Contemporary Control Systems AI-FR Redundant Fibre Hubs - Manual (Page 6)

TD675300-0ID
6
OPERATION
In IDLE state, the AI-FR hub awaits an ARCNET signal with all its
receivers enabled and all transmitters disabled. As soon as one of its ports
senses a signal, the hub becomes ACTIVE, thus keeping that receiving port
enabled while disabling all other receivers. Conversely, all hub transmitters
are enabled except that of the receiving port.
The unlatch delay (which squelches any recirculating message) has been
increased from 5.9 µs to 11.9 µs. Hence, total delay due to fiber
propagation (5 ns/m) and the number of AI-FR hubs (330 ns/hub) must be
less than 11.9 µs. After 11.9 µs have elapsed without a logic “1” being
received, the hub assumes the source has finished transmitting. The hub
then “unlatches,” switching from the ACTIVE to the IDLE state.
With a fiber 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 2.
Fault Monitoring
If something disrupts normal operation, a watchdog timer (sensing no hub
activity after a preset time) automatically resets its timing circuitry to ensure
that the hub reestablishes communication.
If a fiber fails, the AI-FR will sense no activity within 250 ms and open the
screw terminals of the front-panel relay connection (refer to figure 6 below).
By monitoring these terminals, fiber faults can be detected. If the hub locks
on one port for more than 1.2 seconds (possibly due to circulating noise), a
second watchdog timer interprets this as a lockup, clears the hub and
restores normal operation.
Figure 6 — Fiber Ring Field Connections