W89C926 Datasheet, PDF (27/48 Pages) Winbond – PCMCIA ETHERNET NETWORK TWISTED PAIR INTERFACE CONTROLLER

W89C926 PENTIC+
interface for Cheapernet. The SNA contains three blocks: a phase locked loop (PLL), a Manchester
encoder/decoder, and a collision decoder as well as crystal/oscillator logic.
TP or
Coax
AUI
Interface
PLL
Osc/
Crystal
Transmit
Logic
L
C
E
Receive
Logic
The Manchester encoder/decoder handles code interpretation between NRZ signals and Manchester
coded signals. The PLL locks the receiving signals with an internal voltage control oscillator (VCO) so
that network noise can be eliminated before the signals enter the core coprocessor. The collision de-
coder detects whether a collision has occurred on the network. The oscillator logic supplies the
PENTIC+ with the required 20 MHz clock. This clock also supplies the SNA clocking system.
TWISTED PAIR INTERFACE MODULE FUNCTION
Transmit Driver
There are two signals for data transmission: the true and complement Manchester differential data
(TXO+/-). These two signals are resistively combined to form a pre-equalized differential pair, which
is then passed to the twisted-pair cable via a transmitter filter and an optional common mode choke.
Smart Squelch
The main function of this block is to determine when valid data are present on the differential
receiving inputs (RXI+/-). To ensure that impulse noise on the medium will not be taken to be valid
data, this circuit adopts a combination of amplitude and timing measurements to determine the
validity of the input signals. To qualify incoming data, the smart squelch circuitry monitors the signals
for three peaks of alternating polarity that occur within a 400 nS window. Once this condition has been
satisfied, the squelch level is reduced to minimize the noise effect and the chances of causing
premature Start Of Idle (SOI) pulse detection. If the receiver detects activity on the receive line while
packets are being transmitted, incoming data are qualified on five peaks of alternating polarity so as
to prevent false collisions caused by impulse noise. The squelch function returns to its squelch state
under any of the following conditions:
• A normal SOI signal
• An inverted SOI signal
• A missing SOI signal
A missing SOI signal is assumed when no transitions have occurred on the receiver for 175 nS after a
packet has arrived. In this case, a normal SOI signal is generated and appended to the data.
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Publication Release Date: January 1996
Revision A1