message path.
To conserve connections, configure one message to read from or write to
multiple devices. You can programmatically change the target of a MSG
instruction to optimize program size.
Determining Whether to Cache Message Connections
When you configure a MSG instruction, you can choose whether or not to
cache the connection.
Message Types
Message Type Communication
Method
Connected
Message
Message Can
Be Cached
CIP data table read or write N/A
Yes
(1)
(1)
Starting with version 16 of RSLogix 5000 programming software, you can initiate these messages as
unconnected.
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 N/A
Optional
(2)
(2)
You can connect CIP generic messages. However, for most applications we recommend you leave CIP generic
messages unconnected.
Yes
(3)
(3)
Consider caching only if the target module requires a connection.
Block-transfer read or write N/A Yes Yes
Caching Message Connections
Message Execution Function
Repeatedly Cache the connection.
This keeps the connection open and optimizes execution time. Opening and closing a connection each time
the message executes increases execution time.
Infrequently Do not cache the connection.
This closes the connection upon completion of the message, which frees up that connection for other uses.