Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 126)

Publication 1785-UM012D-EN-P - July 2005
8-8 Communicating with Extended-Local I/O
Example Calculations:
Here is an example system that provides sample calculations of a worst case
block-transfer completion time and the completion time of the modules in
chassis 2.
Considerations for Extended-local Racks
The following are programming considerations when you are transferring
block data in an extended-local rack:
• Block-transfer instructions to any of the following modules that reside
in an extended-local rack will result in frequent checksum errors:
– 1771-OFE1, -OFE2, and -OFE3 modules, all versions prior to series
B, revision B
– IMC-120 and IMC-123, all versions
The logic scan completes in 15 ms. Housekeeping completes in
approximately 6 ms (as calculated in the formula on page
NO TAG). The longest block-transfer request is 20 words.
Extended-Local I/O Link
PLC-5/40L
Processor
Channel 2
Processor-
Resident
I/O
No BT
modules
2 BT
modules 1 BT
modules
No BT
modules
Extended-Local I/O Chassis 1 Extended-Local I/O Chassis 2 Extended-Local I/O Chassis 3
1771-ALX
Adapter
Module 1771-ALX
Adapter
Module 1771-ALX
Adapter
Module
D (ms) = (2 2) (2) + (0.1 20 )]
D = 10 ms
T = 10 1
T = 10 ms
Worst-case time (T) = D X R
D = 2E L + (0.1W) and R = 1 Because 10 < 15 (which is the logic scan)
D (ms) = (2 2) (1) + (0.1 20 )]
D = 6 ms
T = 6 1
T = 6 ms
Completion time (T) for module in chassis 2 transfer: = D X R
Block-transfer length = 20
D = 2E M + (0.1W) and R = 1 Because 6 < 15 (which is the logic scan)