Leds; 10Base-T Interface; Aui Interface; 10Base-2 Interface - Cirrus Logic CRYSTAL LAN CS890 Technical Reference Manual

2.2.2.6 LEDs

A pair of LEDs are provided in the refer-
ence design to indicate link OK and line
active status. The pair of LEDs are pack-
aged one on the top of the other at location
LED1. The top LED is driven by the LIN-
KLED* pin while the bottom LED is driven
by the LANLED* pin of the CS8900. The
top LED lights up when the CS8900 has
the link pulse. The bottom LED lights up
when the CS8900 transmits or receives a
packet or senses a collision. The LEDs are
directly driven by the CS8900. Two 680
ohm resistors limit the current flowing
through the LED circuitry.

2.2.2.7 10BASE-T Interface

The 10BASE-T interface for the CS8900 is
straight forward. Please refer to Figure
2.2.5 for connections and components of
this circuit. Transmit and receive signal
lines from the CS8900 are connected to an
isolation transformer at location T3. This
isolation transformer has a 1:1 ratio be-
tween the primary and the secondary
windings on the receive side and 1:√2
(1:1.41) ratio between the primary and sec-
ondary windings for the transmit lines. Re-
sistor R2 provides termination for the re-
ceive lines. Resistors R4 and R5 are in se-
ries with the differential pair of transmit
lines for impedance matching.

2.2.2.8 AUI Interface

Please refer to Figure 2.2.6 for connection
of AUI signals to the CS8900. The AUI
lines from the 15-pin sub-D connector
(location J2) are connected to the CS8900
through an isolation transformer at T2.
18
CS8900 Technical Reference Manual
This isolation transformer has three wind-
ings for three pairs of differential AUI sig-
nals: transmit, receive and collision. All
three windings have a turns ratio of 1:1
between the primary and secondary wind-
ings. Circuitry consisting of R6, R7 and
C14 provides impedance termination for
the collision differential pair.
consisting of R8, R9 and C15 provides im-
pedance termination for the receive differ-
ential pair. The +12 volt power going out
to the AUI connector is safeguarded by the
fuse at F1. The AUI interface at J2 can be
used to connect external Media Access
Units (MAU). These MAUs allow the AUI
interfaced to be used to interface with
10BASE-5 or 10BASE-F.

2.2.2.9 10BASE-2 Interface

A Crystal Semiconductor 10BASE-2 trans-
ceiver IC, the CS83C92C, is used to gen-
erate a 10BASE-2 interface for the refer-
ence design. Please refer to Figure 2.2.7
for details about the components and con-
nection.
A 12 volt to -9 volt DC to DC voltage con-
verter (location U5) is used to generate an
isolated -9 volt supply for the CS83C92C.
The DC-DC converter used in the reference
design has an enable pin. This enable pin
is connected to the HC1 pin of the CS8900.
Usually the DC-DC converter is disabled
when the 10BASE-2 interface is not used.
This not only reduces power used by the
adapter card but also eliminates any noise
the 10BASE-2 circuitry can induce on the
10BASE-T or AUI interface that may be in
use. This reference design uses a "low"
enable DC-DC converter. That is, the DC-
DC converter is enabled when the enable
pin is logic low. However, the board can
Circuitry
AN83REV1