Spectrum Controls 1756sc-OF8H Series A - Manual (Page 36)

22 ControlLogix ™ Analog HART Modules
Modules in a Remote
Chassis
If an input module resides in a networked chassis, the role of the RPI
and the module’s RTS behavior change slightly with respect to getting
data to the owner. The RPI and RTS intervals still define when the
module will multicast data within its own chassis (as described in the
previous section), but only the value of the RPI determines how often
the owner controller will receive it over the network.
When an RPI value is specified for an input module in a remote chassis,
in addition to instructing the module to multicast data within its own
chassis, the RPI also “reserves” a spot in the stream of data flowing
across the ControlNet network.
The timing of this “reserved” spot may or may not coincide with the
exact value of the RPI, but the control system will guarantee that the
owner controller will receive data at least as often as the specified
RPI.
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