Rockwell Automation 1771-QC , D17716.5.25 SER/B SERVO POS - Manual (Page 172)

Chapter 8
8Ć11
4. Calculate worst case time between write transfers:
Since each chassis contains one block transfer module, each 1771-M3
controller is serviced once every four remote I/O scans. Because the
1771-M3 controller module uses bidirectional block transfer, a read
transfer occurs between consecutive write transfers. Consequently, a write
transfer for a given 1771-M3 controller occurs every eight system scans.
In those eight scans, four write and four read transfers occur:
Time Between Write Transfers = 4TW + 4TR
= 4(123) + 4(87)
= 840ms
PLCĆ2/30 Local System
In a PLC-2/30 local system, all block-transfer modules are serviced once
in each system scan. Time between consecutive block transfers for a
given module is, consequently, system scan time plus the sum of the
individual block transfer times of the modules in the system.
Time Between Transfers = (system scan time) + T1 + T2...+Tn
where T1, T2..., Tn are individual block transfer times.
System scan time for a PLC-2/30 local system is program scan time (PS)
plus processor I/O scan time (PIO):
PS = (5ms/K word) x (program length)
PIO = (1ms/chassis) x (number of chassis)
You can calculate individual block-transfer times with this formula:
T = 0.1ms + (0.075ms/word) x (block length)
The same formula applies to both read and write transfers.
As an example, consider a PLC-2/30 local system with four I/O chassis,
each of which holds one 1771-M3 controller module and no other
block-transfer modules. User program length is 4k words. To calculate
the worst case time between consecutive write block transfers for one of
the 1771-M3 controllers follow these steps: