Introduction
The Eurotherm Process Automation distributed control system uses the ALIN (Arcnet Local Instrument Network) as a control network to link control units together. This network is able to support up to 16 nodes with a nominal maximum cable length of 100 meters. The network can be expanded by the use of active hubs which support both twisted-pair and fiber-optic technology. These active hubs support multiple cable connections

Redundant connection for dual server workstations

and allow many more nodes to be connected using a combination of bus and star topology. The ALIN may be run over much longer distances by using a compact active hub at either end of a fiber-optic link to convert the media from twisted-pair to glass or plastic fiber-optic cable. With the use of multiple fiber-optic hubs, the maximum distance over which a network may run is extended to 2.7 kilometers.

The use of this medium is also beneficial in areas with a high degree of electrical noise and inter-building communication or areas with earth potential differences.

Redundant Connection For Dual Server Workstation
The integrity of an ALIN segment may be protected against the failure of a cable by the use of an active hub as a buffer/network isolator. Thus two server workstations connections may each be connected via an active hub. In the event of failure of the primary network, hubs or server workstation itself, the operator is assured access via the secondary server and its independent network connection.

Network Architecture
The active hub may be considered a buffer/isolator supporting multiple network connections using either twisted-pair or fiber-optic cables. It allows mixed star/bus topologies both for expanding the node capacity of local network clusters, and for extending the network length over considerable distances.

INSTALLATION
ALIN twisted-pair cabling
Connection to the ALIN network is made in 100 ohms shielded category 5 cable using either screw terminals, or an RJ45 connector which is also used to connect to the active hub. The integrity of the cable shield should be maintained throughout the system by having all foil screens or drain wires connected together. A terminal on each T640 loop controller or T221 gateway provides a convenient tie point for the drain wires, while the shield of the RJ45 connectors maintains continuity across T103/T303 units.

Arcnet transceivers
The transceivers used on the ALIN nodes have better performance than conventional twisted-pair bus transceivers and a total of 16 nodes can be supported on one bus segment. However when using active hubs the ARCNET standard of eight nodes per segment should not be exceeded.

Termination
The twisted pair port on the active hub appears as a balanced twisted-pair port with a high input impedance. This port is not a termination and allows the hub to be fitted anywhere in a segment. If the active-hub is connected to the end of an ALIN segment, an end-of-line terminator (LA 249397) must be fitted in the spare RJ45 jack.

Connecting to the compact active hub
Refer to the diagram below when connecting the active hub to either the end of an ALIN segment or between two ALIN segments. Observe correct polarity of signals since reverse polarity will cause unreliable operation.

Owing to the nature of fiber-optic communications, a duplex arrangement is needed thus necessitating two cores per connection (1 transmit/1 receive).

Expansion to fiber-optic cables
The compact active hubs provide three ports (3 twisted or 1 twisted-pair with 2 fiber-optic). The twisted pair ports can extend the bus by 122 meters, but the fiber-optic ports can extend the bus up to 2.7km per link.

Fiber-optic cable types and connectors
There are three common diameters of fiber-optic cable that are supported by the active hubs, namely 50/125, 62.5/125 and 100/140 microns.