PACSystems™ RX3i and RSTi-EP CPU Reference Manual Appendix A
GFK-2222AK October 2019
Serial I/O, SNP & RTU Protocols 202
A-3.4 I/O Scan and I/O Fault Sweep Impact
The I/O scan sweep impact has two parts, Local I/O and Genius I/O. The equation for
computing I/O scan sweep impact is:
Sweep Impact of Local I/O Modules
The I/O scan of I/O modules is impacted as much by location and reference address of a
module as it is by the number of modules. The I/O scan has several basic parts.
Each expansion rack is selected separately because of the addressing of expansion
racks on the VME bus. This results in a fixed overhead per expansion rack,
regardless of the number of modules in that rack.
Each Local I/O module has a fixed setup scan time.
The actual transfer of bytes is much faster for modules located in the main rack
than for those in expansion racks. The byte transfer time differences will be
accounted for by using different times for I/O modules in the main rack versus
expansion racks.
In addition, analog input expander modules (the same as Genius blocks) have the ability
to be grouped into a single transfer as long as consecutive reference addresses are used
for modules that have consecutive slot addresses. Each sequence of consecutively
addressed modules is called a scan segment. There is a time penalty for each additional
scan segment.
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