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 fiber optic cables.