Digi NS9750 - Manual (Page 351)

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E t h e r n e t C o m m u n i c a t i o n M o d u l e
Transmit packet processor
Transmit frames are transferred from system memory to the transmit packet
processor into a 256-byte TX_FIFO. Because various parts of the transmit frame can
reside in different buffers in system memory, several buffer descriptors can be used
to transfer the frame.
All buffer descriptors (that is, up to 64) are found in a local TX buffer descriptor RAM.
Figure 67 shows the transmit buffer descriptor format.
Figure 67: Transmit buffer descriptor format
Buffer length This is a dual use field:
When the buffer descriptor is read from system memory, buffer length
indicates the maximum sized frame, in bytes, that can be stored in this
buffer ring.
When the RX_RD logic writes the descriptor back from the receive status
FIFO into system memory at the end of the frame, the buffer length is the
actual frame length, in bytes.Only the lower 11 bits of this field are valid,
since the maximum legal frame size for Ethernet is 1522 bytes.
Field Description
Field Description
W WRAP bit, which, when set, tells the TX_WR logic that this is the last buffer
descriptor within the continuous list of descriptors in the TX buffer descriptor
RAM. The next buffer descriptor is found using the initial buffer descriptor
pointer in the TX Buffer Descriptor Pointer register (TXPTR; see "TX Buffer
Descriptor Pointer register," beginning on page 389).
When the WRAP bit is not set, the next buffer descriptor is located at the next
entry in the TX buffer descriptor RAM.
Destination Address (not used)
Buffer Length (11-bits used)
Status
Source Address
0
OFFSET + 0
OFFSET + 4
OFFSET + 8
OFFSET + C F I L W Reserved
31 15 16 30 29 28