Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 181)

ASCII Module
PLC-2 Family Processors
Appendix A
AĆ13
2. Calculate the block transfer times T for the read and write operation.
T = 0.1 + (0.075ms/word x 64 words)
= 0.1 + 4.8
4.9ms (read)
T = 0.1 + (0.075ms/word x 2 words)
= 0.1 + 0.15
= 0.25ms (write)
3. Calculate the worst case system time ST between 2 consecutive read
block transfers.
The module toggles to a read operation in the scan following completion
of the write operation and vice versa.
ST = PS+ PIO + T(1) +T(2) + T(3) +T(4)(writes)
PS+ PIO + T(1) +T(2) + T(3) +T(4)(reads)
ST = 2PS + 2PIO + 4T(read) + 4T(write)
= 2(20) + 2(2) + 4(4.9) + 4(0.15)
= 40 + 4 + 19.6 + 0.6
= 64.2ms
This is the worst case time between two consecutive read block transfers
in the 4-chassis local configuration described in example problem 2
(enabled ASCII module in each chassis).
MiniĆPLCĆ2/15 Controller
The program scan and I/O scan are consecutive and are considered as a
single processor scan. The Mini-PLC-2/15 scan time varies typically
from 18 to 24ms for a 1K program and one I/O chassis. 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 two steps.
The facts of the problem are:
Processor Scan time, PS = 24ms
Number of Words Transferred, W = 64(read) or 2(write)