IBM42M10SCYAA10
IBM42M10LCYAA20
1063Mb/s Gigabit Link Module
Open Fibre Control
A safety interlock is provided by the Open Fibre Control (OFC) module on these GLMs. Two GLMs must be
connected in a point-to-point duplex configuration as shown below for proper operation. The OFC module
pulses the laser at a low duty cycle while the fiber is open. This produces Class 1 optical power in the open
fiber.
The two Transition Detectors shown in the Block Diagram on page 4 detect a minimum AC level of modulated
light entering the photoreceiver. These redundant signals shut down the link if either fiber path is open. When
the fibers are reconnected, the OFC returns the laser to continuous power. The timing for this sequence is
shown on page 6.
GLM Fiber Connection
Link Unusable
GLM 1
Laser Fault
T
Duplex Fiber Path
R
R
GLM 2
T
Link Unusable
Laser Fault
If the fiber path is opened (both fiber paths are broken), the OFC on each GLM detects the loss of signal and
turns off its laser. Each GLM enters the Inactive mode where it pulses its laser at a very low duty cycle. This
pulsing behavior provides a safe (laser safety Class 1) laser emission level in the fiber cable. The pulse dura-
tion of the laser is roughly equal to the Decode 1 timing modified by the transmitter turn-on/turn-off delays. If
the GLM does not receive a responding optical signal during the Decode 1 time period, it continues to emit
this pulse every 10.1 sec.
When the fiber path is restored as shown in OFC Timing: Reconnect on page 6, the pulse from one of the
GLMs appears at the other GLM. The presence of light is detected and recognized and immediately an
optical pulse is returned. (The initiating GLM is referred to as the “master” and the responding GLM is referred
to as the “slave”.) This returned optical pulse (emitted by the slave) is received by the initiating (or master)
GLM. If this returned optical pulse is detected and recognized by the master during the Decode 1 time
interval, the master GLM proceeds to a second timing interval (or state), after the first timing interval expires.
During this second timing interval the master GLM waits to see the absence of light (“the lights go off”). This
second timing interval is equal to the Decode 2 timing. If the received optical signal goes off during this
Decode 2 interval, the master GLM proceeds to a third timing interval after the second interval expires. At the
start of this third timing interval, the master GLM energizes its laser where it expects to see the presence of
light within a period of time equal to the Decode 3 interval. If the slave GLM complies and energizes its laser
and it is received and recognized by the master GLM within the Decode 3 timing interval, the link becomes
Active. This “on-off-on” handshake is the basic link initiation sequence.
The responding and initiating (slave and master) GLM cards have nominally identical timing intervals (Tdcd1,
Tdcd2 and Tdcd3). Any GLM card in the Disconnect state functions as a master, sending out an optical pulse
every 10.1 sec. The width of this pulse is nominally equal to the Decode 1 time interval. If an optical pulse is
received outside this Decode 1 interval, the card immediately becomes a slave, responding with the “on-off-
on” handshake sequence.
glm1063.02.fm.02
Feb. 15, 2000
Page 5 of 30
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