OPTICAL SYSTEMS DESIGN
OSD1250 OPERATOR MANUAL
PAGE 7
DOC ID: 10106110
The primary method of managing data traffic is the use of RTS/DCD. In RS232 and RS422, the RTS signal envelopes the user
data. In RS485, a voltage ‘window’ comparator is used to detect when the copper pair is driven out of the idle state. The output
of window comparator is used to provide a pseudo RTS signal. The RTS signal is used to enable data buffers at the remote end
as DCD.
The OSD1250 data interface is via a high density DB26 female connector. Essential modem control signals are available as
RS232 or RS422 signals. Translation between RS232 and RS422 is possible. The OSD1250 generates Data Set Ready only
when it detects the adjacent units are in their correct operating state. The OSD1250 does use Data Terminal Ready as true input
to determine if DTE is operational. If user hardware does not use DSR/DTR then a wire link should be used between these 2
signals on the interface connector.
Data received from the fiber is sent to the receiver outputs and tri-stated or continuously driven depending on each mode.
Output signals from the interface like DSR are only ‘true’ if the OSD1250 links are operational,
The Net Error signal has been included to indicate a Link failure. It can be used to energise a relay for normal operation. A
network or power failure will cause a relay to be de-energised. The maximum rating for relay is 30V at 100ma.
The receivers will deliver the correct idle logic state output without any signals connected at inputs. MOV and diode protection
is included on all outputs.
1.6
FRAME STRUCTURE
The simplest and most robust way of transmitting the data over the fiber is to encode it using methods like Bi-phase or
Manchester coding. This enables simple and effective timing recovery and bit error monitoring over the link..
The OSD1250 uses a 32MHz primary clock. The data is transmitted as manchester encoded data at 16Mbps. The high speed
sampling of user data is at 8Mbps. The remaining 8Mbps is further divided into a 4Mbps channel for sampling RTS and 8
channels each at 0.5Mbps. The 0.5Mbps channels are used for synchronisation, network status bits, and three low speed data
channels.
The frame is multiplexed such that user data is sampled every second bit. Every second alternate bit is RTS/DCD. The
remaining bits reflect the 8 channels at 0.5Mbps.
