729
6. After holding -Lock to
Reference low for 500 µsec it
should then be driven high. This
causes the module to phase and
frequency lock onto the incoming
data stream within 2500 bit times
(2.4 µsec).
7. After 2500 bit times, the
modules should be in bit syn-
chronization, but not yet byte
synchronization. The Receive
Byte Clock (RBC0) should be
running at 53.125 MHz.
8. Finally, drive the data lines
Tx[00:19] with a K28.5 (comma
or byte sync) character. Upon
detection of this character, the
receiver will drive the Comma
Detect line high, the clocks will
align to the byte boundary, and
the receive data lines (Rx[00:19])
will have valid data. The link is
now ready for data transmission.
TX_SI Operation
In normal operation, pin Tx_SI
should be held low. In this mode,
the data at Tx[00:19] is serialized
and driven over the fiber optic
link. With pin Tx_SI driven high,
however, the data at Tx[00:19] is
serialized and driven out the
±Serial Data Out lines and the
data applied to the ±Serial Data
In lines are driven over the fiber
optic link.
EWRAP Operation
To aid in link diagnostics, the
modules have the capability of
wrapping the local transmit data
electrically back to the local
deserializer. This feature is
enabled by driving the EWRAP
pin high. When enabled, EWRAP
causes the laser to turn off within
20 µsec. The OFC circuit goes
into its low duty cycle “on-off-on”
handshake mode. The link will
need to be stepped through the
synchronization procedure
outlined above to return to
normal operation after EWRAP is
brought low.
Enable Comma Detect
In the synchronization procedure
above, the Enable Comma Detect
(EN_CDET) signal is driven high
to allow the receiver to reset and
align its boundaries properly
when a K28.5 character is
transmitted. This line can be kept
in a high state and the receiver
will reset on every K28.5
character it detects. This feature
can be disabled, after initial
synchronization, by driving
Enable Comma Detect to a low
state.
Open Fiber Control
The purpose of the Open Fiber
Control (OFC) integrated circuit
is to ensure user safety. This
circuit uses the dual loss of light
signals from the receiver I.C. and
the laser Fault detection signal
from the transmitter I.C. to
determine if the laser and the
fiber link are properly connected
and functioning normally. Should
a Fault condition be determined,
all laser transmission is shut
down.
A safety interlock is provided by
the HGLM-1063 module. HGLM-
1063 modules (or equivalent)
must be connected in full point-
to-point configuration as shown
in Figure 3 for proper operation.
The Open Fiber Control System
(OFCS) of the HGLM-1063
deactivates the laser signal
whenever there is an interruption
or loss of signal of either laser
drive circuit.
For example, in Figure 3, if Path
A is opened through a cut or
other physical damage to the
fiber, or if the fiber is discon-
nected at either the transmitting
port of HGLM1 or the receiving
port of HGLM2, the OFCS detects
the loss of signal. The Link
Unusable line of HGLM2 goes
high, signaling the system of an
open fiber condition. The OFCS
then shuts down the laser of
HGLM2. HGLM1 in turn detects
this loss of signal, raises its Link
Unusable line, and shuts down its
laser. The OFC pulses the laser of
HGLM2 at a very low duty cycle.
Simultaneously, the OFCS of
HGLM1 detects the low duty
cycle operation of HGLM2 and
places its laser in the same low
duty cycle pulsing mode. It takes
less than 2 msec to shut down all
laser transmission and results in a
safe (Class I) laser emission level
in Path A, the open path.
While Path A is still open,
HGLM1 launches a pulse
synchronously with the pulse it
receives on Path B from HGLM2.
However, HGLM2 receives no
pulse (because Path A is open)
and continues in an inactive
mode. HGLM2 will continue
launching “inquiry” pulses once
approximately every 10.1 seconds
along Path B.
After Path A is restored, HGLM2
will receive pulses along Path A,
synchronous with its transmit
pulses along Path B. A completed
link of Path A and Path B is
verified by both HGLM1 and
HGLM2. HGLM2 will verify its
link by deactivating its laser and
confirm that its receive signal
disappears. HGLM1 also
performs the same verification
check, deactivating its laser.
Once both HGLM modules have