Introduction
The Eurotherm Process Automation distributed control system uses the ALIN (Arcnet Local Instrument Network) as a control network to link control units together. The network can be expanded by the use of active hubs which support both twisted-pair and fiber-optic technology. These modular active hubs (Mod-hubs) support multiple cable connections and allow many more nodes to be connected using a combination of bus and star topology.

Redundant connection for dual server workstations

Each cable connected to the Mod-hub can support up to eight nodes. The ALIN may be run over much longer distances by using a Mod-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 five kilometers.

The use of this medium is also beneficial in applications requiring intrinsically safe equipment and 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 Mod-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 similar to the RJ11 connector used to connect to the Modhub. 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. The Mod-hub has a conventional Arcnet transceiver and up to 8 nodes may be connected to a bus segment connected to a Mod-hub. The connector for the twisted-pair star (-TPS) port is a 4-wire, 6-position RJ11 which is connected to the EPA RJ45 connector system via an adapter cable.

Termination
The -TPS port on the Mod-hub appears as a balanced twisted-pair port with an input impedance of 93 ohms. This port effectively terminates one end of an ALIN segment since the characteristic impedance of the ALIN cable is 100 ohms. The -TPS port on a Mod-hub expansion module must therefore be connected to the end of an ALIN segment, remembering to omit the end-of-line terminator.

Connecting to the Mod-hub
Refer to the diagram below when connecting the Mod-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.

If a connection must be made to the middle of an ALIN segment, cut the ALIN cable, thereby creating two ALIN segments, and use two -TPS ports which each effectively terminate one end of a segment. The far end of each segment will still require passive termination.

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 Mod-hub expansion modules provide four ports each (either twisted-pair or fiber-optic) and up to four expansion modules can be inserted into a Mod-hub-16 chassis. Mixed media modules exist with both twisted-pair and fiber-optic ports. These modules may represent the least expensive method of bridging cable technologies.

Fibre-optic cable types and connectors
There are three common diameters of fiber optic cable that are supported by the Mod-hubs, namely 50/125, 62.5/125 and 100/140 microns. There are also two common types of connecter, namely ST and SMA. The first of these is a twist and lock type and gives repeatable performance at a slightly higher price. The SMA type connector is less expensive but is a twist on only and gives a higher insertion loss and lower repeatability since the connecter is not guaranteed to be in the same place with respect to the fibre after each mating.