Westermo LR-01 - Manual (Page 14)

14 6608-2201
3.4 Channel delay
When two or more LR-01 units are used on a LonWorks free topology network they
form a physical repeater link. As with a normal FTT-FTT physical repeater, the LR-01
link also forms a new channel segment and regenerates the signal allowing more nodes
to be installed.
It is important that the user is aware of the limitations involved with a physical extension
of an FTT channel. The LR-01 units can transparently forward the LonTalk
® packets and
also assure that the required signal level is kept throughout the channel extension.
However, cable length between the units will impose an extra propagation delay on the
channel that could conflict with the media access timing used by the LonTalk
® protocol.
Increasing the propagation delay results in a higher probability for packet collisions,
especially for a busy channel.
A normal FTT channel is dimensioned for one physical repeater allowing a maximum
distance of 5 400 metres (2 x 2 700 metres).
For a busy channel it is better to use the LR-11 router instead of the LR-01. With the
LR-11 router the delay is not a problem since it uses a fibre optic channel where the
propagation delay is accounted for.
It is recommended to use a L
ON
W
ORKS
® protocol analyser to verify the network per-
formance during high peak channel access. If an increased number of packet collisions
and retries are detected, the options are to either use the router model (LR-11) or
modify the communication parametres on the nodes to allow for an extra propagation
delay. The trade-off for changing the communication parametres is a reduced channel
performance. See application notes AN-01201A for details about changing communica-
tion parametres on the nodes.
Having the above limitations in mind, we recommend using any of following equations
to determine total fibre distance and number of units:
For example, the above equation would allow 10 LR-01 units to use a maximum fibre
optic cable length of 23.4 km.
According to the EIA-709.3, the delay through a repeater link must not exceed 36 micro-
seconds. In many cases this requirement is met if the following relation equation is used:
For example, the above equation would allow two LR-01 units to use a maximum fibre
optic cable length of 5.6 km.
The above discussion considers the limitation in maximum distance due to protocol
parametres involving media access and network idle detection. As with all fibre optic
products, the maximum distance is also dependent on the available power budget
between the nodes (see section 3.5).
Fibre distance (m) / 200 + Number of units < 127
Max number of units = 10
Fibre distance (m) / 200 + Number of units < 30
Max number of units = 10