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AM79C100

Twisted-Pair Ethernet Transceiver (TPEX)

Advanced Micro Devices 

FUNCTIONAL DESCRIPTION

The Twisted-Pair Ethernet Transceiver Plus (TPEX

Plus) complies with the requirements specified by the

IEEE 802.3 standard for the attachment unit interface

(AUI) and the 10BASE-T standard for a twisted-pair

medium attachment unit (MAU). TPEX Plus also imple-

ments a number of features in addition to the IEEE

802.3 standard. An outline of the functions of the

Am79C100 is given below.

Attachment Unit Interface (DO±, DI±, CI±)

The AUI electrical and functional characteristics com-

ply with those specified within the IEEE 802.3 docu-

ments, Sections 7 and 14. The AUI pins can be wired

to an isolation transformer, for a remote MAU applica-

tion, or directly to another device (e.g., Am7992B serial

interface adapter), in the case of a local DTE applica-

tion. The end-of-packet SQE Test function (heartbeat)

can be disabled to allow the device to be employed 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± and TXP± pins. The driver circuitry

provides the necessary electrical driving capability and

the predistortion control for transmitting signals over

maximum length twisted-pair cable, as specified by the

IEEE 802.3 10BASE-T standard. During transmission,

data is looped back to the DI± differential circuit, indi-

cating normal operation. The transmit function for data

output and loopback operations meets the propagation

delays and jitter specified by the standard. During nor-

mal transmission, and providing that TPEX Plus is not

in a Link Fail or Jabber state, the XMT pin will be driven

LOW, and can be used to drive a status LED directly.

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. The receiver squelch level drops to approximately

half its threshold value after unsquelch to allow recep-

tion of minimum amplitude signals and to mitigate car-

rier fade in the event of worst-case signal attenuation

and crosstalk noise conditions. During receive, the

RCV pin is driven LOW and can be used to drive a sta-

tus LED directly.

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 speci-

fied. The TPEX Plus receiver squelch levels are defined

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

typical for the type of receive filters/transformers rec-

ommended (see also Table 1).

Normal 10BASE-T-compatible receive thresholds are

employed when the LRT pin is inactive (HIGH). When

the LRT pin is externally pulled LOW, the Low Receive

Threshold option is invoked, and the sensitivity of the

TPEX Plus receiver is increased. This allows longer

line lengths to be employed, exceeding the 100 m tar-

get distance of normal 10BASE-T (assuming typical 24

AWG cable). The additional cable distance contributes

directly to increased signal attenuation and reduced

signal amplitude at the TPEX Plus receiver. However,

from a system perspective, making the receiver more

sensitive means that it is also more susceptible to

extraneous noise, primarily caused by coupling from

co-resident services (crosstalk). For this reason, it is

recommended that when using the Low Receive

Threshold option, the service should be installed on

4-pair cable only. Multipair cables within the same outer

sheath have lower crosstalk attenuation, may allow

noise emitted from adjacent pairs to couple into the re-

ceive pair, and be of sufficient amplitude to falsely un-

squelch the TPEX Plus.

Link Test Function

The Link Test function is implemented as specified by

the 10BASE-T standard. During periods of transmit

pair inactivity, “link beat” pulses will be sent periodically

over the twisted-pair medium to allow constant monitor-

ing of medium integrity.

When the Link Test function is enabled, the absence of

link beat pulses and receive data on the RXD± pair will

cause the TPEX Plus to go into a Link Fail state. In the

Link Fail state, data transmission, data reception, data

loopback, and collision detection functions are disabled

and remain disabled until valid data or >5 consecutive

link 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. With Link Test disabled, the

data driver, receiver, and loopback functions, as well as

collision detection, remain enabled irrespective of the

presence or absence of data or link pulses on the

RXD± pair.

Polarity Detection and Reversal

The TPEX Plus 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 al-

lows data packets received from a reverse-wired RXD±

input pair to be corrected in the TPEX Plus prior to

transfer to the DTE via the AUI interface (DI±). The

Am79C100

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