Rockwell Automation 1771-QA Stepper Positioning Assembly User Manual - Manual (Page 60)

3–38 Programming and Operation
Publication 1771-UM002A–EN–P – May 2000
The method for calculating the worst case time between block
transfers will be covered for the following cases: PLC-2/30 remote
and local systems, and Mini-PLC-2/15 controller.
PLC-2/30 (PLC-2/20) Remote System
The system scan time for a remote PLC-2/30 or PLC-2/20 system is
the sum of the processor scan time, the processor I/O scan time
(between processor and remote distribution panel), and the remote
distribution panel I/O scan time. Assume that for a remote system,
the remote distribution panel can process only one block transfer
operation per remote distribution panel scan.
The procedure for calculating the worst case time between transfers
under normal operating conditions 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/1K words) x (number of program
words)
•
Processor I/O Scan PIO = (0.5ms/rack number) x (declared
rack numbers)
•
Remote Distribution I/O Scan RIO = (7ms/chassis) x (number
of chassis)
•
Number of Words Transferred W = default values of the
module
3. Calculate the block transfer time TW for the write operation and
TR for a read operation.
TW = PS + PIO + 2 RIO + .5W + 13
TR = PS + PIO + 2 RIO + .5W + 4
These equations are valid for up to 10,000 cable feet between the
remote distribution panel and remote chassis and for a baud rate of
57.6k, or 5,000 cable feet at 115k baud rate.
4. Calculate the worst case system time ST between transfers.
ST = sum of transfer times of all block transfer modules in a
system taken worst case (read or write).