COM20020ILJP datasheet(42/72 Pages) MICROCHIP

42 / 72 page

5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM

Datasheet

Revision 12-05-06

Page 42

SMSC COM20020I Rev D

DATASHEET

SID

DID

COUNT = 256-N

NOT USED

DATA BYTE 1

DATA BYTE 2

DATA BYTE N-1

DATA BYTE N

NOT USED

SID

DID

COUNT = 512-N

NOT USED

DATA BYTE 1

DATA BYTE 2

DATA BYTE N-1

DATA BYTE N

SHORT PACKET

FORMAT

LONG PACKET

FORMAT

ADDRESS

COUNT

255

511

N = DATA PACKET LENGTH

SID = SOURCE ID

DID = DESTINATION ID

(DID = 0 FOR BROADCASTS)

COUNT

511

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

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