Rockwell Automation 1771-DE Absolute Encoder Module User Manual - Manual (Page 48)

Appendix A
AĆ7
Processor I/O Scan Time (PIO) = (0.5 ms/rack number) x (4 rack
numbers) = 2 ms
Number of Words Transferred (W) = 2 (read) or 20 (write)
2. Calculate the block-transfer times (T) for the read and write
operation.
T =0.08 ms/word x 2 words
= .16 ms (read)
T =0.08 ms/word x 20 words
= 1.6 ms (write)
3. Calculate the worst case system time (ST) between 2 consecutive
block-transfer-read operations.
The module toggles to a read operation in the scan following completion
of the write operation and vice versa.
ST =PS + PIO + T(1) + T(2) + T(3) + T(4)(writes)
PS + PIO + T(1) + T(2) + T(3) + T(4)(reads)
ST =2PS + 2PIO + 4T(read) + 4T(write)
=2(20) + 2(2) + 4(.16) + 4(1.6)
=40 + 4 + .64 + 6.4
=51.04 ms
This is the worst case time between two consecutive block-transfer-read
operations in the 4-chassis local configuration described in example 2
(one enabled encoder module in each chassis).
MiniĆPLCĆ2/15 Controller
The Mini-PLC-2/15 scan is 15 ms for 1K program. Its I/O scan time is 5
ms. Each block-transfer module is updated during a program scan.
You can calculate the worst case time between transfers in two steps.
The facts are:
Processor scan time (PS) = 15 ms/1K words
Processor I/O scan time (PIO) = 5 ms
Number of words transferred (W) = 2 (read) or 20 (write)
1. Calculate the block-transfer time (T) for the read and write operation.
T =0.08 ms/word x number of words transferred