FINAL

Publication# 07506 Rev: E Amendment/0
Issue Date: May 1994

Am7996

IEEE 802.3/Ethernet/Cheapernet Transceiver

DISTINCTIVE CHARACTERISTICS

Compatible with Ethernet Version 2 and
IEEE 802.3 10BASE-5 and10BASE-2
specifications

Pin-selectable SQE Test (heartbeat) option

Internal jabber controller prevents excessive
transmission time

Noise rejection filter ensures that only valid data
is transmitted onto the network

Collision detection on both transmit and receive
data

Collision detect threshold levels adjustable for
other networking applications

GENERAL DESCRIPTION

The Am7996 IEEE 802.3/Ethernet/Cheapernet Trans-
ceiver suppor ts Ethernet Version 2, IEEE 802.3
10BASE-5 and IEEE 802.3 10BASE-2--Cheapernet)
transceiver applications. Transmit, receive, and colli-
sion detect functions at the coaxial media interface to
the data terminal equipment (DTE) are all performed by
this single device.

In an IEEE 802.3 (10BASE-5)/Ethernet application, the
Am7996 interfaces the coaxial (0.4

diameter) media to the

DTE through an isolating pulse transformer and the 78

attachment unit interface (AUI) cable. In IEEE 802.3
10BASE-2--Cheapernet applications, the Am7996 typi-
cally resides inside the DTE with its signals to the DTE iso-
lated and the coaxial (0.2

diameter) media directly

connected to the DTE. Transceiver power and ground in
both applications are isolated from that of the DTE.

The Am7996's tap driver provides controlled skew and
current drive for data signaling onto the media. The jabber
controller prevents the node from transmitting exces-
sively. While transmitting, collisions on the media are de-
tected if one or more additional stations are transmitting.

The Am7996 features an optional SQE Test function
that provides a signal on the Cl pair at the end of every
transmission. The SQE Test indicates the operational
status of the Cl pair to the DTE. It can also serve as an
acknowledgment to the node that packet transmission
onto the coax was completed.

BLOCK DIAGRAM

Line

Driver

Line

Driver

Transmit

Data

Amplifier

Control

Logic

Carrier

Detect
Circuit

Receive

Data

Amplifier

Collision

Detect
Circuit

Jabber

Timer

SQE Test

Generator

Transmit

Squelch

Input

Buffer

SQE TEST

AUI Interface

DO+

Control

Logic

Tap

Driver

TXT

Coaxial Media Interface

Input

Buffer

CI+

DI+

RXT

07506E-1

AMD

Am7996

RELATED PRODUCTS

Part No. Description

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Am79C940 Media Access Controller for Ethernet (MACE

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Am79C900 Integrated Local Area Communications Controller

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Am79C960 PCnet-ISA Single-Chip Ethernet Controller (for ISA bus)

Am79C961 PCnet-ISA

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CONNECTION DIAGRAMS

07506E-2

07506E-3

Notes:
Pin 1 is marked for orientation.
NC = No Connection

DIP PLCC

CC1

CI+

DI+

REF

SQE^TEST

DO+

CC2

COLL OSC

COL

RXT

TAP SHIELD

TX+

TXT

DI+

REF

SQE^TEST

DO+

TX+

TXT

TAP SHIELD

RXT

COL

COLL OSC

CI+

CC1

CC2

2 1 20 19

9 10 11 12 13

4 Am7996

PIN DESCRIPTION

Attachment Unit Interface (AUI)

Dl+, Dl

Receive Line Output (Differential Outputs)

This pair is intended to operate into terminated 78

transmission lines. Signals at RXT meeting bandwidth
requirements and carrier sense levels are outputted at
D l

. S i g n a l i n g a t D l

m e e t s r e q u i r e m e n t s o f

IEEE 802.3, Rev. D.

Cl+, Cl

Collision Line Output (Differential Outputs)

This pair is intended to operate into terminated 78

transmission lines. Signal Quality Error (SQE), de-
tected at DO

inputs (excessive transmissions) or RXT

input (during a collision), outputs the 10 MHz internal
oscillator signal to the AUI interface. For proper compo-
nent values at COLL OSC, signaling at Cl

meets re-

quirements of IEEE 802.3, Rev. D.

