COM20051I Datasheet, PDF (36/74 Pages) SMSC Corporation – Integrated Microcontroller and ARCNET (ANSI 878.1) Interface

Register. This allows a finer resolution of the buffer pages without affecting software compatibility. This scheme is
useful for applications which frequently use packet sizes of 256 bytes or less, especially for microcontroller systems
with limited memory capacity. The remaining portions of the buffer pages which are not allocated for current transmit
or receive packets may be used as temporary storage for previous network data, packets to be sent later, or as extra
memory for the system, which may be indirectly accessed.
If the device is configured to handle both long and short packets (see "Define Configuration" command), then the
receive page 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 page may be made 256 bytes long, leaving at least 256 bytes free at any given
time. Please note that it is the responsibility of software to reserve 512 bytes for the receive page if the device is
configured to handle long packets. The ARCNET core 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, allowing two receive and two transmit packets.
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 ARCNET core 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 11. 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
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 11 – RAM BUFFER PACKET CONFIGURATION
SMSC DS – COM20051I
Page 36
Rev. 03/27/2000