COM20020I_0609 Datasheet, PDF (38/69 Pages) SMSC Corporation – 5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM

5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip 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 9 – Ram Buffer Packet Configuration
The SID in Address 0 is used by the receiving node to reply to the transmitting node. The COM20020I 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.
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 3V automatically sends a FREE
BUFFER ENQUIRY to the destination node in order to send the message. At this point, one of four possibilities may
SMSC COM20020I 3.3V Rev.E
Page 38
DATASHEET
Revision 09-11-06