Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 124)

Publication 1785-UM012D-EN-P - July 2005
8-6 Communicating with Extended-Local I/O
The time in ms that it takes to scan extended-local I/O chassis depends on the
number of 1771-ALX adapter modules and the number of extended- local
I/O racks. The formula used to calculate the total time to scan extended-local
I/O chassis is:
extended-local I/O scan time = (0.32 ms x A)+(0.13 ms x L)
where:
A
= the number of 1771-ALX modules and
L
= the number of racks in the extended-local I/O system
Transferring Block Data
Requests for block-transfer data occur during the logic scan. Concurrent with
the execution of the program logic, block-transfer requests are forwarded to
the appropriate 1771-ALX adapter module(s) and data is transferred.
A 1771-ALX adapter module may start block-transfer operations to multiple
slots and have block-transfer data transactions on-going in parallel within the
I/O chassis.
The block-transfer duration shown above does not affect logic scan time. This
transfer of data occurs concurrent with execution of program logic.
Block-transfer duration is the time interval between the enabling of the
block-transfer instruction and the receipt of the done bit.
EXAMPLE
If you have three 1771-ALX modules in three chassis and a
total of 4 racks, the total time is calculated as follows:
extended-local I/O scan time = (0.32 ms x 3)+(0.13 ms x
4)
extended-local I/O scan time = 1.48 ms
housekeeping time = 1.48 ms (extended-local I/O) + 4.50
ms(other housekeeping)
housekeeping time = 5.98 ms