COM20020I Datasheet, PDF (42/72 Pages) SMSC Corporation – 5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM

5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
Datasheet
ADDRESS
0
1
2
COUNT
255
511
SHORT PACKET
FORMAT
SID
DID
COUNT = 256-N
NOT USED
DATA BYTE 1
DATA BYTE 2
DATA BYTE N-1
DATA BYTE N
NOT USED
ADDRESS
0
1
2
3
COUNT
511
LONG PACKET
FORMAT
SID
DID
0
COUNT = 512-N
NOT USED
DATA BYTE 1
DATA BYTE 2
DATA BYTE N-1
DATA BYTE N
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
DATASHEET
SMSC COM20020I Rev D