DP83902A datasheet(13/70 Pages) NSC

13 / 70 page

50 TransmitReceive Packet

EncapsulationDecapsulation

(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

LENGTHTYPE 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 (X32 a X26 a X23 a X22 a X16 a

X12 a X11 a X10 a X8 a X7 a X5 a X4 a X2 a X1 a 1)

polynomial is used for the CRC calculations

60 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

IO 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

TLF11157 – 8

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