83C694D Datasheet, PDF (10/33 Pages) SMSC Corporation – Twisted-Pair Interface and Manchester Encoder/Decoder

ARCHITECTURE
TPX2+
TPX1+
TPX1-
TPX2-
261Ω
65Ω
65Ω
261Ω
TPX+
2.4ΚΩ
TPX-
Transmit
Filter
Common Mode
Choke
Isolation
Transformer
100 nsec
50 nsec
83C694D
Twisted Pair
Cable
FIGURE 2-6. TWISTED-PAIR TRANSMIT PATH AND TIMING
The signal received from the unshielded cable can
be noisy, so minimum voltage and timing limits must
be met before the receiver logic is enabled. A
"smart squelch"^© digital noise filter is used in addi-
tion to the analog squelch circuit in the receiver.
The smart squelch circuit provides extra protection
against false collisions and false link connections.
If the input polarity is reversed, it will be automat-
ically detected and corrected. When this happens,
the TPOL output pin will go high to signal the
controller or to turn off the polarity indicator LED.
The phase-locked loop and Manchester decoder
are the same circuits used by the AUI receiver.
An external interface circuit for TPR+ and TPR-
might be designed like Figure 2-5.
2.7 LOOPBACK FUNCTION
When the loopback input goes high it causes the
83C694D to send serial data from the transmit data
input through the encoder, and back through the
phase-locked loop and decoder to the receive data
output. The transmit driver is in the idle state during
loopback mode and the receiver circuitry and colli-
sion detection are disabled. Loopback can be en-
abled during either AUI or TP (10BaseT) operation.
Transmit data is always looped back during TP
operation, simulating the physical broadcast char-
acteristic of 802.3 coaxial cable networks.
TP+
TP-
Twisted Pair
Cable
Common Mode
Choke
Receive
Filter
Isolation
Transformer
TPR+
100Ω 1%
TPR-
FIGURE 2-5. TWISTED-PAIR RECEIVE PATH
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