User Manual

20 Rockwell Automation Publication 1756-UM005B-EN-P - January 2013
Chapter 2 High-speed Analog I/O Operation in the ControlLogix System
Differences between
Inputs and Outputs
The ControlLogix high-speed analog I/O module uses both inputs and outputs.
However, there are significant differences between how each channel type
operates.
Module Input Operation
In traditional I/O systems, controllers poll module inputs to obtain their status.
The owner-controller does not poll the ControlLogix high-speed analog inputs
once a connection is established. Rather, the module multi-casts its input data
periodically. Multicast frequency depends on module configuration, such as RTS
and RPI rates.
In Figure 2
, the RTS value is 20 ms and the RPI value is 5 ms. Only every fourth
multicast contains updated channel data.
Figure 2 - Input Data Update Rate
IMPORTANT
The module only sends data at the RPI in these scenarios:
• RPI < RTS. In this case, the module multicasts at both the RTS rate and the
RPI rate. Their respective values dictate how often the owner-controller
receives data and how many multicasts from the module contain updated
channel data.
• If the RPI > RTS, each multicast from the module has updated channel
data. In effect, the module is only multicasting at the RTS rate.
• The module is operating in a mode where inputs are not being sampled, for
example calibration.
RTS
20 ms—Updated Input Data
RPI
5 ms—Updated and Old Data,
Depending on Time
5 10152025303540455055 60657075 80
Time (ms)
Updated input channel data is received at 0 ms, 20 ms, 40 ms, 60 ms, and 80 ms. The data
received at other RPI times repeats the most previous RTS. For example, data received at
30 ms repeats that received at 20 ms.