Rockwell Automation 1771-QC , D17716.5.25 SER/B SERVO POS - Manual (Page 173)

Chapter 8
8Ć12
1. Write down known values:
program length = 4K words
number of chassis = 4
block length = 64 words write, 10 words read
2. Calculate system values:
program scan time
PS = (5ms/K word)(4K words) = 20ms
processor I/O Scan time
PIO = (1ms/chassis)(4 Chassis) = 4ms
3. Calculate individual block transfer times:
Write
TW = 0.1ms + [(0.075ms/word)(64 words)] = 4.9ms
Read
TR = 0.1ms + [(0.075ms/word)(10 words)] = 0.85 ms
These times apply to all four 1771-M3 controller modules in the system.
If the system includes other block-transfer modules, you will have to
make separate calculations for each.
4. Calculate the time between consecutive write transfers. Since a read
transfer occurs between write transfers, you must include 2 system
scans, 4 write transfer times and 4 read transfer times in the formula:
Time between write transfers = 2(PS + PIO) + 4 TR + 4 TW
= 2(24ms) + 4(0.85ms) + 4(4.9ms)
= 71ms
MiniĆPLCĆ2/15 Controller
Block transfer timing for the Mini-PLC-2/15 controller is similar to that
for the PLC-2/30 local system. The program scan and processor I/O scan
are consecutive and are considered as one scan. Scan time typically
varies from 18 to 24ms per 1K word of user program:
processor scan time
PS = (24ms/K word) x (program length)
Individual block transfer times can be calculated from this formula:
T = 0.1ms + (0.16ms/word) x (block length)