Rockwell Automation 1756-OF8H ControlLogix HART Analog I/O Modules User Manual
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Rockwell Automation Publication 1756-UM533C-EN-P - February 2011
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ControlLogix Module Operation Chapter 3
Best Case Scenario - Real Time Sample
In the best case scenario, the module performs a real time sample multicast with
updated channel data just before the reserved network slot is made available. In
this case, the remotely-located owner-controller receives the data almost
immediately.
Worst Case Scenario - Real Time Sample
In the worst case scenario, the module performs a real time sample multicast just
after the reserved network slot has passed. In this case, the owner-controller does
not receive updated data until the next scheduled network slot.
Because it is the requested packet interval and not the real time sample that
dictates when the module’s data is sent over the network, we recommend the
requested packet interval value be set less than or equal to the real time sample to
make sure that updated channel data is received by the owner-controller with
each receipt of data.
Remote Input Modules Connected Via EtherNet/IP Network
When remote analog input modules are connected to the owner-controller via an
EtherNet/IP network, data is transferred to the owner-controller in the following
way:
• At the RTS or RPI (whichever is faster), the module broadcasts data
within its own chassis.
• The 1756 Ethernet bridge module in the remote chassis immediately
sends the module’s data over the network to the owner-controller as
long it has not sent data within a time frame that is one-quarter the
value of the analog input module’s RPI.
For example, if an analog input module uses an RPI = 100 ms, the
Ethernet module sends module data immediately on receiving it if
another data packet was not sent within the last 25 ms.
The Ethernet module will either multicast the module’s data to all
devices on the network or unicast to a specific owner-controller
depending on the setting of the Unicast box, as shown on
.
TIP
For more information, see the Guidelines to Specify an RPI
Rate for I/O Modules section in the Logix5000 Controllers
Design Considerations Reference Manual,
public