AM7996DCTR AMD [Advanced Micro Devices], AM7996DCTR Datasheet - Page 5

• Manufacturer Part Number: AM7996DCTR
• Description: IEEE 802.3/Ethernet/Cheapernet Transceiver
• Manufacturer: AMD [Advanced Micro Devices]

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
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
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
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
EE
, the Am7996 generates
Am7996
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
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
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
CC
.
TX–
EE
.The Am7996 senses
. The SQE Test signal
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