5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
Datasheet
LONG PACKET
SHORT PACKET
FORMAT
FORMAT
ADDRESS
0
ADDRESS
SID
DID
SID
0
1
2
3
DID
1
2
COUNT = 256-N
0
COUNT = 512-N
NOT USED
DATA BYTE 1
DATA BYTE 2
NOT USED
COUNT
DATA BYTE 1
DATA BYTE 2
COUNT
DATA BYTE N-1
DATA BYTE N
255
511
DATA BYTE N-1
DATA BYTE N
NOT USED
511
N = DATA PACKET LENGTH
SID = SOURCE ID
DID = DESTINATION ID
(DID = 0 FOR BROADCASTS)
Figure 6.2 - RAM Buffer Packet Configuration
6.4.2 Transmit Sequence
During a transmit sequence, the microcontroller selects a 256 or 512 byte segment of the RAM buffer and
writes into it. The appropriate buffer size is specified in the "Define Configuration" command. When long
packets are enabled, the COM20020ID interprets the packet as either a long or short packet, depending
on whether the buffer address 2 contains a zero or non-zero value. The format of the buffer is shown in
Figure 6.2. Address 0 contains the Source Identifier (SID); Address 1 contains the Destination Identifier
(DID); Address 2 (COUNT) contains, for short packets, the value 256-N, where N represents the number
of information bytes in the message, or for long packets, the value 0, indicating that it is indeed a long
packet. In the latter case, Address 3 (COUNT) would contain the value 512-N, where N represents the
Revision 12-05-06
Page 42
SMSC COM20020I Rev D
DATASHEET
SMSC [ SMSC CORPORATION ]
