Ring topology no longer considered unconventional
11 April 2001
Computer/Embedded Technology
A combination of original design and skill now allows an ArcNet control system to be cabled as a ring under certain conditions without causing unreliable network operation. Contemporary Controls' engineers developed the AI-FR to provide redundant cabling so continuity can be preserved in case of cable failure. If one cable becomes disabled for any reason, another is still able to transmit messages.
The AI-FR is defined as a redundant fibre ring with a local drop. It is constructed with two or three-port hubs incorporating the principle of fault monitoring and fibre-optic cables connected in a ring topology. A fibre backbone ring up to 2 km in size and employing up to four AI-FR hubs is possible. By decreasing the total fibre ring size, additional AI-FR hubs can be attached to the network allowing more devices to share the redundant ring.
In idle start, the AI-FR awaits a signal with all its receivers enabled and all transmitters disabled. As soon as one of the ports senses a signal, the hub becomes active - while the other two ports become transmitters with their receivers disabled. Activity from the two backbone ports will travel along the ring in two directions - clockwise and counterclockwise.
With a fibre ring properly configured, activity will be present on both backbone ports. The AI-FR will latch the first active port and block the other. If both ports receive simultaneous data, priority is given to port two.
The unlatch delay (which squelches any recirculating messages) has been increased from 5,9 to 11,9 µs for greater distance. This delay must exceed the total delay due to fibre propagation (5 ns/m) and the number of AI-FR hubs (330 ns/hub). The hub now assumes transmission is complete if no data is received after 11,9 µs and reverts to its inactive state.
In case operation is disrupted, the AI-FR incorporates a fault relay by which fibre ring continuity can be monitored. Using this relay, a fibre failure can be quickly detected and the problem location identified and the repair scheduled - all while data traffic is maintained.
The hub is available in two versions. The AI-FR/CXB hub provides fibre 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.
For further information contact Electronic Products Design, (012) 665 9700.
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