Hardware reference guide

Modbus Plus NOM
310
35013379 02 October 2007
Repairing the
Break in the
Cable
Because the NOM25200 will stop transmitting in the direction from which it is not
receiving the signal, the replacement of a broken fiber optic cable and the
reconnection of it will not re-establish communication over that segment. The hot
swap of only one NOM252 at the repaired connections is required to complete the
connection.
For the self healing ring configuration, the repair of the first break in the fiber optic
network has to be scheduled to the time when one of the units on either side of the
repaired break can be hot-swapped without creating the problem by disconnecting
the node.
Calculating
Number of
Modules in a
Fiber Network
Calculate the number of NOM25200 modules in a fiber network using the following
method.
Note: The break of any fiber connectors or fiber optic cables is the equivalent to
the break of the trunk cable in a Modbus Plus network on copper.
Note: Self healing configurations are not considered redundant networks. High
system availability can be achieved with redundant networks.
Step Action
1 The total allowable pulse width distortions and jitter are limited to 20% of the bit
period and is 200 nsec for the full fiber optic network.
2 The jitter contributed by the NOM252 is 5 nsec max.
3 Jitter contributed by fiber optic repeaters (if used) is 40 nsec.
4 The formula to determine the number (N) of chained repeaters is:
where "L" is the total cable length (km), and "X" is the jitter (added by the fiber
optic cable) in nsec/km:
X = 3 ns/km for 50/125 μm
5 ns/km for 62.5/125 μm
7.5 ns/km for 100/140 μm
N
200nsec X L()nsec–40nsec–
5nsec
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This document provided by Barr-Thorp Electric Co., Inc. 800-473-9123 www.barr-thorp.com