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| Multidrop
is a common form of control for network access which
is used when a large number of units have to communicate
with a main computer or control system, e.g. for access
control, scale systems, collection of measurement data
or status control. The main computer polls, or asks,
each unit in turn whether it has a message to send. Since
the units are intelligent and recognize their own address,
fully transparent modems are used. RS-422/485 are typical
multidrop-adapted standards. One disadvantage of the
multidrop system is that a master/slave system is used
where only the main computer is active. Units cannot
transmit data on their own initiative and have to wait
to be polled by the main computer. For this reason, multidrop
systems are more suitable for PDS, indoor climate control
and automation than for computer to computer communication.
Westermo has a wide range of communications products
for multidrop systems, from the synchronous MA-32 to
the asynchronous line sharing LA-01 modem which allows
for network distances of up to 18 km in full duplex over
4-wires. The TD-23 allows distances of up to 25 km in
half duplex over 2-wires. The LD-63 allows multidrop
applications on fibre optic cables. |
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