Rockwell Automation 1756-L55M12, Selection Manual, Page: 59 / 88

57
1756-SG001
G
-EN-P —
March
200
4
How a Logix
System Uses
Connections
A Logix system uses a connection to establish a communication link between two
devices. Connections can be:
y
controller to local I/O modules or local communication modules
y
controller to remote I/O or remote communication modules
y
controller to remote I/O (rack optimized) modules
y
produced and consumed tags
y
messages
You indirectly determine the number of connections the controller uses by
configuring the controller to communicate with other devices in the system.
Connections are allocations of resources that provide more reliable communications
between devices than unconnected messages. A Logix system supports:
The communication module you select determines the number of connections you
have available for I/O and messages.
How you configure connections determines how many remote devices a
communication card can support.
Method Description
scheduled connection
y
level of determinism
y
unique to ControlNet
A scheduled connection is unique to ControlNet communications. A scheduled connection lets
you send and receive data repeatedly at a predetermined interval, which is the requested
packet interval (RPI). For example, a connection to an I/O module is a scheduled connection
because you repeatedly receive data from the module at a specified interval. Other scheduled
connections include connections to:
y
communication devices
y
produced/consumed tags
On a ControlNet network, you must use RSNetWorx for ControlNet to enable all scheduled
connections and establish a network update time (NUT).
unscheduled connection
y
deterministic
y
used by both ControlNet and EtherNet/IP
An unscheduled connection is a message transfer between controllers that is triggered by the
requested packet interval (RPI) or the program (such as a MSG instruction). Unscheduled
messaging lets you send and receive data when needed.
All EtherNet/IP connections are unscheduled.
unconnected message
y
least deterministic
An unconnected message is a message that does not require connection resources. An
unconnected message is sent as a single request/response.
This communication module:
1756-CNB
1756-ENBT
Supports this number of connections:
64
(any combination of scheduled and unscheduled)
128
(any combination of scheduled and unscheduled)
The EtherNet/IP module does not distinguish between scheduled and
unscheduled connections.