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AM79C98

Twisted-Pair Ethernet Transceiver (TPEX)

Advanced Micro Devices 

FUNCTIONAL DESCRIPTION

The Twisted-Pair Ethernet Transceiver (TPEX) com-

plies with the requirements specified by the IEEE 802.3

standard for the attachment unit interface (AUI) and the

standard for 10BASE-T medium attachment unit

(MAU). TPEX also implements a number of features in

addition to the IEEE 802.3 standard. An outline of func-

tions implemented by the Am79C98 is given below.

Attachment Unit Interface

(DO+/–, DI+/–, CI+/–)

The AUI electrical and functional characteristics com-

ply with those specified by the IEEE 802.3, Sections 7

and 14 (drafted). The AUI pins can be wired directly to

the isolation transformer, for a remote MAU application,

or to another device (e.g., Am7992 serial interface

adapter). The end-of-packet SQE Test function (heart-

beat) can be disabled to allow the device to be em-

ployed in a repeater application.

Twisted-Pair Transmit Function

Data transmission to the 10BASE-T medium occurs

when valid AUI signals appear on the DO+/–differential

pair. This data stream is routed to the differential driver

circuitry in the TXD+/– pins. The driver circuitry pro-

vides necessary electrical driving capability and pre-

distortion control for transmitting signals over

maximum-length twisted-pair cable, as specified by the

IEEE 802.3 10BASE-T standard. The transmit function

meets the propagation delays and jitter specified by the

standard. During transmission, the XMT pin is driven

HIGH and can be used for status information.

Twisted-Pair Receive Function

The receiver complies with the receiver specifications

of the IEEE 802.3 10BASE-T standard, including noise

immunity and received signal rejection criteria (“Smart

Squelch”). Signals meeting these criteria appearing at

the RXD+/– differential input pair are routed to the DI+/–

outputs. The receiver function meets the propagation

delays and jitter requirements specified by the stan-

dard. Receiver squelch level drops to approximately

half its threshold value after unsquelch to allow recep-

tion of minimum amplitude signals and to offset carrier

fade in the event of worst-case signal attenuation and

crosstalk noise conditions. During receive, the RCV pin

is driven HIGH and can be used for status information.

Link Test Function

The Link Test function is implemented as specified by

the IEEE 802.3 10BASE-T standard. During periods of

transmit pair inactivity, Link Test pulses will be periodi-

cally sent over the twisted-pair medium to allow con-

stant monitoring of medium integrity. When the Link

Test function is enabled, the absence of Link Test

pulses on the RXD+/– pair will cause the TPEX to go

into a Link Fail state. In Link Fail state, data transmission,

data reception, and the collision detection functions are

disabled, and remain disabled until valid data or >2

consecutive Link Test pulses appear on the RXD+/–

pair. During Link Fail, the LNKST pin is internally pulled

HIGH. When the link is identified as functional, the

LNKST pin is driven LOW and is capable of directly

driving a “link OK” LED. In order to interoperate with

systems that do not implement Link Test, this function

can be disabled by grounding the LNKST pin. When

disabled, the driver and receiver functions remain en-

abled irrespective of the presence or absence of data

or Link Test pulses on the RXD+/– pair. The transmitter

continues to generate Link Test pulses in the absence

of transmit data even if the Link Test function is

disabled.

Polarity Detection and Reversal

The TPEX receive function includes the ability to invert

the polarity of the signals appearing at the RXD± pair if

the polarity of the received signal is reversed (such as

in the case of a wiring error). This feature allows data

packets received from a reverse-wired RXD± input pair

to be corrected in the TPEX prior to transfer to the DTE

via the AUI interface (DI±). The polarity detection func-

tion is activated following reset or Link Fail, and will re-

verse the receive polarity based on both the polarity of

any previous Link Test pulses and the polarity of subse-

quent packets with a valid end transmit delimiter (ETD).

When in the Link Fail state, TPEX will recognize Link

Test pulses of either positive or negative polarity. Exit

from the Link Fail state is caused by the reception of

five to six consecutive Link Test pulses of identical po-

larity. On entry to the Link Pass state, the polarity of the

last five Link Test pulses is used to determine the initial

receive polarity configuration and the receiver is recon-

figured to subsequently recognize only Link Test pulses

of the previously established polarity. This link pulse

algorithm is employed only until ETD polarity determi-

nation is made, as described later in this section.

Positive Link Test pulses are defined as received sig-

nals with a positive amplitude greater than 520 mV and

a pulse width of 60 ns to 200 ns. This positive excursion

may be followed by a negative excursion. This definition

is consistent with the expected received signal at a cor-

rectly wired receiver when a Link Test pulse that fits the

template of Figure 14-12 in the 10BASE-T standard is

generated at a transmitter and passed through 100 m

of twisted-pair cable.

Negative Link Test pulses are defined as received sig-

nals with a negative amplitude greater than 520 mV

and a pulse width of 60 ns to 200 ns. This negative ex-

cursion may be followed by a positive excursion. This

definition is consistent with the expected received signal

at a reverse wired receiver when a Link Test pulse that

fits the template of Figure 14-12 in the 10BASE-T

8

Am79C98

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