5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
If the device is configured to handle both long and short packets (see "Define Configuration" command), then receive
pages should always be 512 bytes long because the user never knows what the length of the receive packet will be. In
this case, the transmit pages may be made 256 bytes long, leaving at least 512 bytes free at any given time. Even if the
Command Chaining operation is being used, 512 bytes is still guaranteed to be free because Command Chaining only
requires two pages for transmit and two for receive (in this case, two 256 byte pages for transmit and two 512 byte
pages for receive, leaving 512 bytes free). Please note that it is the responsibility of software to reserve 512 bytes for
each receive page if the device is configured to handle long packets. The COM20020I does not check page boundaries
during reception. If the device is configured to handle only short packets, then both transmit and receive pages may be
allocated as 256 bytes long, freeing at least 1KByte at any given time.
Even if the Command Chaining operation is being used, 1KByte is still guaranteed to be free because Command
Chaining only requires two pages for transmit and two for receive (in this case, a total of four 256 byte pages, leaving 1K
free).
The general rule which may be applied to determine where in RAM a page begins is as follows:
Address = (nn x 512) + (f x 256).
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
COM20020I 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 9. 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.
SMSC COM20020I 3.3V
Page 37
Revision 12-06-06
DATASHEET
SMSC [ SMSC CORPORATION ]
