Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual - Manual (Page 118)

Performance and Theory of Operations
Chapter 7
7-13
Extended-Local I/O
Processors that have extended-local I/O capability scan the extended-local
I/O chassis (on channel 2) during the housekeeping portion of the program
scan. Extended-local I/O discrete data is exchanged between the processor
data-table image and the I/O in the extended-local I/O chassis. The time
that it takes to scan extended-local I/O chassis is added to the
housekeeping time. See Figure 7.2.
Figure 7.2
Extended-Local I/O Scan Time
Processor
Checks
Remote
I/O Buffer Extended-
Update
Local I/O
Scan
Housekeeping
Logic Scan
Program Scan
+
The time in ms that it takes to scan extended-local I/O chassis depends on
the number of 1771-ALX adapter modules and the number of extended-
local I/O logical racks. The formula used to calculate the total time to scan
extended-local I/O chassis is:
extended-local I/O scan time = (0.32 ms x A) + (0.13 ms x L)
Where: Is the:
A number of 1771-ALX modules
L number of logical racks in the extended-local I/O system
If you have three 1771-ALX modules in three chassis and a total of 4
logical racks, for example, the total time is calculated as follows:
extended-local I/O scan time = (0.32 ms x 3) + (0.13 ms x 4)
or
extended-local I/O scan time = 1.48 ms
housekeeping time = 1.48 ms (extended-local I/O) + 4.50 ms
(other housekeeping) or
housekeeping time = 5.98 ms