synchronisation the machine cycle reverts detected then ERD will go High and will remain High
automatically to normal.
until ERD is initialised. 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 is not a substitute for BER
tests.
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 ACS103 by connecting DM1, DM2 and DM3
together, and attaching that node to an RC
arrangement, i.e. with the capacitor to 5 V and the
resistor to ground. This creates a 5 V à 0 V ramp on
power-up. The RC time-constant should be Ca. 1
second.
LED Considerations
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. The emission
spectrum of an LED is a function of 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 ACS103 and Acapella
will be glad to assist with contact names and
addresses on request.
Random Lock
2
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 Suppliers:
both ends of the link are driven by identical clock
ABB-Hafo (e.g. 1A-212-connector)
frequencies (0 ppm).
Random lock mode is
Acapella
GCA
(e.g. A-connector)
(e.g. 1A-212-connector)
compatible with drift lock available on the ACS100.
Honeywell (e.g. HFE4214-013, HFE4404-013)
Optek (e.g. OPF372, OPF322)
Mixing Lock modes
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. Normally, random lock will be
the preferred mode. The effect of mixing locking
modes on locking speed is tabulated below:
Most suppliers support the standard range of fiber
connectors, e.g. ST, SMA and FC.
Power Supply Decoupling
The ACS103 contains a highly sensitive amplifier,
capable of responding to extremely low current
levels. To exploit this 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.
Device A Device B Locking Speed
Mode
Mode
Drift
Drift
Drift
Active
Random
Not allowed
Random
Random
Drift
Active
Drift
Random
Active
Random
Random
Active
Active
Random
Figure 3. shows the recommended power supply
decoupling. The LED should be sited very close to
the LDN and LDP pins. A generous ground plane
should be provided, especially around the sensitive
LDN and LDP pins. The modem should be protected
from EMI/RFI sources in the standard ways.
LIN (Lock Indicator)
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.
Link Budgets
The link budget is the difference between the power
coupled to the fiber via the transmit LED and the
power required to realise a current of 650 nA
(minimum amplifier sensitivity) via the receive
LED.The link budget is normally specified in dB or
dBm, and represents the maximum attenuation
allowed between communicating LEDs. The budget
is utilised in terms of the cable length, cable
connectors and splices. It usually includes an
operating margin to allow for degradation in LED
performance.
ERD (Error Detector)
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 ACS103
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 format. If an error is
ACS103 Issue 2.03 May 1996.
5
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