18
Rockwell Automation Publication 1756-UM010C-EN-P - June 2021
Chapter 2 Configurable Flowmeter Operation in the ControlLogix System
Connections
Logix controllers make connections to 1756-CFM modules to exchange data.
The controller can make either of these connections to a module:
•
- Only one controller can make this connection to a
module.
•
- Multiple controllers can make this connection
to a module simultaneously.
Direct Connections
A
direct connection
is a real-time data transfer link between the controller and
the device that occupies the slot that the configuration data references. When
module configuration data is downloaded to an owner-controller, the
controller attempts to establish a direct connection to each of the modules
referenced by the data. One of these events occurs:
•
If the data is appropriate to the module found in the slot, a connection is
made and operation begins.
•
If the configuration data is not appropriate, the data is rejected and an
error message displays in the software. In this case, the configuration
data can be inappropriate for any of a number of reasons. For example, a
module’s configuration data can be appropriate except for a mismatch in
electronic keying that prevents normal operation.
The controller maintains and monitors its connection with a module. Any
break in the connection, such as removal of the module from the chassis while
under power, causes the controller to set fault status bits in the data area
associated with the module. The programming software can monitor this data
area to announce the modules’ failures.
Listen-Only Connections
Any controller in the system can
listen
to the data from any 1756-CFM module
even if the controller does not own the module (in other words, it does not have
to hold the module’s configuration data to listen to the module).
During the module creation process in the programming software, you can
specify the ‘Listen-Only’ Communication Format. For more information on
Communication Format, see
Choosing ‘Listen-Only’ mode allows the controller and module to establish
communications without the controller sending any configuration data. In
this instance, another controller owns the module.
IMPORTANT
Each 1756-CFM module requires one connection.
IMPORTANT
Controllers using the Listen-Only mode continue to receive data multicast
from the module as long as a connection between an owner and module
is maintained.
If the connection between all owners and the module is broken, the
module stops multicasting data and connections to all ‘Listening
controllers’ are also broken.
