Optical Systems OSD1250, Руководство оператора

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OPTICAL SYSTEMS DESIGN
OSD1250 OPERATOR MANUAL
PAGE 5 DOC ID: 10106110
1.4 TYPICAL DESIGN APPLICATION
1.5 THEORY OF OPERATION
A simplified block diagram of the OSD1250 is shown in Figure 1.
The unit is designed to operate either in bus or ring topologies (see Figure 5 & Figure 6).
In all cases only one unit in the network is designated the Bus Master and acts as both the start and end of the bus/ring, i.e.
signals do not pass through from Channel 1 to Channel 2 (and vice versa) as they do at the slaves.
In the case of a bus topology, the unit(s) located at the extremities of the system require only one Channel: the unused Channel
is fitted with protective caps. Note that the Bus Master unit does not have to be at the extremity of the bus.
A further consideration is the need to ensure that faults don't cause the bus to be monopolised. This could be due to a software
fault in a remote unit causing transmission or it may be a fault within the optical modem forcing a logical "l" to be
continuously transmitted. The unit therefore has three anti-streaming (or anti-jabber) circuits, which inhibit transmission if the
preset time limit of 300ms is exceeded and then enable transmission after the streaming signal is removed. The OSD1250
removes a streaming signal for a minimum of 1 second. This ensures that a faulty unit cannot monopolise the bus for more than
30% of the time. The OSD1250 does not require any special settings for user data rates from 300bps to 2Mbps (25% distortion
max). The OSD1250 is best suited to polled data protocols.
A concern with systems like this which continuously regenerate the signals is that system timing can be lost or degraded if an
intermediate unit fails. The OSD1250 overcomes this by having a fixed crystal oscillator at each node. When the incoming data
exhibits either a very high bit error rate (this rate is around 1x 10
-2
), or the incoming optical signal is lost, the associated
transmitter’s timing is switched over to the fixed oscillator and the receiver recovered clock is also stabilised with the fixed
oscillator.
BUS CONFIGURATION
OSD1250
1 or 2 fibers 1 or 2 fibers
OSD1250 OSD1250
RS232 / RS422 / RS485
RS232 / RS422 / RS485
RS232 / RS422 / RS485
RING CONFIGURATION
OSD1250
1 or 2 fibers
OSD1250
RS232 / RS422 / RS485 RS232 / RS422 / RS485
1 or 2 fibers