Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 176)

ASCII Module
PLC-2 Family Processors
Appendix A
AĆ8
PLCĆ2/30 (PLCĆ2/20) 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 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 = (0.5ms/rack number) x (declared rack
numbers)
Remote Distribution I/O Scan, RIO = (7ms/chassis) x (number of
chasses)
Number of Words Transferred, W = 64 words for one operation, 2
words for the other
2. Calculate the block transfer time TW for the write operation and TR
for a read operation.
TW = PS + PIO + 2 RIO + 0.5W + 13
TR = PS + PIO + 2 RIO + 0.5W + 4
These equations are valid for up to 10,000 cable feet between the remote
distribution panel and remote I/O chassis and for a baud rate of 57.6k, or
5,000 cable feet at 115k baud rate.
3. 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).