Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 291)

Publication 1785-UM012D-EN-P - July 2005
Maximizing System Performance C-5
Putting Block-Transfer Modules in Controller-Resident Chassis
Because controller-resident racks cannot be updated until after active
block-transfers are completed, putting block-transfer modules in the
controller-resident chassis can affect housekeeping by a worst-case time of
approximately 100
µ s per one word of block-transfer data. Note that this
estimate is based on a worst-case scenario. Typically, the effect, if any, on
housekeeping will be minimal.
Using Global Status Flag Files
The global status flag files are updated during housekeeping. This increases
housekeeping time as follows:
• each global status flag file on a channel (for example, channel 1A or 1B)
adds 3ms
• housekeeping time does not increase more than 6ms, even if there are
more than two global status flag files
If you need two global status flag files, split them across two channels.
Calculating Throughput
Throughput is the time that it takes for an output to be energized after its
associated input has been energized. You need to consider the following
components when evaluating throughput:
• input and output module delay
• I/O backplane transfer
• remote I/O scan time
• controller time
To calculate throughput, use the following equation:
Input
Card
Delay
I/O Backplane
Worst-Case
Remote I/O
Scan Time Worst-Case
Processor
Time
+ + +
Worst-Case
Remote I/O
Scan Time
+ I/O Backplane +
Output
Card
Delay
+