ARCNET should be cabled as either a star or bus network but never as a ring. However, a ring topology is possible under some situations. The ring is merited when the goal is providing redundant cabling so that continuity is preserved in case of cable failure. If one cable within the ring becomes disabled for any reason, another path remains available to pass messages. It was Contemporary Controls' goal that that the AI-FR Series be designed and manufactured to achieve fiber optic redundancy in an otherwise non-redundant networking technology.
As an application-specific hub, the AI-FR assures the integrity of the redundant fiber backbone. Redundant network topology, defined as a fiber optic ring with a local drop, is established with three-port hubs—each incorporating the monitoring principle. A fault is noted by the opening of the fault relay contacts while communication is maintained.
The AI-FR operates from either low-voltage AC (8–24 VAC) or DC (10–36 VDC) power. Redundant power sources can be connected for critical applications.
LED indicators aid troubleshooting. Port activity LEDs report ARCNET traffic received. The status LED will flash at a periodic rate when the AI-FR is correctly powered but in idle condition. When lit continuously, this LED indicates that ARCNET traffic is being received and faithfully regenerated to the other hub ports. The RECON LED will flash to show a routine network reconfiguration as an ARCNET node enters or exists the network.
This hub is available in two models. The AI-FR/CXB provides fiber ring continuity with a coaxial bus local drop while the AI-FR/TB5 accommodates a twisted-pair bus local drop. Units can be panel or DIN-rail mounted.
A redundant fiber ring is made up of two or more AI-FR hubs and fiber optic cables attached in a ring topology. Each hub has a local drop through which standard ARCNET devices may access the backbone and benefit from the redundant cabling. Signal delay constraints enforce a 2-km limit on the ring circumference when no more than five AI-FR hubs are being used. By reducing the circumference, more hubs can be employed—allowing additional devices to share the redundant ring.
With no ARCNET signal present, the three-port AI-FR hub has all its receivers enabled and all its transmitters disabled. When a drop port senses a signal, it becomes the sole receiving port and its transmitter is disabled—while the other two backbone ports become transmitters with their receivers disabled. With a fiber ring properly configured, activity from the two backbone ports will travel along the ring clockwise and counterclockwise. A receiving AI-FR located on the ring will latch onto activity sensed by one of its backbone ports and ignore the redundant transmission seen on the other port. In case both ports receive simultaneous data, priority is given to port 2. If the ring is broken at any one point, the network will function in a traditional star topology.
The unlatch delay (which squelches any recirculating message) has been increased from 5.9 µs to 11.9 µs for greater distance. This delay must exceed total delay due to fiber propagation (5 ns/m) and the number of AI-FR hubs (330 nm/hub). If no data is received after 11.9 µs, the hub assumes that transmission is complete and reverts to its quiescent state with all receivers enabled and all transmitters disabled.
The AI-FR hub's most important requirement is to provide reliable data transfer. The failure of fiber cables is a worry especially if the cabling is installed in underground or hard-to-service locations. In the event of a fiber failure, the AI-FR fiber ring maintains uninterrupted communication by virtue of its redundant data path.
In addition to preserving data flow, the AI-FR includes a fault relay by which fiber ring continuity can be monitored. Using this relay, a fiber failure can be quickly detected, the problem location identified and the repair scheduled—all while data traffic is maintained.
Access to the fault relay contacts is accomplished by a pair of terminals on the AI-FR's front panel. By connecting these terminals to a workstation or other user-provided equipment, data path integrity is monitored. When open, the relay contacts of a particular hub indicate one of three important fault conditions: an open in a fiber attached to the hub, a loss of power to the hub, or a failure of the hub itself.
An AI-FR hub should sense data on both of its fiber ports, but if the signal is constantly absent on one of the receiving fibers for a period of 1.6 seconds, the fault relay contacts will open. Therefore, an open relay occurring at only one hub will indicate one bad fiber in either of the duplex pairs attached to that hub. If a complete duplex pair fails, neither of the two hubs connected by the pair will receive data along that path—and the opening of both fault relays in the two hubs will indicate a complete failure of the fiber pair which connects them.
When power is provided to the AI-FR, the fault relay contacts close as normal operation is assumed. If power to the AI-FR is interrupted, the relay contacts will open and report a problem. If the hub malfunctions and fails to keep the relay contacts closed, the open contacts will indicate the need for service. If a hub fails, either from power loss or for other reasons, the two hubs which normally communicate with it will fail to receive data from the downed hub. They each will indicate a fault and its source.
||Fiber ring backbone with coaxial bus drop|
||Fiber ring backbone with twisted-pair bus drop|
||Wall-mount plug-in transformer 120 VAC (nom) input/24 VAC (nom) output (details...)|
||Wall-mount plug-in transformer 230 VAC (nom) input/24 VAC (nom) output (details...)|
||BNC "T" connector|
||93 ohm BNC terminator|
||100 ohm RJ-45 terminator|
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