DO+, DO

Transmit Input (Differential Inputs)

A pair of internally biased line receivers consisting of a
squelch detect receiver with offset and noise filtering
and a data receiver with zero offset for data signal pro-
cessing. Signals meeting squelch requirements are
waveshaped and output at TXT.

Coaxial Media Interface (TAP)

RXT

Media Signal Receiver Input (Input)

RXT connects to the media through a 4:1 attenuator of
100 k

total resistance (25 k

and 75 k

in series).

Return for the attenuator is V

COL

. RXT is an analog

input with internal AC coupling for Manchester data sig-
nals and direct coupling for Carrier Detect and SQE av-
erage level detection. Signals at RXT meeting carrier
squelch enable data to the Dl

outputs. Data signals

are AC coupled to Dl

with a 150 ns time constant,

high-pass filter. Signals meeting SQE levels enable
COLL OSC frequency to Cl

outputs.

TXT

Tap Node Driver (Input/Output)

A controlled bandwidth current source and sense am-
plifier. This l/O port is to be connected to the media
through an isolation network and a low-pass filter. Sig-
nals meeting DO

squelch and jabber timing require-

ments are output at TXT as a controlled rise and fall
time current pulse. When operated into a double termi-
nated 50

transmission line, signaling meets

IEEE 802.3, Rev. D recommendations for amplitude,
pulse-width distortion, rise and fall times, and harmonic
content. The sense amplifier monitors TXT faults and
inhibits transmission.

Global Signals

REF

Timing Reference Set (Input)

REF

is a compensated voltage reference input with

respect to V

. When a resistor is connected between

REF

and V

, then internal transmit and receive

squelch timing, SQE oscillator frequency, and receive
and SQE output drive levels are set. SQE frequency
set is also determined by components connected be-
tween V

CC1

and COLL OSC.

SQE TEST

Signal Quality Error Test Enable (Input)

The SQE Test function is enabled by connecting the
SQE TEST pin to V

and disabled by connecting to V

TX+

, V

TX

Tap Node Driver Current Set (Inputs)

A reference input for transmission level and external re-
dundant jabber. Transmit level is set by an external re-
sistor between V

TX+

and V

(for an 80 mA peak level,

R = 9.09

). V

may be operated between V

and

+ 1 V. When the voltage at V

goes more positive

than V

+ 2 V, TXT is disabled and an SQE message

is output at the Cl pair.

TAP SHIELD

Low-Noise Media Cable Return (Input)

This input is the return for V

COL

reference and the re-

ceive signal from the media. External connection is to
a positive power supply.

COL

SQE Reference Voltage (Bias Supply)

SQE sense voltage and RXT input amplifier reference.
An internally set analog reference for SQE level and data
signal set at 1.600 V nominal with a source resistance
of 150

nominal. This reference should be filtered with

respect to TAP SHIELD (see Applications section for ad-
justing threshold levels for other applications).

COLL OSC

SQE Timing Set (Input)

Timing input for SQE oscillator. For a properly set input
at VC

REF

, SQE oscillator period is set at 2.1RC. For a

10 MHz SQE oscillator frequency, R should be 1 k

and

C 47 pF, including interconnect and device capacitance.

CC1

Positive Logic Supply

CC2

SQE Timing Reference (Positive Supply Voltage)

Timing reference return for SQE oscillator and analog
signal ground.

Negative Logic Supply and IC Substrate

Am7996 5

FUNCTIONAL DESCRIPTION

The Am7996 IEEE 802.3/Ethernet/Cheapernet Trans-
ceiver consists of four sections: 1) Transmit--receives
signals from DTE and sends it to the coaxial medium;
2) Receive--obtains data from media and sends it to
DTE; 3) Collision Detect--indicates to DTE any colli-
sion on the media; and 4) Jabber--guards medium
from node transmissions that are excessive in length.

Transmit

The Am7996 receives differential signals from the DTE
(in the case of Am7990 family applications, from the
Am7992 --serial interface adapter--SIA). For IEEE
(10BASE-5)/Ethernet applications, this signal is re-
ceived through the AUI cable and isolation transformer.
In IEEE 802.3 10BASE-2--Cheapernet applications,
the AUI cable is optional.

