Technical data

10 Rockwell Automation Publication 1769-TD005I-EN-P - August 2014
CompactLogix Controllers Specifications
CompactLogix 5370 L1 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. You set an RPI rate for each I/O module in your system.
CompactLogix 5370 L1 controllers always attempt to scan an I/O module at the configured RPI rate. For individual I/O
modules, a Module RPI Overlap minor fault occurs if there are enough I/O modules with RPI rates set too fast that they
cannot all be serviced in the allotted interval.
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 1734 POINT I/O modules’ RPI rates are set
• Number of other 1734 POINT I/O modules in the system
• Types of other 1734 POINT I/O modules in the system
• Application user task priorities
In general, follow these guidelines when setting the RPI rates in a CompactLogix 5370 L1 control system:
• For digital modules:
– 1…2 modules can be scanned in 2 ms.
– 3…4 modules can be scanned in 4 ms.
– 5…8 modules can be scanned in 8 ms.
• For specialty and analog modules (except 1734-485ASC modules):
– 1 module can be scanned at 20 ms.
– For each additional module, add 20 ms.
For example, if a CompactLogix 5370 L1 control system uses two analog modules, the module can be scanned in
40 ms.
• For 1734-485ASC modules, the total data size for all ASC modules determines the RPI rates:
– For total data size less than 20 bytes, each module can be scanned in 20 ms.
– For data size greater than 20 bytes, use the size value as the RPI.
For example, if the total data size is 40 bytes, each ASC module can be scanned in 40 ms.
You are not required to set individual 1734 POINT I/O modules’ RPI values to the values listed previously. For example, if
your application scans one or two modules, you do not have to use RPI rates of 2 ms. Remember, though, that higher RPI
rates result in scanning the data less frequently.
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.
IMPORTANT
When considering digital I/O modules, remember that they can be the embedded I/O module on the controller or
1734 POINT I/O modules that are used as local expansion modules. Therefore, the consideration for using two modules can
be the embedded I/O module and a 1734 POINT I/O module or two 1734 POINT I/O modules.