COM20019I-HT datasheet(37/65 Pages) SMSC

37 / 65 page

Low Cost ARCNET (ANSI 878.1) Controller with 2K x 8 On-Board RAM

SMSC COM20019I

Page 37

Rev. 03-07-06

DATASHEET

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

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

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