Optelecom 9525A, Installation And Operation Manual

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To use this feature an active RTS signal is required. If the RTS input cannot be actively controlled, the
electrical side anti-streaming feature must be disabled. MODES switch numbers 1, 2, and 3 allow the
user to select one of seven timeout periods, or disable the feature. To select a timeout, calculate the
longest transmission burst expected by the terminal and pick the next highest timeout period. If this is
not practical in your system, just disable it.
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). The anti-streaming
feature will prevent such an error from "locking up" the network.
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 that
leaves the receiver output constantly high, the receiver output will be disconnected from the circuit
and the FAULT indicator will illuminate. Mode switch numbers 4 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
Another optional feature, enabled or disabled by MODES 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 traveling on the optical trunk or ring either from the master or from
another terminal. The circuit uses the “Priority Select” timers to determine the CTS disable period.
Whatever Priority Select timeout is selected applies. The selected rates should be sufficient to keep
CTS disabled for a period of one byte. For ASCII character transmission at rates below 600 baud, this
feature should not be used.
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 RX1 or RX2. 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.