Motorola MPC8260 - Manual (Page 669)

MOTOROLA Chapter 24. SCC Ethernet Mode 24-1
Chapter 24
SCC Ethernet Mode
240
240
The Ethernet IEEE 802.3 protocol is a widely used LAN protocol based on the carrier sense
multiple access/collision detect (CSMA/CD) approach. Because Ethernet and IEEE 802.3
protocols are similar and can coexist on the same LAN, both are referred to as Ethernet in
this manual, unless otherwise noted. Figure 24-1 shows Ethernet and IEEE 802.3 frame
structure.
Figure 24-1. Ethernet Frame Structure
The frame begins with a 7-byte preamble of alternating ones and zeros. Because the frame
is Manchester encoded, the preamble gives receiving stations a known pattern on which to
lock. The start frame delimiter follows the preamble, signifying the beginning of the frame.
The 48-bit destination address is next, followed by the 48-bit source address. Original
versions of the IEEE 802.3 speciÞcation allowed 16-bit addressing, but this addressing has
never been widely used and is not supported.
The next Þeld is the Ethernet type Þeld/IEEE 802.3 length Þeld. The type Þeld signiÞes the
protocol used in the rest of the frame and the length Þeld speciÞes the length of the data
portion of the frame. For Ethernet and IEEE 802.3 frames to coexist on the same LAN, the
length Þeld of the frame must always be different from any type Þelds used in Ethernet. This
limits the length of the data portion of the frame to 1,500 bytes and total frame length to
1,518 bytes. The last 4 bytes of the frame are the frame check sequence (FCS), a standard
32-bit CCITT-CRC polynomial used in many protocols.
When a station needs to transmit, it checks for LAN activity. When the LAN is silent for a
speciÞed period, the station starts sending. At that time, the station continually checks for
collisions on the LAN; if one is found, the station forces a jam pattern (all ones) on its frame
and stops sending. Most collisions occur close to the beginning of a frame. The station waits
Preamble
Start Frame
Data
Delimiter
Destination
Address
Type/
Length
Source
Address
Frame Check
Sequence
7 Bytes 1 Byte 6 Bytes 6 Bytes 2 Bytes 46–1500 Bytes 4 Bytes
Frame Length is 64–1518 Bytes
NOTE: The lsb of each octet is transmitted first.