Bradley Smoker PLC-5 user manual Considerations for Extended-local Racks

8-8Communicating 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.

PLC-5/40L Processor

Channel 2

Processor-

Resident

I/O

No BT modules

Extended-Local I/O Chassis 1

1771-ALX
Adapter	2 BT
Module
	modules

Extended-Local I/O Chassis 2

1771-ALX
Adapter	1 BT
Module
	modules

Extended-Local I/O Chassis 3

1771-ALX
Adapter	No BT
Module
	modules

Extended-Local I/O Link

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.

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)	(2) + (0.1	20 )]
D = 10 ms
T = 10		1
T = 10 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)
D (ms) = (2			2)	(1) + (0.1	20 )]
D = 6	ms
T = 6		1
T = 6	ms

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

Publication 1785-UM012D-EN-P - July 2005