Technical data

14 Rockwell Automation Publication 1769-TD005I-EN-P - August 2014
CompactLogix Controllers Specifications
The specific configuration parameters for a system determine the impact on actual RPI rates. These configuration factors
can impact the effective scan frequency for any individual embedded or local expansion module:
• Rates at which embedded I/O modules’ RPI values are set
• Number of embedded I/O modules that are used in the system
• Types of embedded I/O modules that are used in the system
• Rates at which 1769 Compact I/O modules’ RPI values are set
• Number of 1769 Compact I/O modules in the system
• Types of 1769 Compact I/O modules in the system
• Application user task priorities
Table 8
describes RPI rate guidelines.
You can set individual 1769 Compact I/O modules’ RPI rates higher than those listed in Ta ble 8
. The RPI shows how
quickly modules can be scanned, not how quickly an application can use the data. The RPI is asynchronous to the program
scan. Other factors, such as program execution duration, affect I/O throughput.
Table 8 - RPI Rate Guidelines
Type of Module Guidelines
Digital and analog (any mix) The following guidelines apply:
• 1…2 modules can be scanned in 0.5 ms.
• 3…4 modules can be scanned in 1 ms.
• 5…6 modules can be scanned in 2 ms.
• Some input modules have a fixed 8 ms filter, so selecting a faster RPI has no effect.
Specialty The following conditions apply:
• For every full-sized 1769-SDN module in the system, increase every other module's RPI by 2 ms.
• For every 1769-HSC module in the system, increase every other module's RPI by 1 ms.
• For every full-sized 1769-ASCII module system, increase every other module's RPI by 1 ms.
• For every 1769-SM2 module in the system, increase every other module's RPI by 2 ms.
For example, if the system includes four I/O modules that are configured with an RPI = 1 ms and you add a 1769-SDN module to the system,
increase the RPI value for all four I/O modules by 2 ms. Therefore, when the 1769-SDN module is added to the system, the four I/O modules use
an RPI = 3 ms.
If, in the same system, you add a second 1769-SDN module, the four I/O modules’ RPI value is increased to 5 ms.
IMPORTANT
When considering the number of I/O modules, remember that they can be the embedded I/O modules on the controller or 1769 Compact I/O modules that are
used as local expansion modules.
Therefore, the consideration for using modules can be any of the following system configurations:
• Only embedded I/O modules
• Only 1769 Compact I/O modules
• Some combination of embedded I/O modules and 1769 Compact I/O modules