Westermo LRW-102 - Manual (Page 13)

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LON Channel delay
The LON TP/FT-10 (EIA-709.3) specification states a maximum delay of 36 μs and a maxi-
mum of one repeater between any two nodes. Using the LRW-102 this delay restriction
will normally be met if the total fibre cable length is restricted to 5.6 km 3.5 mile).
When two or more LRW-102 is used on a LON ® free topology network they form a
physical repeater link. As with a normal FTT-FTT physical repeater, the LRW-102 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 LRW-102 units can transparently forward LonTalk
® packet 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, espe-
cially for a busy channel.
A normal FTT channel is dimensioned for one physical repeater allowing a maximum dis-
tance of 5400 metres (2 x 2700 metres) or 3.4 mi (2 x 1.7 mi).
For a busy channel it is recommended to use the LRW-112 router instead for the
LRW-102 converter. With the LRW-112 router, the delay is not a problem since it uses a
dedicated fibre optic channel where the propagation delay is accounted for.
It is recommended to use a LON ® protocol analyser to verify network performance dur-
ing high peak channel access. If an increased number of packets collisions and retries are
found we recommend using the LRW-112 router instead.
According to the EIA-709.3, the delay through a repeater link must not exceed 36 μs.
To honour that requirement the following relation should be used:
Fibre distance (m) / 200 + Number of units < 36
Recommended max number of units = 10
For example, the above relation would allow two LRW-102 units to use a maximum fibre
optic cable length of 6.2 km (3.8 mi).
The EIA-709.3 recommendation is strict and shall handle 100% load on a network,
decreasing the load gives possibility to increase the allowed delay in the network.
Westermo has made tests with 60% load on the TP/FT-10 network and up to 60% load
it is possible to increase the allowed delay to 127 μs.
The following relation can be used on networks with up to 60% load
Fibre distance (m) / 200 + Number of units < 127
Recommended max number of units = 10
For example, the above equation would allow 10 LRW-102 units to use a maximum fibre
optic cable length of 23.4 km (15 miles).
The above discussion considers the limitation in maximum distance due to protocol
parameters involving media access and network idle detection. As with all fibre optic
products, the maximum distance is also dependent on the over all power budget
(see page 15).