Rockwell Automation 1786-RPFRXL ControlNet Fiber Ring Modules Installation Instructions - Manual (Page 23)

ControlNet Fiber-optic Ring Repeater Modules 23
Rockwell Automation Publication 1786-IN003D-EN-P - April 2011
EXAMPLE Determining worst-case delay
To determine the worst-case delay in a ring topology, first
disregard the shortest fiber segment in the system.
In Figure 4 on page 22 , the shortest segment is segment H, with
the 200 m fiber. Remove segment H. You will see that the
worst-case delay is now between nodes 1 and 2.
You must account for worst-case delays introduced by physical
media when setting up the media configuration screen in
RSNetWorx software. If too many components with too great a
delay are entered into RSNetWorx for ControlNet software, the
delay becomes too great for the bandwidth RSNetWorx software
has available. This affects system performance and limits
network length. If you do not account for all media components in
the worst-case delay path, erratic network operation will result.
Refer to the documentation supplied with RSNetWorx for
ControlNet software for more information.
This example shows you in a simple way how to account for
system delays. In this example, you enter the total length of all
media components between nodes 1 and 2 into RSNetWorx for
ControlNet software. The totals of the components between
nodes 1 and 2 are as follows, as specified in
Table 1 on page 21 :
Coax media delay: 200 m (A) + 20 m (G) = 220 m x 4.17 ns
Fiber media delay: 2 km (B) + 1 km (D) + 5 km (F) = 8 km x 5.01 ns
1786-RPA/B module delay: 1 (at node 1) + 1 (at node 3) +
1 (at node 4) +1 (at node 2) = 4 x 901 ns
1786-RPFRL/B or 1786-RPFRXL/B module: 1 (at node 1) +
1 (at node 3) + 1 (at node 4) +1 (at node 2) = 4 x 100 ns
In summary:
Worst-case delay = 220 x 4.17 + 8000 x 5.01 + 4 (901)
+ 4 (100) = 45 µs
This delay is acceptable because 45 µs is less than the maximum
allowable delay of 121 µs.