![](http://datasheet.mmic.net.cn/390000/TNETA1560_datasheet_16838729/TNETA1560_34.png)
TNETA1560
ATM SEGMENTATION AND REASSEMBLY DEVICE
WITH SBUS HOST INTERFACE
SDNS010C – JANUARY 1994 – REVISED OCTOBER 1995
34
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
PRINCIPLES OF OPERATION
RX DMA word 1 – current-buffer pointer
Unused (bits 31 – 28)
Current-buffer pointer – 16 byte aligned (bits 27 – 0)
The current-buffer pointer is 28 bits, which implies that the buffer is aligned to 16-byte boundaries. This is a
dynamic field that is updated with every RCB-to-SBus transaction.
RX DMA word 2 – start-of-buffer pointer
Unused (bits 31 – 28)
Start-of-buffer pointer – 16 byte aligned (bits 27 – 0)
The start-of-buffer pointer is 28 bits because the buffer is aligned to 16-byte boundaries. This field is copied from
the corresponding 28-bit field in word 0 of a free-buffer-ring entry.
RX DMA word 3 – configuration
Configuration (bits 31 – 23)
Unused (bits 22 – 11)
Null-AAL packet length (bits 10 – 0)
OWN bit position (bit 31)
The OWN bit is set high for each valid receive channel. It is copied into the corresponding OWN bit location in
word 0 at the start of each new packet to indicate that the DMA channel is active.
VC_ON (bit 30)
The VC_ON bit enables packet-reassembly processing. The bit is set in the default mode to indicate that the
VC is enabled. The SAR discards cells received on the corresponding VC when the VC_ON bit is deasserted
on a per-cell basis.
buffer type – small or big (bit 29)
The SAR supports only two buffer sizes on receive: small and big. The host determines the sizes of the small
and big buffers. The buffer-type bit is used to select between a buffer pointer from the small free-buffer ring or
the big free-buffer ring for each new packet, which allows the host to target small or big buffers for all packets
on a given VC. The small free-buffer ring is used when the bit is set and the big free-buffer ring is used in the
default (zero) state.
null-AAL indication (bit 28)
This field is set to indicate that null-AAL packets are received on this BWG (VC). The null-AAL packet-length
field in bits (10 – 0) is used to determine the end of a packet. CRC errors are ignored for null-AAL packets. The
CRC-error indicator in the receive completion ring is not used.
AAL3/4 indication (bit 25)
This field is set to indicate that AAL3/4 packets are received on this BWG (VC). This indicates the EOM field
in byte 6 (bit 6 of an ATM cell is used as the EOP indicator). CRC errors are ignored for AAL3/4 packets. The
CRC-error indicator in the receive completion ring is not used.
end-of-packet wait (bit 24)
This bit must be set to zero by the device driver during initialization. This gives the SAR the responsibility of
setting the bit to one in DMA word 0 (when this feature is enabled). This bit is a status bit used by the TNETA1560
during operation.