5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
LONG PACKET
FORMAT
SHORT PACKET
FORMAT
ADDRESS
0
ADDRESS
SID
DID
SID
0
1
2
3
DID
1
2
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 9 – RAM BUFFER PACKET CONFIGURATION
The SID in Address 0 is used by the receiving node to reply to the transmitting node. The COM20020I 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.
Once the packet is written into the buffer, the microcontroller awaits a logic "1" on the TA bit, indicating that a previous
transmit command has concluded and another may be issued. Each time the message is loaded and a transmit
command issued, it will take a variable amount of time before the message is transmitted, depending on the traffic on
the network and the location of the token at the time the transmit command was issued. The conclusion of the Transmit
Command will generate an interrupt if the Interrupt Mask allows it. If the device is configured for the Command Chaining
operation, please see the Command Chaining section for further detail on the transmit sequence. Once the TA bit
becomes a logic "1", the microcontroller may issue the "Enable Transmit from Page fnn" command, which resets the TA
and TMA bits to logic "0". If the message is not a BROADCAST, the COM20020I automatically sends a FREE BUFFER
ENQUIRY to the destination node in order to send the message. At this point, one of four possibilities may occur.
Revision 12-06-06
38
SMSC COM20020I 3.3V
DATASHEET
SMSC [ SMSC CORPORATION ]
