ACS101A Semtech Corporation, ACS101A Datasheet - Page 4

• Manufacturer Part Number: ACS101A
• Description: Acapella Optical Modem ic
• Manufacturer: Semtech Corporation

In remote loopback mode the near-end modem sends a request to
the far-end modem to loopback its received data, thus returning
the data. The far-end modem also outputs the received data at its
RxD. Both modems are exercised completely, as well as the
LEDs and the fiber optic link.
established, the DCDB of the near-end (initiating) modem is Low,
and DCDB on the far-end modem is set High. Any data appearing
on the TxD input of the far-end modem is ignored.
When RSS = 0, RTS and DTR are looped back to CTS and DSR
respectively.
Communicating modems attain a stable state where the 'transmit
window' of one modem coincides with the 'receive window' of the
other allowing for delay through the optical link. Adjustments to
machine cycles are made automatically during operation to
compensate for differences in crystal frequencies which cause
loss of synchronisation.
Using drift lock, synchronisation described above depends on a
difference in the crystal frequencies at each end of the link, the
greater the difference the faster the locking. Therefore, if the
difference between crystal frequencies is very small (a few ppm),
automatic locking may take tens of seconds.
Active lock mode may be used to accelerate synchronisation of a
pair of communicating modems.
modems with less than 250 ms delay, by adjusting the machine
cycles of the modem. Active lock reduces the machine cycle of
the device by 0.5 % ensuring rapid lock. After synchronisation the
machine cycle reverts to normal.
Note that only one device can be configured in active lock at any
one time, and thus the DM pins must not be permanently wired
High on both devices in a production system. Active lock mode is
usually invoked on power-up.
One common way of temporarily invoking active lock is to adopt
the standard RC time-constant method. This is achieved on the
ACS101A by connecting DM1, DM2 and DM3 together, and
attaching that node to an RC arrangement, i.e. with the capacitor
to 5V and the resistor to ground. This creates a 5V
power-up. The RC time-constant should be Ca. 1 second.
This is a new mode of operation (over the ACS100), both ends of a
link can be permanently configured in this mode (i.e. with hard-
wired DM1-DM3 pins), which will achieve lock in typically 1
second, and worst case 5 seconds.
Like active lock, random lock will operate even when both ends of
the link are driven by identical clock frequencies (0ppm
difference).
available on the ACS100.
It is possible to mix all combinations of locking modes once the
modems are locked, however, prior to synchronisation two
modems configured in active lock will not operate.
ACS101A Issue 1.2 October 1999.
Drift Lock
Active Lock Mode
Random Lock
Remote Loopback
Mixing Lock modes
Random lock mode is compatible with drift lock
This mode synchronises the
Once remote loopback is
0V ramp on
Normally,
4
random lock will be the preferred mode. The effect of mixing
locking modes on locking speed is tabulated below:
Device A
Mode
Drift
Drift
Drift
Active
Active
Random
LIN goes High when synchronisation or “lock” is achieved. Lock is
normally an invert of DCDB. But unlike DCDB is not affected by
the status of RTS and DTR, or the selected diagnostic mode.
This signal can be used to give an indication of the quality of the
optical link. Even when a DC signal is applied to the TxD and
TxCL inputs, the ACS101A transmits approximately 256 kbps over
the link in each direction. This control data is used to maintain the
timing and the relative positioning of transmit and receive
windows. The transmit data and the control data is constantly
monitored to make sure it is compatible with the 3B4B coding
rules. If an infringement of the rules is detected then ERD will go
High and will remain High until reset. ERD may be initialised by
applying reset (DM1-DM3) or PORB, or by removing the fiber-optic
cable from one side of the link thereby forcing the device
temporarily out of lock. ERD is only an indication and should not
be considered as a substitute for Bit Error Rate (BER) tests.
Since LEDs from different suppliers may emit different
wavelengths, it is recommended that the LEDs in a
communicating pair of modems are obtained from the same
supplier.
temperature, so a temperature difference between the ends of a
link reduces the responsivity of the receiving diode. This results in
a reduction in the link budget. Information is given in the LED
suppliers’ data sheets. The following manufacturers have LEDs
that have been successfully tested with the ACS101A, and
Acapella will be glad to assist with contact names and addresses
on request:
MITEL
Acapella
Optek Technology
Most suppliers support the standard range of fiber connectors ,
e.g. ST, SMA & FC.
The ACS101A contains a highly sensitive amplifier, capable of
responding to extremely low current levels.
sensitivity it is important to reduce external noise to a low level
compared to the input signal from the LED. The modem should
have an independent power trace to the point where power enters
the board.
LIN (Lock Indicator)
ERD (Error Detector)
LED Considerations
Power Supply Decoupling
Suppliers
The emission spectrum of an LED is a function of
Device B
Mode
Drift
Active
Random
Active
Random
Random
(e.g. A- connector )
(e.g. OPF372, OPF322)
(e.g. 1A-212- connector )
Locking Speed
Drift
Active
Random
Not allowed
Random
Random
To exploit this