AM79C970A datasheet(85/219 Pages) AMD

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P R E L I M I N A R Y

AMD

Am79C970A

PCnet-PCI II

DI+

DI-

40.2

Ω

40.2

Ω

0.01

µF

0.1

µF

AUI Isolation

Transformer

19436A-40

Figure 37. AUI Differential Input Termination

Collision Detection

A MAU detects the collision condition on the network

and generates a 10 MHz differential signal at the

± inputs. This collision signal passes through an input

stage which detects signal levels and pulse duration.

When the signal is detected by the MENDEC it sets the

ICLSN line HIGH. The condition continues for approxi-

mately 1.5 bit times after the last LOW-to-HIGH transi-

tion on CI

±.

Twisted-pair Transceiver

This section describes operation of the Twisted Pair

Transceiver (T-MAU) when operating in half-duplex

mode.

When

half-duplex

mode,

the

T-MAU

implements the Medium Attachment Unit (MAU) func-

tions for the Twisted Pair Medium as specified by the

supplement to IEEE 802.3 standard (Type 10BASE-T).

When operating in full-duplex mode, the MAC engine

behavior

changes

described

the

section

“Full-Duplex Operation”.

The T-MAU provides twisted pair driver and receiver cir-

cuits, including on-board transmit digital predistortion

and receiver squelch and a number of additional fea-

tures including Link Status indication, Automatic

Twisted Pair Receive Polarity Detection/Correction and

Indication, Receive Carrier Sense, Transmit Active and

Collision Present indication.

Twisted Pair Transmit Function

The differential driver circuitry in the TXD

± and TXP±

pins provides the necessary electrical driving capability

and the pre-distortion control for transmitting signals

over maximum length Twisted Pair cable, as specified

by the 10BASE-T supplement to the ISO 8802-3

(IEEE/ANSI 802.3) Standard. The transmit function for

data output meets the propagation delays and jitter

specified by the standard.

Twisted Pair Receive Function

The receiver complies with the receiver specifications of

the ISO 8802-3 (IEEE/ANSI 802.3) 10BASE-T Stan-

dard, including noise immunity and received signal re-

jection criteria (“Smart Squelch”). Signals meeting

these criteria appearing at the RXD

± differential input

pair are routed to the MENDEC. The receiver function

meets the propagation delays and jitter requirements

specified by the standard. The receiver squelch level

drops to half its threshold value after unsquelch to allow

reception of minimum amplitude signals and to offset

carrier fade in the event of worst case signal attenuation

and crosstalk noise conditions.

Note that the 10BASE-T Standard defines the receive

input amplitude at the external Media Dependent Inter-

face (MDI). Filter and transformer loss are not specified.

The T-MAU receiver squelch levels are defined to

account for a 1 dB insertion loss at 10 MHz, which is typi-

cal for the type of receive filters/transformers employed.

Normal 10BASE-T compatible receive thresholds are

employed when the LRT bit (CSR15, bit 9) is cleared to

ZERO. When the LRT bit is set to ONE, the Low Receive

Threshold option is invoked, and the sensitivity of the

T-MAU receiver is increased. This allows longer line

lengths to be employed, exceeding the 100 m target dis-

tance of normal 10BASE-T (assuming typical 24 AWG

cable). The increased receiver sensitivity compensates

for the increased signal attenuation caused by the addi-

tional cable distance.

However, making the receiver more sensitive means

that it is also more susceptible to extraneous noise, pri-

marily caused by coupling from co-resident services

(crosstalk). For this reason, it is recommended that

when using the Low Receive Threshold option that the

service should be installed on 4-pair cable only.

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