COM20051ILJP datasheet(13/74 Pages) SMSC

13 / 74 page

SMSC DS – COM20051I

Page 13

Rev. 03/27/2000

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 COM20051I's internal microcoded sequencer

called ARCNET network core. 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. All timing details in the discussion of ARCNET protocol are based on the 2.5 Mbps

data rate.

DATA RATES

The ARCNET core is capable of supporting data rates from 156.25 Kbps to 5 Mbps. For slower or faster data rates,

an internal Programmable clock divider scales down the clock frequency. Thus all timeout values are scaled up

as shown in the following table:

DATA

RATE

TIMEOUT SCALING

FACTOR

(MULTIPLY BY)

CLOCK

PRE-

SCALER

40 MHz CLOCK

DIV. TO 20 MHz

40 MHz UN-

DIVIDED

40 MHz CLOCK

DIV. TO 20 MHz

40 MHz UN-

DIVIDED

!16

!32

!64

!128

25Mbps

1.25Mbps

625Kbps

3125Kbps

156.25

Kbps

5Mbps

25Mbps

1.25Mbps

625Kbps

3125Kbbps

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 COM20051I 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, it starts

an internal timeout

equal to 146 s times the quantity 255 minus its own ID.

The COM20051I starts network

reconfiguration by sending an invitation to transmit

(TOKEN) first

to itself and then to all other nodes by

incrementing the destination Node ID value. If the timeout expires with no line activity, the ARCNET core starts

sending INVITATION TO TRANSMIT with the Destination ID (DID) equal to the currently stored NID. Within a given

network, only one node will timeout (the one with the highest ID number).

After sending the INVITATION TO

TRANSMIT, the COM20051I waits for activity on the line.

If there is no activity for 74.7 S, the COM20051I

increments the NID value and transmits another INVITATION TO TRANSMIT using the NID equal to the DID. If

activity appears before the 74.7 S timeout expires, the COM20051I releases control of the line. During NETWORK

RECONFIGURATION, INVITATIONS TO TRANSMIT are sent to all NIDs (1-255).

COM20051ILJP Datasheet(PDF) 13 Page - SMSC Corporation

COM20051ILJP Datasheet(HTML) 13 Page - SMSC Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74

Datasheet Download

Link URL