83C694D Datasheet, PDF (9/33 Pages) SMSC Corporation – Twisted-Pair Interface and Manchester Encoder/Decoder

83C694D
+12V (circuit board)
510Ω (1/4 watt)
ARCHITECTURE
anode
cathode
5.1V zener
ground
(circuit board)
0.1 µ F
10ΚΩ
31.6ΚΩ
BSR
OSR
VCC
VCC
(pin 32) (pin 35)
83C694C
FIGURE 2-4. ZENER DIODE VOLTAGE REGULATION FOR 12 VOLTS
It is also helpful to place a decoupling capacitor
between the diode’s cathode and ground as shown
in Figure 2-4.
2.4 COLLISION TRANSLATOR
When the 83C694D is used as an AUI device, a
separate Ethernet transceiver detects collisions on
the coaxial cable and generates a 10 MHz signal,
which is monitored by the 83C694D through the
collision detect pins. The presence of the signal
activates the collision detect (CD) pin connected to
the 83C690 causing the controller to stop transmit-
ting. The collision detect output is deactivated
within 160 nsec. after the absence of the 10 MHz
signal. Figure 5-7 illustrates the collision timing. An
external interface circuit for CD+ and CD- is de-
signed exactly like an external interface for RX+
and RX-. See Figure 2-3.
2.5 TP DIFFERENTIAL DRIVER
The TP driver can transmit through up to 100m of
unshielded twisted-pair (UTP) cable. The driver
includes a circuit for transmit equalization, which
attenuates low frequency components of the trans-
mit waveform. This reduces the zero crossing jitter
of the received signal and avoids the use of a
receive equalizer.
There are two pulse widths transmitted: 50 nsec
and 100 nsec. When a pulse width of 100 nsec is
sent, both drivers (TPX1+ and TPX2+) turn on and
drive a high level. This provides a greater amplitude
at the start of the pulse; however, halfway through
the pulse TPX2 turns off, thereby reducing the
amplitude after 50 ns. A narrow pulse is transmitted
at the same amplitude as the first half of the wide
pulses. The resistor ratio is calculated to produce
the best signal wave shape at the receiving end
assuming a UTP cable length of 100 meters.
Figure 2-6 shows the basic twisted-pair transmit
path along with its timing and one possible external
transmit interface design. Typical values for resis-
tance on TPX2 pins are 261Ω, while TPX1 pins use
65Ω. The 2.4KΩ parallel resistor is used to match
the output resistance of the transmitter to the
twisted-pair cable.
At the receive end of the cable, a 100Ω termination
resistor is commonly used. To verify the operation
of the circuit, measure the TPX signals differentially.
In designing the external circuits to connect the
83C694D transmit outputs to the cable, use a trans-
mit filter followed by an isolation transformer and, in
the most practical applications, a common mode
choke for FCC compliance. The common mode
choke may not always be needed in every applica-
tion; however, the isolation transformer is always
needed and the transmit filter is strongly recom-
mended; without it, high frequency radiation may
exceed FCC limits.
2.6 TP DIFFERENTIAL RECEIVER
6