Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 136)

Chapter 10
Maximizing System Performance
10-4
If the communication rate is 115.2 kbps and you want to block transfer 10
words, for example, the interruption of the remote I/O scan is:
(10 x .14) + 2.5 = 1.4 + 2.5 = 3.9 ms
For the particular remote I/O scan in which the block transfer takes place,
3.9 ms will be added to the remote I/O scan time.
Calculating WorstĆCase Remote I/O Scan Time
Since it is impossible to predict which remote I/O scan a block transfer will
occur within, you can only calculate the worst-case remote I/O scan time.
To calculate the worst case, determine the normal I/O (without block
transfers) then add the time of the longest block transfer to each entry in
the scan list. (The processor can only perform 1 block transfer per entry in
the scan list per I/O scan.)
For example, if your system is:
PLC
115.2 kbps
Rack 1
Rack 2
Rack 3
No BTs
BT
30
words
BT
10
words
BT
20
words
WorstĆcase I/O scan:
(3 x 6)
+ (20 x .14) + 2.5
+ 0
+ (30 x .14) + 2.5
3 racks at 115.2 kbpsĊnormal I/O scan
longest BT in rack 1
no BTs in rack 2
longest BT in rack 2
18 + 5.3 + 0 + 6.7 = 30 ms
Optimizing Remote I/O Scan Time
The best way to optimize your scan time is to place your most time-critical
I/O on a separate channel from non-critical I/O. If you only have one
channel available for I/O, however, you can still optimize the scanning by
using the processor’s configurable scan list.
In a normal 4-rack system, the scan list would be:
If you are using 57.6 kbps, the normal I/O scan is 4 racks x 10ms—40ms.
Each entry is of equal priority, so each rack is scanned every 40 ms.
rack 1
rack 2
rack 3
rack 4