Data is received through a noise rejection filter that re-
jects signals with pulse widths less than 7 ns (negative
going), or with levels less than 175 mV peak. Only sig-
nals greater than 275 mV peak from the DTE are en-
abled. This minimizes false starts due to noise and
ensures that no valid packets are missed.

The Am7996's tap driver provides the driving capability
to ensure adequate signal level at the end of the maxi-
mum length network segment (500 meters) under the
worst-case number of connections (100 nodes). Re-
quired rise and fall times of data transmitted on the net-
work are maintained by the Am7996 Tap Driver. The tap
driver's output is connected to the media through exter-
nal isolating diodes. To safeguard network integrity, the
driver is disabled whenever power falls below the mini-
mum operation voltage.

During transmission, the Am7996 Jabber Controller
monitors the duration that the transmit tap driver is ac-
tive and disables the driver if the jabber time is ex-
ceeded. This prevents network tie-up due to a
"babbling" transceiver. Once disabled, the driver is not
reset until 400 ms after the DO pair is idle and there is
no fault on TXT. During the disable time, an SQE signal
is sent on the Cl pair to the DTE.

When SQE TEST is tied to V

, the Am7996 generates

an SQE message at the end of every transmission. This
signal is a self-test indication to the DTE that the media
attachment unit (MAU) collision pair is operational.

Receive and Carrier Detect

Signal is acquired from the tap through a high-
impedance (100 k

) resistive divider. A high input-

impedance (low capacitance, high bandwidth, low
noise) DC-coupled input amplifier in the Am7996 re-
ceives the signal. The received signal passes through
a high-pass filter to minimize inter-symbol distortion,
and then through a data slicer. The Am7996 Carrier
Detect compares received signals to a reference.
Signals meeting carrier squelch requirements enable

data to the differential line driver within five bit times
from the start of the packet.

Received data is transmitted from the Dl pair through
an isolation transformer to the AUI cable (Ethernet/
lEEE 802.3--10BASE-5). In IEEE 802.3 10BASE-2--
Cheapernet, the AUI cable is optional. Following the
last transition of the packet, the Dl pair is held HIGH for
two bit times and then decreases to idle level within
twenty bit times.

Collision Detect

The Am7996 detects collisions on transmit if one or
more additional stations are transmitting on the network.

Received signals are compared against the collision
threshold reference. If the level is more negative than
the reference, an enable signal is generated to the Cl
pair. The collision threshold can be modified by
external components.

The collision oscillator is a 10 MHz oscillator that drives
the differential Cl pair to the DTE through an isolation
transformer.

This signal is gated to the Cl pair whenever there is a
collision, the SQE Test is in progress, or the jabber con-
troller is activated. The oscillator is also utilized in
counting time for the jabber timer and SQE Test.

The Cl

output meets the drive requirements for the

AUI interface. The output stays HIGH for two bit times
at the end of the packet, decreasing to the idle level
within twenty bit times.

Jabber Function

The Am7996 Jabber Timer monitors the activity on the
DO pair and senses TXT faults. It inhibits transmission
if the tap driver is active for longer than the jabber time
(26 ms). An SQE message (10 MHz collision signal), is
enabled on the Cl pair for the fault duration.

After the fault is removed, the jabber timer counts the
unjab time of 400 ms before it enables the driver.

If desired, a redundant jabber function can be imple-
mented externally, and the output driver disabled by re-
moving the driver supply at V

.The Am7996 senses

this condition and forces an SQE message on the Cl
pair during the disable time.

SQE Test

An SQE Test will occur at the end of every transmission
if the SQE TEST pin is tied to V

. The SQE Test signal

is a gated 10 MHz signal to the Cl pair. The SQE Test en-
sures that the twisted-pair assigned for collision notifica-
tion to the DTE is intact and operational. The SQE Test
starts eight bit times after the last transition of the trans-
mitted signal and lasts for a duration of eight bit times.

The SQE Test can be disabled by connecting the
SQE TEST pin to V

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