Optelecom 9525A Installation And Operation Manual page 23

3) Warning: If one of the rings in a fault tolerant ring is broken, a modem on the ring may only
be able to receive an important signal from one direction. In this case, unless a time period
greater than the priority select timeout elapses before the next transmission occurs, the priority
select circuit will not be able to react in time and the first portion of the message could be lost.
Therefore, the priority select timeout limits the speed at which a terminal may respond to a poll
and limits the polling rate. Select the timeout accordingly.
2.4.7 Clear to Send Inhibited by Optical Side Activity
To enable this feature, as explained in Section 2.3.2, set TIMEOUTS switch #6 in
the UP position. To disable it, set switch #6 in the DOWN position. To enable it, set switch #6 in the
UP position.
2.4.8 Ring Propagation Timeout As It Pertains to the ECHO CANCEL Feature and RS485
2-Wire Operation
The Ring Propagation Timeout plays a role in the ECHO CANCEL function as explained in Section 2.3.4.
This timeout is not important unless the unit is operated in Master Fault Tolerant Mode and MODES
switch #7 is set to the DOWN position. This timeout should exceed the time it takes for a signal to
travel from the master unit around the ring and return to the master unit. To calculate the time, figure
(2 microseconds) plus (6.5 microseconds times the number of local modems around the ring) plus (1.5
nanoseconds per foot of fiber in the ring). For example, for a ring with a total length of 3000 feet with
15 local modems, the propagation time would be 2 microsecond + 97.5 microseconds + 4500 ns =
104 microseconds. For this ring, the shortest timeout of 1 millisecond should be chosen. Refer to Table
5.
Table 5 – Ring Propagation Timeout Selection via the TIMEOUTS Dipswitch
Switch # Ring Propagation Timeout
7 8
U U 1 millisecond
D U 4 milliseconds
D D 12 milliseconds
D (DOWN) = ON; U (UP) = OFF
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