Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 293)

Publication 1785-UM012D-EN-P - July 2005
Maximizing System Performance C-7
These three factors affect the remote I/O scan time:
• communication rate
• number of rack entries
• block-transfers
Communication Rate
The communication rate determines the time it takes for the scanner to
communicate with each individual entry in its scan list. The following table
lists the amount of time required to communicate to a device at each
communication rate.
If four full-rack entries are in the scan list, the I/O scan for that channel at
57.6 kbps is 4 x 10 = 40 ms. If you change the communication rate to 230.4
kbps, the I/O scan decreases to
4 x 3 = 12 ms.
Number of Rack Entries
You determine the total remote I/O scan time in the remote I/O system by
this formula:
If one channel has twice as many racks as another, for example, the scan time
for the first channel is twice as long.
To optimize this scan time, divide your I/O racks between multiple channels.
Place your most time-critical I/O on one channel, and non- time-critical I/O
on the other channel. Since all I/O channels are independent, a long remote
I/O scan on one channel will not affect the remote I/O scan on another
channel.
Communication
Rate (kbps):
Time
(ms):
57.6 10
115.2 7
230.4 3
Note that these are full rack times.
Smaller racks will decrease this time.
total remote I/O scan time = # of rack entries X time per rack-entries
in the scan list (see on page 7)