Optelecom 9525AY/SM-FC Installation And Operation Manual page 16

2.3.2 Optical Side Anti-streaming
The optical side anti-streaming feature, when enabled, disconnects an optical receiver from the circuit
in the event that the receiver's output goes high for a period longer than would be expected in normal
data flow. This may occur due to a receiver failure, a transmitter or terminal equipment failure, or if
the receiver input is left open to strong ambient light (850 nm versions only). In cases where more
than one receivers' outputs are routed to the data output (RD) or in cases where a receiver's output is
re-transmitted, a failure of this type could disable one modem, part of the network, or the whole
network, depending on the network architecture, if it were not for the optical anti-streaming circuitry.
The circuit works as follows: As long as the data is high (optical power present at the receiver) the
timeout proceeds. Whenever the data goes low, as it does for all normal zeros, the timeout resets. As
long as the data does not contain a long string of continuous ones that exceeds the timeout period,
the anti-streaming will not be triggered. If there are too many ones in a row or if there is a failure
which leaves the receiver output constantly high, the receiver output will be disconnected from the
circuit and the FAULT indicator will illuminate. Mode switchnumbers4 and 5 allow the user to select
among 3 timeouts or to disable the feature. The longest timeout selection is sufficient to not trigger an
optical side anti-streaming timeout for continuous 8 bit ASCII characters ("quick brown fox") at only
150 baud. Faster data rates allow the use of a shorter timeout.
2.3.3 Clear to Send Inhibited by Optical Side Activity (RS232 mode only)
Another optional feature, enabled or disabled by mode switch number 6, causes the disabling of the
Clear-to-send (CTS) control line output in the presence of fiber side data activity. Normally the system
software will preclude such an occurrence, but as a safeguard this feature will prevent a terminal from
interfering with data already travelling on the optical highway, data either from the master or from
another terminal. The data activity detector has a fixed timeout of about 12 msec, which is sufficient to
keep CTS disabled during zeros in the fiber side data transmission at rates greater or equal to 600
baud. For ASCII character transmission at rates below 600 baud, this feature should not be used.
There is no visual indication of the feature; only the electrical CTS output's going low if already high or
being delayed in going high if there is optical side activity at the time a RTS is given.
PLEASE NOTE: IF THIS OPTION IS ENABLED, A TERMINAL WILL NOT BE ABLE TO RESPOND TO
DATA TRANSMITTED EVEN TO ITSELFUNTIL AFTER THE 12 msec TIMEOUT PERIOD. THEREFORE IT
IS NOT RECOMMENDED TO USE THIS OPTION UNLESS IT MAKES SENSE FOR YOUR SYSTEM.
2.3.4 Priority Select Priority Select is enabled automatically in both Master and Local Fault Tolerant modes
(see Sections 2.2.3 and 2.2.4). The priority select circuitry automatically selects the data that arrives
first, via optical port 1 or 2. In normal operation, since the master transmits both directions on the
ring, each terminal on the ring will receive data from both directions. The optical signal that arrives
first will be selected by the priority select circuitry, and output via the RS232 ports as RD and
RDE. If the data transmission in the first direction fails it will automatically select the data coming from
the other direction the next time it is transmitted.
This feature functions as follows: A flip flop constantly stores which optical receiver has priority, as
indicated by the RX1 and RX2 priority indicators observable with mode switch #4 in the DOWN position.
This flip flop will change states and select the other optical input port upon the arrival of data only if
there is no activity currently detected on the first port. For a period of approximately 12 ms after a
transition from HIGH to LOW on the selected optical input late arriving data from the other port is
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