COM20019I Datasheet, PDF (37/65 Pages) SMSC Corporation – Low Cost ARCNET(ANSI 878.1) Controller with 2k X 8 On-Board RAM

Low Cost ARCNET (ANSI 878.1) Controller with 2K x 8 On-Board RAM
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 COM20019I 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 8. 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 COM20019I 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
SMSC COM20019I
Page 37
DATASHEET
Rev. 04-15-05