COM20019 datasheet(9/81 Pages) SMSC

9 / 81 page

DISCONTINUED DATASHEET

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

COM20019's internal microcoded sequencer. A

processor or intelligent peripheral transmits data

by simply loading a data packet and its destination

ID into the COM20019's internal RAM buffer, and

issuing a command to enable the transmitter.

When the COM20019 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

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 COM20019 to generate an

interrupt to the processor when selected status

bits become true.

Figure 1 is a flow chart

illustrating

the

internal

operation

the

COM20019 connected to a 20 MHz crystal

oscillator.

DATA RATES

The COM20019 is capable of supporting data

rates from 156.25 Kbps to 312.5 Kbps. The

following protocol description assumes a 312.5

Kbps data rate. 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 @ 312.5 Kbps =

656

IDLE LINE Timeout for 156.2 Kbps is

656

s * 2 = 1.3 ms

INTERNAL CLOCK

FREQUENCY

CLOCK

PRESCALER

DATA RATE

TIMEOUT SCALING

FACTOR (MULTIPLY BY)

20 MHz

Div. by 64

Div. by 128

312.5 Kbps

156.25 Kbps

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