Spectrum Controls 1756sc-IF8u - Manual (Page 38)

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ControlLogix ™ Universal Analog Input Module
The “reserved” spot on the network and the module’s RTS are
asynchronous to each other. This means there are Best and Worst Case
scenarios as to when the owner controller will receive updated channel
data from the module in a networked chassis.
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 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.
Because it is the RPI and NOT the RTS which 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.
Listen-Only Mode
Any controller in the system can listen to the data from any I/O module
(e.g. input data or “echoed” output data) even if the controller does not
own the module (i.e. it does not have to hold the module’s configuration
data to listen to the module).
The “listen only” mode is set during the I/O configuration process.
Choosing a ‘Listen-Only’ mode option allows the controller and module
to establish communications without the controller sending any
configuration data. In this instance, another controller owns the module
being listened to.
Important:Controllers using the Listen-Only mode continue to receive
data multicast from the I/O module as long as a connection between an
owner and I/O 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.
Multiple Owners of
Input Modules
Because ‘Listening controllers’ lose their connections to modules when
communications with the owner stop, the ControlLogix system will allow
you to define more than one owner for input modules.
Important: Only input modules can have multiple owners. If multiple
owners are connected to the same input module, they must maintain
identical configuration for that module.
In the example below, Controller A and Controller B have both been
configured to be the owner of the input module.