AM79C970A Advanced Micro Devices, AM79C970A Datasheet - Page 85

• Manufacturer Part Number: AM79C970A
• Description: PCnet-PCI II Single-Chip Full-Duplex Ethernet Controller for PCI Local Bus Product
• Manufacturer: Advanced Micro Devices

Collision Detection
A MAU detects the collision condition on the network
and generates a 10 MHz differential signal at the
CI 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 in 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 as described in 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.
PCnet-PCI II
Figure 37. AUI Differential Input Termination
DI+
DI-
40.2
P R E L I M I N A R Y
Am79C970A
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.
0.01 F
0.1 F
to
40.2
AUI Isolation
Transformer
19436A-40
AMD
85