Rockwell Automation 1771-QA Stepper Positioning Assembly User Manual - Manual (Page 62)

3–40 Programming and Operation
Publication 1771-UM002A–EN–P – May 2000
Processor Scan Time PS = (5ms/lK 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 ms/chassis) x
(4 chassis) = 28ms
Number of Words Transferred = 64 (write) or 10 (read)
3. Calculate the block transfer times, TW for a write and TR for a
read operation.
TW = PS + PIO + 2(RIO) + .5W + 13
TW = 20 + 2 + 2(28) + .5(64) +13
TW = 123ms (write)
TR = PS + PIO + 2(RIO) + .5W + 4
TR = 20 + 2 + 2(28) + .5(10) + 4
TR = 87ms (read)
4. Calculate the worst case system time ST between 2 consecutive
write block transfers.
ST = 4TW + 4TR
= 4(123) + 4(87)
= 492 + 348
= 840ms
This is the worst case time between two consecutive write block
transfers in the 4-chassis remote configuration described in example
problem 1 (enabled stepper assembly in each chassis).
PLC-2/30 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 transfer
can be done in four steps.
1. Write down the known facts.
2. Calculate the system values that are determined by the system
configuration.
•
Program Scan PS = (5ms/lK words) x (number of program
words)
•
Processor I/O Scan PIO (0.5ms/rack number) x (number of
declared rack numbers)
•
Number of words transferred W default values of the module