Echelon LonPoint Module - Manual (Page 30)

3-8 Network Cabling and Connections
distributed configuration. For this reason, it is essential to follow a simple topology
rule when using the TPT/XF-1250 channel.
Referred to as the “8-in-16” topology rule, this rule requires that no more than 8
devices be connected to the TP/XF-1250 channel within any 16 meter length of cable.
This means that no matter where along the bus the 16 meter measurement is taken,
there should be no more than 8 devices. Figure 3.10 provides a diagram of such a
measurement technique.
Figure 3.10 8-in-16 Topology Rule Example
In the example we see an installation with six groups of devices, varying in size from 2
to 7 devices, in a doubly-terminated bus that is terminated with Model 44200
Terminators. By using a 16 meter measurement stick that we can move from side-to-
side over the length of the bus, we can determine whether the 8-in-16 rule has been
met (designated by the word "OK") or violated (shown by the designation "PROBLEM").
In the case of the PROBLEM area, a total of 13 devices are located within a 16 meter
length of the bus, which amounts to five more devcies than are permitted under the 8-
in-16 rule.
There are two solutions that can be applied to situations in which the 8-in-16 rule has
been, or must be, violated by virtue of the installation scenario. The first and simplest
remedy is to insert a Model 42105 LPR-15 Router (TP/XF-1250 to TP/XF-1250) and two
Model 44200 Terminators in the bus to break the network into two channels (figure 3.11).
Since each side of the router comprises a different channel, the bus is effectively split and
the nodes divided between two channels.
Figure 3.11 Using a Router to Meet the 8-in-16 Topology Rule
The second remedy to a violation of the 8-in-16 rule is to add additional cable to the
bus such that the rule is no longer violated (figure 3.12). It is important to ensure
that the maximum bus length (130 meters of 22AWG/0.65mm Level IV twisted pair)
is not exceeded by the additional cable. Due to the complex interactions between the
bus and the devcies with regard to reflections and transmission line delays, it is not
possible to substitute an inductor/capacitor network in lieu of the additional cable to
resolve this rule violation.