
Draft 6/5/00
FX Interface
A-11
Copyright 2000 by LSI Logic Corporation. All rights reserved.
A.8 FX Interface
The FX interface has differential PECL inputs (FXI+/
) and outputs
(FXO+/
) that are typically connected to an external fiber optic
transceiver. The FX interface outputs are designed to drive 100-ohm
differential loads. The FX interface can be directly coupled to either
3.3 V or 5 V fiber optic transceivers with minimum external components,
as described in the following sections.
A.8.1 Connection to 3.3 V Transceivers
The schematic for a typical connection of the device to an external 3.3 V
fiber optic transceiver is shown in
Figure A.4
.
The 70- and 173-ohm resistors on FXO+/
are used for 100 ohm
termination and for biasing the device FXO+/
outputs to match the input
range on the 3.3 V fiber optic transceiver inputs. Place these resistors as
close as possible on the PCB to the fiber optic transceiver inputs. In
addition, keep the parasitic loading on FXO+ and FXO
to a minimum
and matched as well as possible.
The 127 and 83 ohm resistors on FXI+/
and SD/FXDISn are used for
100-ohm termination and for biasing the 3.3 V fiber optic transceiver
outputs to match the input range on the device FX inputs. Place these
resistors as close as possible on the PCB to the device FX inputs. In
addition, keep the parasitic loading on FXI+ and FXI
to a minimum and
matched as well as possible.
In the 3.3 V transceiver application, the SD_THR pin must be tied to
GND, as shown in
Figure A.4
. The SD_THR pin determines the
SD/FXDISn internal buffer ECL trip level. The SD/FXDISn internal buffer
trip level needs to be set to VDD
1.3 V and must be referenced to the
3.3 V supply. Connecting SD_THR to GND causes the device to
internally set the ECL trip point on the SD/FXDISn input to V
DD
1.3 and
references it to the common 3.3 V supply.