Rockwell Automation Publication 1769-UM021G-EN-P - October 2015 167
Use I/O Modules with CompactLogix 5370 L1 Controllers Chapter 7
In general, follow these guidelines when setting the RPI rates in a
CompactLogix 5370 L1 control system:
• For digital I/O 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 I/O modules (except 1734-485ASC modules):
– One 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 RPI values of 1734 POINT I/O modules
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 less frequent data scans.
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.
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.
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