Rockwell Automation 1771-QC , D17716.5.25 SER/B SERVO POS - Manual (Page 169)

Chapter 8
8Ć8
For worst case calculations, use the longest block transfer time.
Block Transfer Sequence
As stated above, the remote I/O scanner can process only one block
transfer per remote I/O scan, worst case. If a system has N I/O chassis
with block-transfer modules, a block transfer for a given chassis occurs
once each N system scans. If a given chassis contains X block-transfer
modules, block transfer for any one of them occurs once each (N) x (X)
remote I/O scans.
For example, consider a system with 4 I/O chassis, each of which contains
one or more block transfer modules:
chassis 1 - modules A, B, C
chassis 2 - modules D, E
chassis 3 - module F
chassis 4 - modules G, H, I
The block-transfer sequence for this system is:
A D F G B E F H C D F I A E F G B D F H C E F I...
Note that block transfer for modules A, B, and C occurs once each 12
system scans (4 chassis x 3 modules). Block transfer for modules D and
E occurs once each 8 system scans (4 chassis x 2 modules), and so on.
Example Calculation
Consider the PLC-2/30 remote system of Figure 8.4. This system has four
I/O chassis, each of which holds one 1771-M3 controller module. The
ladder diagram program is 4K words long. Calculate the worst case time
between write block transfers for one of the 1771-M3 controller modules
in the following steps:
1. Write down known facts:
Program Length = 4K words
Number of Chassis = 4
Block Length - 64 words write, 6 words read