Technical data

28 Rockwell Automation Publication 1769-TD005I-EN-P - August 2014
CompactLogix Controllers Specifications
CompactLogix 5370 L3 Controllers’ Local I/O Performance
The requested packet interval (RPI) defines the frequency at which the controller sends data to and receives data from I/O
modules. In the programming software, you set an RPI rate for each I/O module in your system.
The CompactLogix 5370 L3 controllers always attempt to scan an I/O module at the configured RPI rate. If there is not
enough system bandwidth, that is, if other, higher priority tasks prevent the 1769 Compact I/O subsystem task from
completing before the next scheduled time for it to run again, an I/O Task Overlap minor fault occurs.
For individual I/O modules, a Module RPI Overlap minor fault occurs if there is at least one I/O module which cannot be
serviced within its RPI time.
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 module:
• Rates at which other 1769 Compact I/O modules’ RPI rates are set
• Number of other 1769 Compact I/O modules in the system
• Types of other 1769 Compact I/O modules in the system
• Application user task priorities
You can set individual 1769 Compact I/O modules’ RPI values higher than those listed in Ta ble 18
. For example, if your
application scans one or two modules, you do not have to use RPI values = 0.5 ms. You can set the RPI to a higher value,
such as 1.0 ms, if necessary. Remember, higher RPI values result in scanning the data less frequently.
Table 18 - RPI Rate Guidelines
Type of Module Guidelines
1769 Compact I/O digital and analog
(any mix) modules
The following guidelines apply:
• 1…2 modules can be scanned in 0.5 ms.
• 3…4 modules can be scanned in 1 ms.
• 5…30 modules can be scanned in 2 ms.
• Some input modules have a fixed 8 ms filter, so selecting a faster RPI has no effect.
1769 Compact I/O specialty modules 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 there are four digital I/O modules in the system configured with an RPI = 1 ms and a 1769-SDN module is added to the system, increase the RPI
value for all four digital I/O modules by 2 ms. Therefore, when the 1769-SDN module is added to the system the four digital I/O modules use an RPI = 3 ms.
If, in the same system, you add a second 1769-SDN module, the four digital I/O modules’ RPI value is increased to 5 ms.