Rockwell Automation 1756-XXXX ControlLogix Analog I/O Modules User Manual
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Publication 1756-UM009C-EN-P - December 2010
Chapter 2
Analog I/O Operation in the ControlLogix System
Best Case RTS Scenario
In the best case scenario, the module performs an RTS 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 RTS Scenario
In the worst case scenario, the module performs an RTS multicast just after
the ‘reserved’ network slot has passed. In this case, the owner-controller will
not receive data until the next scheduled network slot.
Remote Input Modules Connected Via the 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 as 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
Because it is the RPI and not the RTS that dictates when the
module’s data will be sent over the network, we recommend
the RPI value be set less than or equal to the RTS to make sure
that updated channel data is received by the owner-controller
with each receipt of data.
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,
publication
.