COM20051 查看數據表（PDF） - SMSC -> Microchip

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COM20051

Integrated Microcontroller and Network Interface with Watchdog Timer and A/D Converter Interface

SMSC -> Microchip 

PROTOCOL DESCRIPTION

NETWORK PROTOCOL

Communication on the network is based on a

token passing protocol. Establishment of the

network configuration and management of the

network protocol are handled entirely by an

internal microcoded sequencer. The 80C32

controller core transmits data by simply loading

a data packet and its destination ID into the

network core's RAM buffer, and issuing a

command to enable the transmitter. When the

ARCNET core next receives the token, it verifies

that the receiving node is ready by first

transmitting a FREE BUFFER ENQUIRY

message. If the receiving node transmits an

ACKnowledge message, the data packet is

transmitted followed by a 16-bit CRC. If the

receiving node cannot accept the packet

(typically its receiver is inhibited), it transmits a

Negative AcKnowledge message and the

transmitter passes the token. Once it has been

established that the receiving node can accept

the packet and transmission is complete, the

receiving node verifies the packet. If the packet

is received successfully, the receiving node

transmits an ACKnowledge message (or nothing

if it is not received successfully) allowing the

transmitter to set the appropriate status bits to

indicate successful or unsuccessful delivery of

the packet. An interrupt mask permits the

ARCNET core to generate an interrupt to the

processor when selected status bits become

true. Figure 4 is a flow chart illustrating the

internal operation of the ARCNET core.

DATA RATES

The ARCNET core is capable of supporting data

rates from 156.25 Kbps to 5 Mbps. The

following protocol description assumes a 2.5

Mbps data rate. For slower data rates, an

internal clock divider scales down the clock

frequency. Thus all timeout values are scaled

up as shown in the following table:

CLOCK

PRESCALER

÷8

÷16

÷32

÷64

÷128

DATA RATE

W/20MHz

CLOCK

2.5 Mbps

1.25 Mbps

625 Kbps

312.5 Kbps

156.25 Kbps

TIMEOUT

SCALING

FACTOR

(MULTIPY BY)

1

2

4

8

16

Example: IDLE LINE Timeout @ 2.5 Mbps = 82

s. IDLE LINE Timeout for 156.2 Kbps is 82 s *

16 = 1.3 ms

For 5 Mbps operation, all timeouts are scaled

DOWN by two.

NETWORK RECONFIGURATION

A significant advantage of the ARCNET is its

ability to adapt to changes on the network.

Whenever a new node is activated or

deactivated,

a

NETWORK

RECONFIGURATION is performed. When a

new ARCNET node is turned on (creating a

new active node on the network), or if the

COM20051+ has not received an INVITATION

TO TRANSMIT for 840mS, or if a software reset

occurs, the ARCNET node causes a NETWORK

RECONFIGURATION by sending a

RECONFIGURE BURST consisting of eight

marks and one space repeated 765 times. The

purpose of this burst is to terminate all activity

on the network. Since this burst is longer than

any other type of transmission, the burst will

interfere with the next INVITATION TO

TRANSMIT, destroy the token and keep any

other node from assuming control of the line.

When any ARCNET node senses an idle line for

greater than 82 S, which occurs only when the

token is lost, each node starts an internal

timeout equal to 146 s times the quantity 255

minus its own ID. It also sets the internally-

14

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