DM9008 datasheet(39/68 Pages) ETC

39 / 68 page

DM9008

ISA/Plug & Play Super Ethernet Contoller

Final

Version :DM9008-DS-F02

June 14, 2000

Preamble and Start of Frame

Delimiter (SFD)

The Manchester encoded alternating 1, 0 preamble field is

used by the ENA to acquire bit synchronization with an

incoming packet. When transmitted, each packet contains 62

bits of alternating 1, 0 preamble. Some of this preamble will be

lost as the packet travels through the network. The preamble

field is stripped by the ENC. Byte alignment is performed with

the Start of Frame Delimiter (SFD) pattern, which consists of

two consecutive 1's. The ENC does not treat the SFD pattern

as a byte; it detects only the two-bit pattern. This allows any

preceding preamble within the SFD to be used for phase

locking.

Destination Address

The destination address indicates the destination of the packet

on the network, and is used to filter unwanted packets from

reaching a node. Three types of address formats are

supported by the DM9008: physical, multicast and broadcast.

The physical address is a unique address that corresponds to

only a single node. All physical addresses have an MSB of "0."

These addresses are compared to the internally stored

physical address registers. Each bit in the destination address

must match the corresponding address of the address register

in order for DM9008 to accept the packet. Multicast addresses

begin with an MSB of "1." The DM9008 filters multicast

addresses using a standard hashing algorithm that maps all

multicast addresses into a 6-bit value. This 6-bit value indexes

a 64-bit array that filters the value. If the address consists of all

1's, it is a broadcast address, indicating that the packet is

intende for all nodes. Promiscuous mode allows reception of

all packets: the destination address is not required to match

any filters. Physical, broadcast, multicast, and promis-cuous

address modes can be selected.

Source Address

The source address is the physical address of the node that

sent the packet. Source addresses cannot be multicast or

broadcast addresses. This field is simply passed to buffer

memory.

Length Field

The 2-byte length field indicates the number of bytes that are

contained in the data field of the packet. This field is not

interpreted by the ENC.

Data Field

The data field consists of anywhere from 46 to 1500 bytes.

Messages longer than 1500 bytes need to be broken into

multiple packets. Messages shorter than 46 bytes will require

padding to bring the data field to the minimum length of 46

bytes. If the data field is padded, the number of valid data bytes

is indicated in the length.

FCS Field

The Frame Check Sequence (FCS) is a 32-bit CRC field

calculated and appended to a packet during transmission to

allow detection of errors when a packet is received. During

reception, error-free packets result in a specific pattern in the

CRC generator. Packets with improper CRC will be rejected.

The AUTODIN II ( X

32 + X26 + X23 + X22 + X16 + X12 + X11 + X10 +

8 + X7 + X5 + X4 + X2 + X1 + 1) polynomial is used for CRC

calculations.

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