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參數(shù)資料
| 型號： | MB86964PFV-G |
| 元件分類： | 微控制器/微處理器 |
| 英文描述： | 2 CHANNEL(S), 10M bps, LOCAL AREA NETWORK CONTROLLER, PQFP100 |
| 封裝： | SHRINK, PLASTIC, QFP-100 |
| 文件頁數(shù)： | 8/52頁 |
| 文件大?。?/td> | 373K |
| 代理商： | MB86964PFV-G |
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MB86964
16
Interpreting the Results
If the count is zero, no fault was detected. If the count is greater
than zero, but smaller than the packet length, a cable fault may
exist. If the count is less than 525, a real collision may have oc-
curred during the test. Real collisions normally occur within
the first 65 bytes of the packet, including preamble. Note the
error status bits, COL and CR LOST. COL high suggests a
cable open, whereas CR LOST suggests a short. Repeat the
measurement several times, discarding any anomalous values,
and average the remaining measurements. A cluster of read-
ings at about the same value is a strong indicator of a valid fault
measurement. If such a cluster of readings occurs, multiply the
average of the cluster by 39 feet to estimate the distance from
the node to the fault. [39 feet = (100 ns x 0.8 x 186,282 miles/
second x 5280 feet/mile)/2; this assumes the network is mostly
coaxial cable with signal propagation speed of approximately
0.8 x C, the speed of light.]
RECEIVER CIRCUITS
The receiver includes a receive state machine, serial-to-parallel
conversion, pipe-line FIFO, preamble recognition, bit and
byte-framing, address filtering, CRC and other error checking.
Additional circuits involved in packet reception are described
in the Transceiver section of this document.
The receiver state machine provides sequencing of events for
the receiver, including idle, busy, address filtering, and data
storage, detects receive error conditions and sets appropriate
bits within the DLC registers. A small data FIFO provides elas-
tic buffering for synchronization with the buffer controller tim-
ing and buffering of data while the buffer controller is servicing
other buffer memory access requests.
Monitoring the Network
Whenever the data link section is enabled (DLC EN bit,
DLCR6<7>, is set to zero), the MB86964 constantly monitors
the network for carrier. Signals that exceed the AC and DC
squelch thresholds of the transceiver cause the internal carrier
sense line to assert, which in turn causes the receiver to attempt
to receive a packet. (The transmitter also uses the carrier sense
function to defer to transmissions from other nodes.)
The receiver monitors the serial data stream from the transceiv-
er for the end-of-preamble bit pattern, a four-bit pattern of 1011
ending the preamble’s pattern of alternating ones and zeros.
This pattern also provides byte and field synchronization for
the receiver; the bit immediately following the end of preamble
is the first bit of the first byte of the packet’s destination address
field.
When packet transmission is unflawed, carrier sense remains
asserted for the duration of the packet, negating just after the
last bit of the CRC field is received, when the transceiver de-
tects the end-of-packet symbol at the end of the packet. Loss of
carrier sense at any other time may also result from a collision
or other network problems.
Address Filtering
A receive packet can be filtered by applying selectable criteria
to the contents of its Destination Address field, which is the
first data-bearing field following immediately after the
Preamble. There are several control bits in the MB86964’s reg-
isters which provide programmablility of the filter criteria. The
contents of the Destination Address field can be of three basic
types. The first data type which can occur in this 48-bit field is a
single node address, the unique, single-node address, globally
registered with the IEEE. This data type is indicated by a “0” in
the first bit position of the address. The second type is the mul-
ticast address, an address for a pre-defined group of nodes, for
example, NetWare 4.0 file servers. The multicast address is in-
dicated by a “1” in the first bit position of the address. The
MB86964 filters multicast addresses using a hash function and
a 64-bit hash table. Thirdly, a broadcast address is defined as a
special case of the multicast address which addresses all nodes
on the local network. This address value consists of all “1”s.
The MB86964 provides programmable address filtering logic
for each of these address types.
Among the address filtering selections possible with the
MB86964 are the following examples:
1. All Pass (no filtering)
2. Node address and broadcast packets only
3. Node address xxx.xxx, multicast addresses yyy.yyy and
zzz.zzz and broadcast packets only
Hash Table
The Hash Table provides a way to filter incoming multicast
packets so the host processor need not process packets that are
not of interest. The principle behind this filtering process is
based on the arrangement of a large number of elements of an
array, or database, to facilitate searching for elements
associated with a given key or datum. The hash function is a
mathematical or logical function that maps all elements in a do-
main onto a smaller domain called the hash table.
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