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參數(shù)資料
| 型號： | AM79C971 |
| 廠商： | Advanced Micro Devices, Inc. |
| 英文描述： | PCnet⑩-FAST Single-Chip Full-Duplex 10/100 Mbps Ethernet Controller for PCI Local Bus |
| 中文描述： | PCnet⑩快速單芯片全雙工10/100 Mbps以太網(wǎng)控制器，PCI總線 |
| 文件頁數(shù)： | 62/265頁 |
| 文件大?。?/td> | 3190K |
| 代理商： | AM79C971 |
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62
Am79C971
Medium Allocation
The IEEE/ANSI 802.3 standard (ISO/IEC 8802-3 1990)
requires that the CSMA/CD MAC monitor the medium
for traffic by watching for carrier activity. When carrier is
detected, the media is considered busy, and the MAC
should defer to the existing message.
The ISO 8802-3 (IEEE/ANSI 802.3) standard also al-
lows optionally a two-part deferral after a receive mes-
sage.
See ANSI/IEEE Std 802.3-1993 Edition, 4.2.3.2.1:
Note:
It is possible for the PLS carrier sense indication
to fail to be asserted during a collision on the media. If
the deference process simply times the inter-Frame
gap based on this indication, it is possible for a short in-
terFrame gap to be generated, leading to a potential re-
ception failure of a subsequent frame. To enhance
system robustness, the following optional measures,
as specified in 4.2.8, are recommended when Inter-
Frame-SpacingPart1 is other than 0:
1. Upon completing a transmission, start timing the in-
terrupted gap, as soon as transmitting and carrier
sense are both false.
2. When timing an inter-frame gap following reception,
reset the inter-frame gap timing if carrier sense be-
comes true during the first 2/3 of the inter-frame gap
timing interval. During the final 1/3 of the interval,
the timer shall not be reset to ensure fair access to
the medium. An initial period shorter than 2/3 of the
interval is permissible including 0.
The MAC engine implements the optional receive two
part deferral algorithm, with an InterFrameSpacing-
Part1 time of 6.0
μ
s. The InterFrameSpacingPart 2 in-
terval is, therefore, 3.4
μ
s.
The Am79C971 controller will perform the two-part
deferral algorithm as specified in Section 4.2.8 (Pro-
cess Deference). The Inter Packet Gap (IPG) timer will
start timing the 9.6
μ
s InterFrameSpacing after the re-
ceive carrier is deasserted. During the first part deferral
(InterFrameSpacingPart1 - IFS1), the Am79C971 con-
troller will defer any pending transmit frame and re-
spond to the receive message. The IPG counter will be
cleared to 0 continuously until the carrier deasserts, at
which point the IPG counter will resume the 9.6
μ
s
count once again. Once the IFS1 period of 6.0
μ
s has
elapsed, the Am79C971 controller will begin timing the
second part deferral (InterFrameSpacingPart2 - IFS2)
of 3.4
μ
s. Once IFS1 has completed and IFS2 has com-
menced, the Am79C971 controller will not defer to a re-
ceive frame if a transmit frame is pending. This means
that the Am79C971 controller will not attempt to receive
the receive frame, since it will start to transmit and gen-
erate a collision at 9.6
μ
s. The Am79C971 controller
will complete the preamble (64-bit) and jam (32-bit) se-
quence before ceasing transmission and invoking the
random backoff algorithm.
The Am79C971 controller allows the user to program
the IPG and the first part deferral (InterFrame-
SpacingPart1 - IFS1) through CSR125. By changing
the IPG default value of 96 bit times (60h), the user can
adjust the fairness or aggressiveness of the
Am79C971 MAC on the network. By programming a
lower number of bit times than the ISO/IEC 8802-3
standard requires, the Am79C971 MAC engine will be-
come more aggressive on the network. This aggressive
nature will give rise to the Am79C971 controller possi-
bly
capturing the network
at times by forcing other less
aggressive compliant nodes to defer. By programming
a larger number of bit times, the Am79C971 MAC will
become less aggressive on the network and may defer
more often than normal. The performance of the
Am79C971 controller may decrease as the IPG value
is increased from the default value, but the resulting be-
havior may improve network performance by reducing
collisions. The Am79C971 controller uses the same
IPG for back-to-back transmits and receive-to-transmit
accesses. Changing IFS1 will alter the period for which
the Am79C971 MAC engine will defer to incoming re-
ceive frames.
CAUTION: Care must be exercised when altering
these parameters
.
Adverse network activity could
result!
This transmit two-part deferral algorithm is imple-
mented as an option which can be disabled using the
DXMT2PD bit in CSR3. The IFS1 programming will
have no effect when DXMT2PD is set to 1, but the IPG
programming value is still valid. Two part deferral after
transmission is useful for ensuring that severe IPG
shrinkage cannot occur in specific circumstances,
causing a transmit message to follow a receive mes-
sage so closely as to make them indistinguishable.
During the time period immediately after a transmission
has been completed, the external transceiver (in the
case of a standard AUI connected device) should gen-
erate the SQE Test message (a nominal 10-MHz burst
of 5 to 15 bit times duration) on the CI
±
pair (within 0.6
to 1.6
μ
s after the transmission ceases). During the
time period in which the SQE Test message is ex-
pected, the Am79C971 controller will not respond to re-
ceive carrier sense.
相關PDF資料 |
PDF描述 |
|---|---|
| AM79C971KCW | IC LOGIC 16211 24-BIT FET BUS SWITCH -40+85C TSSOP-56 35/TUBE |
| AM79C972BKCW | PCnet⑩-FAST+ Enhanced 10/100 Mbps PCI Ethernet Controller with OnNow Support |
| AM79C972BKIW | PCnet⑩-FAST+ Enhanced 10/100 Mbps PCI Ethernet Controller with OnNow Support |
| AM79C972BVCW | PCnet⑩-FAST+ Enhanced 10/100 Mbps PCI Ethernet Controller with OnNow Support |
| AM79C972 | PCnet⑩-FAST+ Enhanced 10/100 Mbps PCI Ethernet Controller with OnNow Support |
相關代理商/技術參數(shù) |
參數(shù)描述 |
|---|---|
| AM79C971AKC | 制造商:Advanced Micro Devices 功能描述: |
| AM79C971AVC | 制造商:Advanced Micro Devices 功能描述:10/100 MBPS ETHERNET - Trays |
| AM79C971AVC\\W | 制造商:Rochester Electronics LLC 功能描述:- Bulk |
| AM79C971AVC\W | 制造商:Rochester Electronics LLC 功能描述:- Bulk |
| AM79C971AWW WAF | 制造商:Advanced Micro Devices 功能描述: |
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