Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 179)

ASCII Module
PLC-2 Family Processors
Appendix A
AĆ11
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 will be
updated during a program scan.
The procedure for calculating the worst case time between transfers can
be done in three steps.
1. Calculate the system values that are determined by the system
configuration.
Program Scan PS = (5ms/1K words) x (number of program words)
Processor I/O Scan PIO = (1ms/rack number) x (number of declared
rack numbers)
Number of words transferred W = 64 (read) or a (write)
2. Calculate the block transfer time T for the read or write operation.
T = 0.1ms + (0.075ms/word x number of words transferred)
The same equation is used for either read or write transfer times.
3. Calculate the worst case system time ST between transfers.
ST = PS + PIO + T(1)(read) + T(2)(read) + T(3)(read) + ...
PS + PIO + T(1)(write) + T(2)(write) + T(3)(write) + ...
= 2(PS +PIO) + T(1)(read) + T(2)(read) + T(3)(read) + ...
T(1)(write) + T(2)(write) + T(3)(write) + ...
Example Problem 2
A PLC-2/30 programmable controller is controlling four I/O racks in a
local configuration (Figure A.5). Otherwise this example problem is
identical to example problem 1.