NE8392C NXP Semiconductors, NE8392C Datasheet - Page 6

• Manufacturer Part Number: NE8392C
• Description: Coaxial Transceiver Interface For Ethernet/thin Ethernet
• Manufacturer: NXP Semiconductors

Philips Semiconductors
The equalizer is a high pass filter that compensates for the low pass
effect of the coaxial cable and results in a flatband response over all
signal frequencies to minimize signal distortion.
The 4-pole Bessel low pass filter extracts the average DC voltage
level on the coaxial cable for use by the receiver squelch and
collision detection circuits.
The receiver squelch circuit prevents noise on the coaxial cable
from falsely triggering the receiver in the absence of a true signal.
At the beginning of a packet, the receiver turns on when the DC
level from the low pass filter is lower than the DC squelch threshold.
For normal signal levels this will take less than 500ns, or 5 bits.
However, at the end of a packet, a fast receiver turn off is needed to
reject both dribble bits on the coaxial cable and spurious responses
due to settling of the on-chip bandpass filter. This is accomplished
by an AC timing circuit that disables the receiver if the signal level
on the coaxial cable remains high for typically 250ns and only
enables the receiver again after approximately 1 s. Figures 3 and 4
illustrate receiver timing.
The differential line driver provides typically 900mV signals to the
DTE with less than 7ns rise and fall times. When in idle state (no
received signal) its outputs provide <20mV differential voltage offset
to minimize DC standing current in the isolation transformer. The
line driver outputs are emitter followers and, for Ethernet
applications where they drive a 78
500 pull-down resistor to V
where the AUI cable is not used, the pull-down resistor can be
increased to 1.5kΩ to save power consumption.
Transmitter Functions
The transmitter has differential inputs and an open collector current
driver output. The differential input common mode voltage is
established by the CTI and should not be altered by external
circuitry. Controlled rise and fall times of 25ns (+5ns) minimize
higher harmonic components in the transmitted spectrum, while
matching of these rise and fall times to typically 2ns minimizes
signal jitter. The drive current levels of the CTI are set by an on-chip
bandgap voltage reference and an external 1% resistor. An on-chip
isolation diode is provided to reduce the transmitter’s coaxial cable
load capacitance. For Thin Ethernet applications, no further external
isolation diode is required, since the NE8392C meets the capacitive
loading specifications. For Ethernet applications a further external
diode should be added to reduce loading capacitance.
The transmitter squelch circuit ensures that the transmitter can only
be enabled by negative-going differential signals of typically greater
than 225mV in magnitude and 15ns in duration. The transmitter will
be disabled at the end of a packet if there are no negative going
signals of greater than 225mV for more than typically 250ns. Figure
5 illustrates transmitter timing.
Collision Functions
The collision detection scheme implemented in the NE8392C is
receive mode detection, which detects a collision between any two
stations on the network with certainty at all times, irrespective of
whether or not the local DTE is producing one of the colliding
1995 May 1
Coaxial transceiver interface for Ethernet/Thin Ethernet
EE
. For Thin Ethernet applications
transmission line, require a
6
signals. This is the only detection scheme allowed by the IEEE
802.3 standard for both repeater and non-repeater nodes.
The collision circuitry consists of the 4-pole Bessel low pass filter, a
comparator, a precision voltage reference that sets up the collision
threshold, a heartbeat generator, a 10MHz oscillator, and a
differential line driver.
The collision comparator monitors the DC level at the output of the
low pass filter and enables the line driver if it is more negative than
the collision threshold. A collision condition is indicated to the DTE
by a 10MHz oscillation signal at the CD outputs and typically occurs
within 700ns of the onset of the collision. The collision signal begins
with a negative-going pulse and ends with a continuous high-to-idle
state longer than 170ns. Figure 6 illustrates collision timing.
At the end of every transmission, the heartbeat generator creates a
pseudo collision to ensure that the collision circuitry is properly
functioning. This pseudo collision consists of a 1µs burst of 10MHz
oscillation at the line driver outputs approximately 1µs after the end
of the transmission. The heartbeat function can be disabled
externally by connecting the HBE (heartbeat enable) to V
allows the CTI to be used in repeater applications. Figure 7
illustrates heartbeat timing.
As with the receiver outputs, the collision outputs also require a pull
down resistor to V
in the idle state to minimize DC standing current in the isolation
transformers.
Jabber Functions
The jabber timer monitors the transmitter and inhibits transmission if
it is active for longer than typically 30ms. The jabber circuit then
enables the collision outputs for the remainder of the data packet
and for typically 450ns (unjab time) after it has ended. At this point
the transmitter becomes uninhibited. Figure 8 illustrates jabber
timing.
Detection of Coaxial Cable Faults
In the NE8392C there is no internal loopback path from the TX
inputs to the RX outputs. This means that, when the local DTE is
transmitting, the signal will only be present at the receiver outputs
RX+ and RX– if it appears on the coaxial cable and is larger than
the receiver squelch threshold V
occurs at the cable connector to the CTI, then no signal will appear
at the receiver outputs. An intelligent DTE can, therefore, detect this
fault. If the fault is an open circuit, then a continuous collision signal
will be sent to the DTE, provided the average DC voltage at the RXI
pin is greater than the typical collision threshold of –1.53V.
If a short or open circuit occurs elsewhere on the coaxial cable, the
resulting reflections can result in an impedance at the CTI of any
value between a short circuit and 50Ω, depending on the distance of
the CTI from the fault. The upper limit of 50Ω results from the fact
that the coaxial cable is terminated in 50Ω at both ends. Faults on
the cable itself are, therefore, not guaranteed to be detected by
simply monitoring the RX and CD pins when in the transmit mode,
and more sophisticated schemes may be necessary.
EE
and maintain <20mV differential voltage offset
RS
. If a short circuit fault condition
Product specification
NE8392C
EE
. This