11
Digital Diagnostic Interface and Serial Identication
The 2-wire serial interface is explicitly dened in the
XFP MSA Rev 4.0. 2-wire timing specications and the
structure of the memory map are per XFP MSA Rev 2.0.
The normal 256 Byte I2C address space is divided into
lower and upper blocks of 128 Bytes. The lower block
of 128 Bytes is always directly available and is used for
diagnostic information providing the opportunity for
Predictive Failure Identification, Compliance Predic-
tion, Fault Isolation and Component Monitoring. The
upper address space tables are used for less frequently
accessed functions such as serial ID, user writeable EE-
PROM, reserved EEPROM and diagnostics and control
spaces for future standards denition, as well as Avago
Technologies-specic functions.
Predictive Failure Identication
The diagnostic information allows the host system to
identify potential link problems. Once identied, a “fail
over” technique can be used to isolate and replace sus-
pect devices before system uptime is impacted.
Compliance Prediction
The real-time diagnostic parameters can be monitored
to alert the system when operating limits are exceeded
and compliance cannot be ensured. As an example, the
real time average receive optical power can be used to
assess the compliance of the cable plant and remote
transmitter.
Fault Isolation
The diagnostic information can allow the host to pin-
point the location of a link problem and accelerate sys-
tem servicing and minimize downtime.
Component Monitoring
As part of host system qualication and verication,
real time transceiver diagnostic information can be
combined with system level monitoring to ensure per-
formance and operating environment are meeting ap-
plication requirements.
Transceiver Internal Temperature
Temperature is measured on the AFBR-720XPDZ us-
ing sensing circuitry mounted on the internal PCB.
The measured temperature will generally be cooler
than laser junction and warmer than XFP case and can
be indirectly correlated to XFP case or laser junction
temperature using thermal resistance and capacitance
modeling. This measurement can be used to observe
drifts in thermal operating point or to detect extreme
temperature uctuations such as a failure in the system
thermal control. For more information on correlating
internal temperature to case or laser junction contact
Avago Technologies.
Transmitter Laser DC Bias Current
Laser bias current is measured using sensing circuitry
located on the transmitter laser driver IC. Normal varia-
tions in laser bias current are expected to accommodate
the impact of changing transceiver temperature and
supply voltage operating points. The AFBR-720XPDZ
uses a closed loop laser bias feedback circuit to main-
tain constant optical power. This circuit compensates
for normal laser parametric variations in quantum ef-
ciency, forward voltage and lasing threshold due to
changing transceiver operating points.
Transmitted Average Optical Output Power
Variations in average optical power are not expected
under normal operation because the AFBR-720XPDZ
uses a closed loop laser bias feedback circuit to main-
tain constant optical power. This circuit compensates
for normal laser parametric variations due to changing
transceiver operating points. Only under extreme laser
bias conditions will signicant drifting in transmitted
average optical power be observable. Therefore it is rec-
ommended Tx average optical power be used for fault
isolation, rather than predictive failure purposes.
Auxilliary Monitors
Received average optical power measurements are a
valuable asset for installers to verify cable plant compli-
ance. Drifts in average power can be observed from the
cable plant and remote transmitter for potential predic-
tive failure use. Received average optical power can be
used for fault isolation.
Auxilliary Monitors
In addition to the parameters mentioned above, 3.3V
Supply Voltage (AUX1) is also reported as auxilliary pa-
rameter 1.
