 |
 |
|
|
 |
| 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. |
|
|
| |
|
|
|
|
|
|
|
|
|
