Transmit packet processor – Digi NS9750 User Manual
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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
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