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Appendix A
A 2
Program Scan (PS) = (5 ms/1K words) x (number of program
words)
Processor I/O Scan (PIO) = (0.5 ms/rack number) x (declared rack
numbers)
Remote Distribution I/O Scan (RIO) = (7 ms/chassis) x (number of
chassis)
Number of Words Transferred (W) = 20 words for one write
operation, two words for one read operation
2.
Calculate the block-transfer time for a write operation (TW) and for
a read operation (TR).
TW = (PS + PIO + 2 RIO + 0.5W + 13) ms
TR = (PS + PIO + 2 RIO + 0.5W +4) ms
These equations are valid for up to 10,000 cable feet between the remote
distribution panel and remote I/O chassis for a baud rate of 57.6kBd or
5,000 cable feet at 115kBd.
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)
Example 1
A PLC-2/30 programmable controller is controlling 4 I/O chassis
in a remote configuration with 1 assigned rack number per chassis
(Figure A.1). An encoder module is located in each chassis.
Assume the 2 words are transferred in each read operation, 20
words are transferred in each write operation, and that the ladder
diagram program contains 4K words. There are no other
block-transfer modules in the system.
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