Motorola MC68824 User Manual page 143

II
B.1.9 Abort Sequence
An abort sequence prematurely terminates the transmission of a frame. The abort sequence is
sent by a station that does not wish to continue to send a frame it has already begun sending.
The TBC will automatically generate an abort sequence if the conditions require it to do so. For
example, the TBC will generate an abort sequence if it gets an underrun condition while trans- .
mitting a frame. The abort sequence is a start delimiter immediately followed by an end delimiter
and is represnted as follows:
First Svmbol Transmitted
I
I
E
B.2 ADDRESSING
IEEE 802.4 uses the IEEE addressing notation and two types of addressing: individual and group.
Individual addressing identifies a particular station and group addressing identifies a group of
logically related stations. These two types of addresses are differentiated by the least significant
bit in the destination address field of the frame.
B.2.1 IEEE Addressing
The IEEE globally administered 48-bit address contains 12 hexadecimal digits with each digit
paried in groups of two. The bits within each octet or pair, are transmitted from the least significant
bit to the most significant bit. For example, an address supplied by IEEE is represented as follows:
FO-2E-15-6C-77-9B
It would be transmitted onto the local area network as follows:
First Bit Transmitted
0000 1111 0111 0100 1010 1000 0011 0110 1110 1110 1101 1001
This address would appear in the TBC private area as:
Least Significant Bit
TS Low 2EFO TS Medium 6C15 TS High
9B77
This address would appear in the TBC frame descriptor as:
Offset 28 502E Offset 30 156C Offset 32 779B
B.2.2 Individual Addressing
The individual address, or this station (TS), of a station is set during initialization through the
initialization table. The TBC also contains an individual address mask that allows the station to
accept individually addressed frames for more than one station. The individual address mask is
also set on initialization through the initialization table. This means the TBC can be used in the
configuration below. In this example, station 0036 is the only member of the logical ring and
therefore the only station to receive and send the token. Station 0036 will receive frames addressed
MOTOROLA
8-4
MC68824 USER'S MANUAL