Rockwell Automation 1771-DE Absolute Encoder Module User Manual - Manual (Page 45)

Appendix A
AĆ4
Number of block-transfer words = 2 words (read) or 20 words
(write)
1. Calculate the system values.
Processor Scan Time (PS) = (5ms/1K words) x (4K words) = 20ms
Processor I/O Scan Time (PIO)=(0.5 ms/rack number) x (4 rack
numbers) = 2 ms
Remote Distribution I/O Scan Time (RIO) = (7 mx/chassis) x (4
chassis) = 28 ms
Number of Words Transferred = 2 (read) or 20 (write)
2. Calculate the block-transfer times for a write operation and for a read
operation.
TW = (PS +PIO + 2(RIO) + 0.5W + 13) ms
=(20 + 2 + 2(28) + 0.5(20) + 13) ms
=101 ms (write)
TR =(PS + PIO + 2(RIO) + 0.5W + 4) ms
=(20 + 2 + 2(28) + 0.5(2) + 4 ms
= 83 ms (read)
3. Calculate the worst case system time (ST) between 2 consecutive
block-transfer-read operations.
ST =4TW + 4TR
=4(101) + 4(83)
= 736 ms
This is the worst case time between two consecutive block-transfer-read
operations in the 4-chassis remote configuration described in example 1
(one enabled encoder module in each chassis).
PLCĆ2/30 Local System
The system scan time for a local PLC-2/30 system is the program scan
time plus the processor I/O scan time. Each block-transfer module is
updated during a program scan.
The calculation of the worst case time between transfers can be done in
three steps.