Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 119)

Programming
Chapter 3
3Ć76
Solution , part a
1. Calculate the system values
PS = Program Scan - (5msec/lK)x(8K) = 40msec
PI0 = Processor I/O Scan = (0.5msec/rack number) x(6 rack
numbers)=3msec
RI0 = Remote I/>O Scan = (7msec/chassis)x(6 chassis) = 42msec
W = Number of words transferred = 18 for a read or write (PID
module)
2. Calculate the block transfer time for a single transfer.
TW = PS + PI0 + 2 RI0 + .5W +9
= 40 + 3 + 84 + .5(18) +9
= 145msec
Solution, part b
For the worst case calculation of a load/enter sequence, assume that the
block transfer instruction for the other module will be enabled at the same
time and will have priority during each transfer.
The time required for the other module to perform a 64 word block
transfer is (from equation 4)
T
BTI
= PS + PI0 + 2 RI0 + .5W + 9
= 40 + 3 + 84 + 32 + 9
=168msec
The time required for the PID module to perform a block transfer as
calculated in solution, part a is TW = 145msec.
The total time for a single read or write operation when there is queueing
due to other block transfer modules is (from equation 3).
T
Q
= TW + T
BTI
T
Q
= 145 + 168
T
Q
= 313msec
The total time for the 7-transfer load/enter sequence is (from equation 5)
T
LE
= (# transfers in the sequence x (T
Q
from equation 3)
= 7(313)
= 2191msec
= 2.2 seconds