5.0 Transmit/Receive Packet
Encapsulation/Decapsulation
(Continued)
SOURCE ADDRESS
The source address is the physical address of the node that
sent the packet. Source addresses cannot be multicast or
broadcast addresses. This field is simply passed to buffer
memory.
LENGTH/TYPE FIELD
The 2-byte length field indicates the number of bytes that
are contained in the data field of the packet. This field is not
interpreted by the ST-NIC.
DATA FIELD
The data field consists of anywhere from 46 to 1500 bytes.
Messages longer than 1500 bytes need to be broken into
multiple packets. Messages shorter than 46 bytes will re-
quire appending a pad to bring the data field to the minimum
length of 46 bytes. If the data field is padded, the number of
valid data bytes is indicated in the length field.
The ST-NIC
does not strip or append pad bytes for short packets,
or check for oversize packets.
FCS FIELD
The Frame Check Sequence (FCS) is a 32-bit CRC field
calculated and appended to a packet during transmission to
allow detection of errors when a packet is received. During
reception, error free packets result in a specific pattern in
the CRC generator. Packets with improper CRC will be re-
jected. The AUTODIN II (X
32
a
X
26
a
X
23
a
X
22
a
X
16
a
X
12
a
X
11
a
X
10
a
X
8
a
X
7
a
X
5
a
X
4
a
X
2
a
X
1
a
1)
polynomial is used for the CRC calculations.
6.0 Direct Memory Access
Control (DMA)
The DMA capabilities of the ST-NIC greatly simplify the use
of the DP83902A in typical configurations. The local DMA
channel transfers data between the FIFO and memory. On
transmission, the packet is DMAed from memory to the
FIFO in bursts. Should a collision occur (up to 15 times), the
packet is retransmitted with no processor intervention. On
reception, packets are DMAed from the FIFO to the receive
buffer ring (as explained below).
dA remote DMA channel is also provided on the ST-NIC to
accomplish transfers between a buffer memory and system
memory. The two DMA channels can alternatively be com-
bined to form a single 32-bit address with 8- or 16-bit data.
DUAL DMA CONFIGURATION
An example configuration using both the local and remote
DMA channels is shown below. Network activity is isolated
on a local bus, where the ST-NIC’s local DMA channel per-
forms burst transfers between the buffer memory and the
ST-NIC’s FIFO. The Remote DMA transfers data between
the buffer memory and the host memory via a bidirectional
I/O port. The Remote DMA provides local addressing capa-
bility and is used as a slave DMA by the host. Host side
addressing must be provided by a host DMA or the CPU.
The ST-NIC allows Local and Remote DMA operations to
be interleaved.
SINGLE CHANNEL DMA OPERATION
If desirable, the two DMA channels can be combined to
provide a 32-bit DMA address. The upper 16 bits of the
32-bit address are static and are used to point to a 64 kbyte
(or 32k word) page of memory where packets are to be
received and transmitted.
Dual Bus System
TL/F/11157 – 8
13
Obsolete