COM20020D datasheet(8/72 Pages) SMSC

8 / 72 page

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 the

COM20020D's internal microcoded sequencer. A

processor or intelligent peripheral transmits data

by simply loading a data packet and its destination

ID into the COM20020D's internal RAM buffer,

and issuing a command to enable the transmitter.

When the COM20020D next receives the token, it

verifies that the receiving node is ready by first

transmitting

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

indicate

successful or unsuccessful delivery of the packet.

An interrupt mask permits the COM20020D to

generate an interrupt to the processor when

selected status bits become true. Figure 1 is a

flow chart illustrating the internal operation of the

COM20020D connected to a 20 MHz crystal

oscillator.

DATA RATES

The COM20020D is capable of supporting data

rates from 156.25 Kbps to 5 Mbps. The following

protocol description assumes a 5 Mbps data rate.

To attain the faster data rates, the clock frequency

may be doubled by the internal clock multiplier

(see next section). For slower data rates, an

internal clock divider scales down the clock

frequency. Thus all timeout values are scaled as

shown in the following table:

Example: IDLE LINE Timeout @ 5 Mbps = 41

µs.

IDLE LINE Timeout for 156.2 Kbps is 41

µs * 32 =

1.3 ms

INTERNAL

CLOCK

FREQUENCY

CLOCK

PRESCALER

DATA RATE

TIMEOUT SCALING

FACTOR (MULTIPLY BY)

40 MHz

Div. by 8

5 Mbps

20 MHz

Div. by 8

Div. by 16

Div. by 32

Div. by 64

Div. by 128

2.5 Mbps

1.25 Mbps

625 Kbps

312.5 Kbps

156.25 Kbps

COM20020D датащи(PDF) 8 Page - SMSC Corporation

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