Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 115)

Programming
Chapter 3
3Ć72
to in section titled Status Block: W57-W74 word W60 Loop
Time/Diagnostic. Refer to Figure 3.11.
System timing refers to the time required to scan the ladder diagram
program and to communicate between PC processor and PID module.
System timing depends on the kind of PC processor used, the number of
words transferred, the number of I/O racks in the system, the number of
enabled block transfer instructions in the ladder diagram program during
any program scan and the length of user program.
PLC-2/30 REMOTE SYSTEM
The system scan time for a remote PLC-2/30 system must include 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
(cat. no. 1771-SD) can process only one block transfer operation per
processor I/O scan.
The procedure for calculating the worst case time for a read or write block
transfer 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 = (5msec/lK words) x (number of program
words)
Processor I/O Scan PI0 = (0.5msec/rack number) x (assigned rack
numbers)
Remote Distribution I/O Scan RI0 = (7msec/chassis) x (number of
chassis)
Number of Words Transferred W = 18 for a read or write.
(Although this number can be 17, 18 or 19, 18 has been chosen for
simplicity.)
2. Calculate the block transfer time TW for the write or read operation
which are essentially the same. This time includes system timing
and module timing if greater than 100msec.
TW = PS + PI0 + 2 RI0 + .5W + 9 (equation 1)
NOTE: If the calculated time TW is less than 100msec. an alternative
equation must be used to calculate the time for a single write or read
operation.