dp83901a National Semiconductor Corporation, dp83901a Datasheet - Page 8

• Manufacturer Part Number: dp83901a
• Description: Serial Network Interface Controller
• Manufacturer: National Semiconductor Corporation

5 0 Transmit Receive Packet Encapsulation Decapsulation
A standard IEEE 802 3 packet consists of the following
fields preamble Start of Frame Delimiter (SFD) destination
address source address length data and Frame Check
Sequence (FCS) The typical format is shown in Figure 2
The packets are Manchester encoded and decoded by the
ENDEC module and transferred serially to the NIC module
using NRZ data with a clock All fields are of fixed length
except for the data field The SNIC generates and appends
the preamble SFD and FCS field during transmission The
Preamble and SFD fields are stripped during reception (The
CRC is passed through to buffer memory during reception )
PREAMBLE AND START OF FRAME DELIMITER (SFD)
The Manchester encoded alternating 1 0 preamble field is
used by the ENDEC 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 NIC module Byte align-
ment is performed with the Start of Frame Delimiter (SFD)
pattern which consists of two consecutive 1’s The SNIC
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 pack-
ets from reaching a node There are three types of address
formats supported by the SNIC physical multicast and
broadcast The physical address is a unique address that
corresponds only to 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 in order for the SNIC to
accept the packet Multicast addresses begin with an MSB
FIGURE 2
8
of ‘‘1’’ The SNIC filters multicast addresses using a stan-
dard 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 intended for
all nodes A promiscuous mode allows reception of all pack-
ets the destination address is not required to match any
filters Physical broadcast multicast and promiscuous ad-
dress 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 SNIC
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 re-
quire appending a pad 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 field The SNIC
does not strip or append pad bytes for short packets
or check for oversize packets
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 re-
jected The AUTODIN II (X
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polynomial is used for the CRC calculations
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TL F 10469 – 5
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