218 Rockwell Automation Publication 1769-UM021I-EN-P - May 2018
Chapter 8 Use I/O Modules with CompactLogix 5370 L2 Controllers
Tab le 25 describes RPI rate guidelines.
You can set RPI rates higher for individual Compact I/O modules than the
rates listed in Tabl e 2 5
. 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 25 - Requested Packet Interval Guidelines
Type of Module Guidelines
(1)
(1) The guidelines in Tabl e 25 do not factor in the following items, which affect CMX5370 controller CPU loading:
I/O RPI timing does not affect the task priority. Event and periodic tasks have higher priority than I/O and user tasks.
IOT (Immediate Output Instruction)
Messaging
CompactBus browsing such as accessing DeviceNet network through 1769-SDN using CMX5370 Ethernet or USB connection
Module RPI guidelines can require adjustment (increase of 1 ms or more) if CMX5370 controller application includes one or
more of the listings in Table 2 5
. Monitor controller minor faults to determine if Module RPI overlaps have occurred.
All digital 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.
Mix of digital and
analog or all analog
The following guidelines apply:
• 1…2 modules can be scanned in 0.5 ms.
• 3…4 modules can be scanned in 1 ms.
• 5…13 modules can be scanned in 2 ms.
• 14…30 modules can be scanned in 3 ms.
Specialty The following conditions apply:
• For every 1769-SDN module in the system, increase every RPI by 2 ms for every other
module.
• For every 1769-HSC module in the system, increase every RPI by 1 ms for every other
module.
• For every 1769-ASCII module in the system, increase every RPI by 1 ms for every other
module.
• For every 1769-SM2 module in the system, increase every RPI by 2 ms for every other
module.
IMPORTANT
When considering the number of I/O modules, remember that they can be the embedded I/O
modules on the controller or 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 Compact I/O modules
• Some combination of embedded I/O modules and Compact I/O modules
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