COM20051 Просмотр технического описания (PDF) - SMSC -> Microchip

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COM20051

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

SMSC -> Microchip 

stored NID (next ID representing the next

possible ID node) equal to its own ID. 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 COM20051+

waits for activity on the line. If there is no

activity for 74.7 S, the COM20051+ 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 COM20051+

releases control of the line. During NETWORK

RECONFIGURATION, INVITATIONS TO

TRANSMIT are sent to all NIDs.

Each COM20051+ on the network will finally

have saved a NID value equal to the ID of the

ARCNET node that it released control to. This

is called the Next ID Value. At this point, control

is passed directly from one node to the next with

no wasted INVITATIONS TO TRANSMIT being

sent to ID's not on the network, until the next

NETWORK RECONFIGURATION occurs.

When a node is powered off, the previous node

attempts to pass the token to it by issuing an

INVITATION TO TRANSMIT. Since this node

does not respond, the previous node times out

and transmits another INVITATION TO

TRANSMIT to an incremented ID and eventually

a response will be received.

The NETWORK RECONFIGURATION time

depends on the number of nodes in the network,

the propagation delay between nodes, and the

highest ID number on the network, but is

typically within the range of 24 to 61 ms for 2.5

Mbps operation.

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 13 illustrates the

position of each byte in the packet with the DID

residing at address 1H of the current page

selected in the "Enable Transmit from Page fnn"

command. Each individual node has the ability

to ignore broadcast messages by setting the

most significant bit of the "Enable Receive to

Page fnn" command (see Table 8) to a logic "0".

EXTENDED TIMEOUT FUNCTION

There are three timeouts associated with the

COM20051+ operation. The values of these

timeouts are controlled by bits 3 and 4 of the

Configuration Register.

Response Time

The Response Time determines the maximum

propagation delay allowed between any two

nodes, and should be chosen to be larger than

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 ARCNET node to start sending a

message in response to a received message)

which is approximately 12.7 S. The round trip

propagation delay is a function of 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 S translates to a distance of about 4

miles. The flow chart in Figure 4 uses a value

of 74.7 S (31 + 31 + 12.7) to determine if any

node will respond.

Idle Time

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

The Idle Time is associated with a NETWORK

RECONFIGURATION. Figure 4 illustrates that

during a NETWORK RECONFIGURATION one

node will continually transmit INVITATIONS TO

TRANSMIT until it encounters an active node.

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