Optelecom 9525AY/SM-FC Installation And Operation Manual page 17

prevented from seizing the priority. This feature will allow guaranteed priority for the shortest
transmission path around the ring as long as the propagation time around the ring is less than 12 ms.
This is equivalent to a ring 500 km long with 500 repeating modems in the path. In the event of a
failure of the data path with has current priority, a wait of at least 12 ms must occur before the other
transmission path is selected.
NOTE: When operation in Master Fault Tolerant mode in a ring architecture, the unit is transmitting in both CW and CCW
directions simultaneously. It also receives its own transmitted data from both the CW and CCW directions. If both fibers in the
ring are routed together, as they are if they share the same cable, the times of arrival of the received signals at the two
receivers may be too close for the priority select circuitry to resolve (<150 nsec). This will cause the priority select to switch
back and forth at random which is unacceptable. To solve this problem the installer may have to add up to 100 feet of
additional fiber to one and only one of the four fibers terminating at this unit.
2.3.5 Data Re-timing
All data received via an optical port is re-timed prior to being sent on to its destinations within the
modem. This is to say that the pulse width distortion is reduced to virtually zero. This feature allows
virtually unlimited repeating avoiding the accumulation of pulse width distortion caused by the optical
transmission.
The circuit outputs pulses which are integer multiples of 8.6805556 µsec. in length, accurately re-
timing all data streams operating at bitrates of
For 9.6 kbaud, N=12. For 57.6 kbaud, N=2. If your system operates at a nonstandard bitrate, contact
the factory. A change in crystal frequency is all that may be required.
2.3.6 Expansion Port
There is a 6-pin modular RJ-12 expansion port that has an RS232 compatible input and output. A
modem with single or dual optic ports can be attached hereto branch in one or two additional
directions. The use of this expansion port allows the user to configure a daisy chain system with
unlimited branches or a fault tolerant ring with unlimited non-fault tolerant branches. See examples in
Section 2.3.
2.3.7 Operating in RS485 2-Wire Mode
When operating in RS485 2-wire mode it is possible to have the electrical data corrupted due to the fact
that the transmit data input and received data output occurson the samepair of wires. To resolve this
problem, there is a circuit that prevents the output of received data onto the 2-wire RS485 bus within a
given period after the data input stops. This is called the Ring Propagation timeout. Refer to Table 2 to
select the proper timeout. The user can select a timeout period of 1 ms, 4 ms, or 12 ms by setting switch
positions 7 and 8. The requirementis that a setting be selected which is greater than the time it forthe data
to propagate over the fiber and through all the units around the ring, back to the master unit. The result
is,then, that the transmitted datawillarrivebackatthe receiver after its trip around the ring before the Ring
Propagation timeout has been completed, preventing the received data from corrupting the transmitted
data. It is suggested that in order to not artificially limit the polling rate that the 1 mstimeout be selected
when operating in all modes otherthanthe MasterFault Tolerant mode.
6
10
N (8.6805556)
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