COM90C66 datasheet(10/76 Pages) SMSC

10 / 76 page

The

time

required

NETWORK

RECONFIGURATION depends on the number of

nodes in the network, the propagation delay

between nodes, and the highest ID number on

the network, but will be in the range of 24 to 61

mS.

BROADCAST MESSAGES

Broadcasting gives a particular node the ability to

transmit a data packet to all nodes on the

network simultaneously. ID zero is reserved for

this feature and no node on the network can be

assigned ID zero. To broadcast a message, the

transmitting node's processor simply loads the

RAM buffer with the data packet and sets the DID

equal to zero. Figure 9 illustrates the position of

each byte in the packet with the DID residing at

address 01 HEX of the current page selected in

the TRANSMIT command. Each individual node

has the ability to ignore broadcast messages by

setting the most significant bit of the ENABLE

RECEIVE TO PAGE nn command (see Table 7)

to logic "0".

EXTENDED TIMEOUT FUNCTION

There are three timeouts associated with the

COM90C66 operation:

Response Time

The Response Time is equal to the round trip

propagation delay between the two furthest

nodes on the network plus the maximum turn

around time (the time it takes a particular

COM90C66 to start sending a message in

response to a received message), which is

approximately

12.7

µS.

The

round

trip

propagation

delay

function

the

transmission media and network topology. For a

typical system using RG62 coax in a baseband

system, a one-way cable propagation delay of 31

µS translates to a distance of about four miles.

The flow chart in Figure 1 uses a value of 74.7

µS (31 + 31 + 12.7) to determine if any node will

respond.

Idle Time

The Idle Time is associated with a NETWORK

RECONFIGURATION.

Figure 1 illustrates that

during a NETWORK RECONFIGURATION, one

node will continually transmit INVITATIONS TO

TRANSMIT until it encounters an active node.

Every other node on the network must distinguish

between this operation and an entirely idle line.

During

NETWORK

RECONFIGURATION,

activity will appear on the line every 82 µS. This

82 µS is equal to the Response Time of 74.7 µS

plus the time it takes the COM90C66 to

retransmit another message (usually another

INVITATION TO TRANSMIT).

Reconfiguration Time

If any node does not receive the token within the

Reconfiguration Time, the node will initiate a

NETWORK RECONFIGURATION.

The ET2 and ET1 bits (bits 3 and 4 of the

Configuration Register) allow the network to

operate over longer distances than the four miles

stated earlier.

The logic levels on these bits

control the maximum distances over which the

COM90C66 can operate by controlling the three

timeout values described above.

See the

description of the ET1 and ET2 bits, found in

Table

for

the

table

containing

the

combinations of these bits.

It should be noted

that

for

proper

network

operation,

all

COM90C66s connected to the same network

must have the same Response Time, Idle Time,

and Reconfiguration Time.

LINE PROTOCOL

The ARCNET line protocol can be described as

isochronous because each byte is preceded by a

start interval and ended with a stop interval.

Unlike

asynchronous

protocols,

there

constant amount of time separating each data

byte.

Each byte takes up exactly 11 clock

intervals with a single clock interval being 400 nS

duration.

result,

one

byte is

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