Rockwell Automation 1771-DCM USER MANUAL 1771-DCM - Manual (Page 39)

Calculating Transfer Time
Chapter 7
7Ć5
PLCĆ2 Family Supervisory Processor
The time required by the supervisory processor is the sum of the program
scan, processor I/O scan, and remote I/O scan. The time is lengthened by
the number of enabled block transfer modules transferring data, one
module after the other, and on the number of words that each module
transfers. Calculate the time for a read or write operation for each enabled
block transfer module as follows:
Scan PLCĆ2/30
Program 5 ms/K Words
Processor I/O 0.5 ms/Rack
Remote I/O 7 ms/Chassis
Time (1 Module) = Program + Processor I/O + 2(Remote I/O) + 0.5W +
10
Where W = number of words transferred and 10 is an average constant
(typically, this constant is 4 for a read and 13 for a write).
If it is possible that all block transfer modules in the supervisory system
could be enabled at the same time, the worst case time for any one
transfer would include the waiting time for all other other block transfers.
PLCĆ2 Family Example Computation
Suppose there are four block transfer modules in addition to the
1771-DCM that could be enabled simultaneously in a PLC-2/30
supervisory system. The local system has no additional block transfer
modules. Assume that the program has 4.8K words, there are six assigned
rack numbers in the remote I/O link, and that each block transfer is 64
words.
1. Calculate the worst case time for the five modules to block transfer
their data.
2. Calculate the worst case time for a block transfer from the
supervisory to the local processor.