Spectrum Controls 1756sc-IF8u - Manual (Page 37)

Chapter 3: Operation within the System
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This configurable parameter instructs the module to perform the
following operations:
1. scan all of its input channels and store the data into on-board memory
2. multicast the updated channel data (as well as other status data) to the
backplane of the local chassis
Requested Packet
Interval (RPI)
This configurable parameter also instructs the module to multicast its
channel and status data to the local chassis backplane.
The RPI instructs the module to multicast the current contents of its
on-board memory when the RPI expires, (i.e. the module does not
update its channels prior to the multicast).
Important: The RPI value is set during the initial module configuration
using RSLogix 5000.
It is important to note that the module will reset the RPI timer each time
an RTS is performed. This operation dictates how and when the owner
controller in the local chassis will receive updated channel data,
depending on the values given to these parameters. If the RTS value is
less than or equal to the RPI, each multicast of data from the module will
have updated channel information. In effect, the module is only
multicasting at the RTS rate.
If the RTS value is greater than the RPI, the module will multicast at
both the RTS rate and the RPI rate. Their respective values will dictate
how often the owner controller will receive data and how many
multicasts from the module contain updated channel data. Note: Even
though data may be transfered at the RPI rate, the data will be indentical
to the previous RTS data transfer.
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.