LAN83C175 Datasheet, PDF (14/92 Pages) SMSC Corporation – Ethernet CARDBUS Integrated Controller With Modem Support

(fragments) are listed. Up to 63 fragments are
allowed. The remaining entries specify the
starting address and length of each buffer.
As in the direct queuing method, the transmit
DMA will copy fragments one at a time into the
local buffer until Transmit Length bytes have
been copied or all of the fragments have been
read. If early transmit is enabled, transmission
will be initiated when enough bytes have been
copied to meet the early transmit threshold.
Otherwise transmission will be initiated when
the entire copy is complete.
When more than one frame is queued, the
transmit DMA will begin copying a second frame
while the first is transmitting. It will continue
copying fragments until the entire frame is
loaded or the first transmission has completed.
If the copy completes while the first
transmission is still in progress, then the
LAN83C175 will stop and wait. When the
transmission is finished, the LAN83C175 will
post the status into the first descriptor and
immediately initiate the second transmission. If
the transmission completes before the copy is
done, the LAN83C175 will pause between
fragments to post the status and then resume
the copy. If the early transmit threshold has
already been exceeded then the second
transmission will be initiated immediately.
Figure 3 on the following page shows a drawing
of the Transmit Buffer Structure.
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