78Q8392L/A03
Low Power Ethernet
Coaxial Transceiver
Page: 2 of 14 © 2008 Teridian Semiconductor Corporation Rev 1.3
FUNCTIONAL DESCRIPTION
The 78Q8392L/A03 incorporates six basic functions
of the Ethernet Transceiver, including receiving,
transmitting, collision signaling, collision detection,
jabber timing, and the heartbeat function. Refer to
Figure 1 for a general system block diagram.
RECEIVER FUNCTIONS
The receiver senses signals through the RXI input,
which minimizes reflections on the transmission
media using a low capacitance, high resistance
input buffer amplifier. The CDS ground input
attaches directly to the input buffer from the coaxial
shield to eliminate ground loop noise.
In addition to the input buffer, the receiver data path
consists of an equalizer, data slicer, receiver
squelch circuitry, and an output line driver.
The equalizer improves the cable-induced jitter; the
data slicer restores equalized received signals to
fast transition signals with binary levels to drive the
receiver line driver; and the receiver line driver
drives the AUI cable through an isolation transformer
that connects to the AUI interface.
Noise on the transmission media is rejected by the
receiver squelch circuitry, which determines valid
data via three criteria: Average DC level, pulse
width and transition period. The DC voltage level is
detected and compared to a set level in the receiver
comparator circuit. The pulse width must be greater
than 20 ns to pass the narrow pulse filter; the
transition timer outputs a true level on the RX Data
Valid line provided the time between transitions is
less than about 200 ns. As long as a valid RXI signal
is detected, the output line driver remains enabled.
The transition timer disables the line driver when
there are no further transitions on the data medium,
and the RX+, RX- pins go to a zero differential
voltage state (Figure 3).
TRANSMITTER FUNCTIONS
The transmitter data path consists of a transmit input
buffer, pulse-shaping filter, transmit squelch
circuitry and transmit output line driver. The
self-biasing transmit input buffer receives data
through an isolation transformer and translates the
AUI differential analog signal to a square pulse
suitable for driving the pulse shaping filter.
The filter outputs a correctly shaped and band
limited signal to the transmit output driver, which
drives the transmission medium through a high
impedance current source. When the transmitter is
off, the capacitance of the transmit driver is isolated
from the transmission media by an external diode
with a low capacitance node. The shield of the
transmission media serves as the ground return for
the transmitter function.
A transmit squelch circuit, which consists of a pulse
threshold detector, a pulse width detector, and a
pulse duration timer, is used to suppress noise, as
well as crosstalk on the AUI cable. The squelch
circuitry disables the transmit driver if the signal at
TX+ or TX- is smaller than the pulse threshold.
Pulse noise is rejected by a pulse width detector that
passes only pulses with durations greater than 20
ns. The pulse duration timer disables the transmit
driver if no pulses are received for two-bit periods
following valid pulses. At the end of a transmission,
the pulse duration timer disables the transmitter and
triggers the blanking timer, used to block “dribble”
bits.
COLLISION DETECTION
A collision occurs when two or more transmitters
simultaneously transmit on the transmission media.
A collision is detected by comparing the average DC
level of the transmission media to a collision
threshold. The received signal at RXI is buffered and
sent through a low pass filter, then compared in the
collision threshold circuit. If the average DC level
exceeds a collision threshold, a 10 MHz signal is
output on the CD± pins.
COLLISION SIGNALING
When collision signaling is enabled (HBE pin is high
and the average DC level on RXI exceeds the
collision threshold V
CD
), a 10 MHz signal is sent from
the CD± pins through an isolation transformer to the
DTE. When the function is disabled, this output goes
to a zero differential state. The 10 MHz signal output
from the CD± pins indicates a collision on the
transmission media, a heartbeat function, or that the
transmitter is in jabber mode.
