Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual - Manual (Page 121)

Performance and Theory of Operations
Chapter 7
7-16
Formula 1—Worst-case time to complete all block-transfers in
extended-local I/O system where block-transfer duration (in ms) = D x R
D = 2 E x L + (0.1 W)
Where: Is the:
E number of extended-local chassis with block-transfer modules
L largest number of block-transfer modules in any extended-local I/O chassis
W number of words in the longest block-transfer request
D (ms) = (2 x 2) x (2) + (0.1 x 20 ) or D = 10 ms
R = 1 (when D < logic scan time) OR
R = Logic Scan + Housekeeping
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Logic Scan
In this example,
R = 1 (because the value of D (10 ms) < 15 ms logic scan time)
block-transfer duration (ms) = 10 x 1 or 10 ms
The block-transfer duration shown above does not affect logic scan time.
This transfer of data occurs concurrent with execution of program logic.
Formula 2—Time for any one block-transfer in extended-local I/O system
(calculate for chassis 2 with block-transfer request of 20 words) where
block-transfer duration (in ms) = D x R
D (ms) = 2C x M + (0.1W)
Where: Is the:
C number of extended-local chassis with 1771-ALX adapter modules
and block-transfer modules
M number of block-transfer modules in the chassis of the module
being calculated
W number of words in block-transfer request being calculated
D (ms) = (2 x 2) x (1) + (0.1 x 20 ) or D = 6 ms
R = 1 (when D < logic scan time) OR
R = Logic Scan + Housekeeping
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Logic Scan