COM20019 Datasheet, PDF (39/81 Pages) SMSC Corporation – Low Cost ARCNET (ANSI 878.1) Controller with 2K x 8 On-Board RAM

The software flow for controlling the Configuration,
Node ID, Tentative ID, and Next ID registers is
generally limited to the initialization sequence and
the maintenance of the network map.
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,
Additionally, it is necessary to understand the
details of how the other Internal Registers are
used in the transmit and receive sequences and to
know how the internal RAM buffer is properly set
up. The sequence of events that tie these actions
together is discussed as follows.
Selecting RAM Page Size
During normal operation, the 2K x 8 of RAM is
divided into four pages of 512 bytes each. The
page to be used is specified in the "Enable
Transmit (Receive) from (to) Page fnn" command,
where "nn" specifies page 0, 1, 2, or 3. This
allows the user to have constant control over the
allocation of RAM.
When the Offset bit "f" (bit 5 of the "Enable
Transmit (Receive) from (to) Page fnn" command
word) is set to logic "1", an offset of 256 bytes is
added to the page specified. For example: to
transmit from the second half of page 0, the
command "Enable Transmit from Page fnn"
(fnn=100 in this case) is issued by writing 0010
0011 to the Command 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 receive pages should always be
512 bytes long because the user never knows
what the length of the receive packet will be. In
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