10 Mbps ARCNET (ANSI 878.1) Controller with 2Kx8 On-Board RAM
Datasheet
LONG PACKET
FORMAT
SHORT PACKET
FORMAT
ADDRESS
0
ADDRESS
0
SID
DID
SID
DID
1
2
1
2
3
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 6.3 - RAM Buffer Packet Configuration
6.3.5 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 COM20022I 3V 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 5.7. 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
number of information bytes in the message. The SID in Address 0 is used by the receiving node to reply
to the transmitting node. The COM20022I 3V puts the local ID in this location, therefore it is not necessary
to write into this location. Please note that a short packet may contain between 1 and 253 data bytes,
while a long packet may contain between 257 and 508 data bytes. A minimum value of 257 exists on a
long packet so that the COUNT is expressible in eight bits. This leaves three exception packet lengths
which do not fit into either a short or long packet; packet lengths of 254, 255, or 256 bytes. If packets of
these lengths must be sent, the user must add dummy bytes to the packet in order to make the packet fit
into a long packet.
SMSC COM20022I 3V
Page 49
Revision 02-27-06
DATASHEET
SMSC [ SMSC CORPORATION ]
