COM20051I Datasheet, PDF (15/74 Pages) SMSC Corporation – Integrated Microcontroller and ARCNET (ANSI 878.1) Interface

Each COM20051I 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.
BROADCAST MESSAGES
Broadcasting gives a particular node the ability to transmit a data packet to all nodes on the network simultaneously.
NID=0 is reserved for this feature and no node on the network can be assigned NID=0. To broadcast a message,
the transmitting node's processor simply loads the RAM buffer with the data packet and sets the DID (Destination ID)
equal to zero. Figure 12 illustrates the position of each byte in the packet with the DID residing at address 1Hex 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 COM20051I operation. The values of these timeouts are controlled by
bits 3 and 4 of the Configuration Register and bit 5 of the Setup Register (see register description for details).
Response Time (ET1, ET2)
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 3 uses a value of 74.7 S
(31 + 31 + 12.7) to determine if any node will respond.
Idle Time (ET1, ET2)
The Idle Time is associated with the NETWORK RECONFIGURATION. Figure 3 and Figure 4 illustrate that during a
NETWORK RECONFIGURATION one node will continually transmit INVITATIONS TO TRANSMIT until it
encounters an active node. All other nodes on the network must distinguish between this operation and an entirely
idle line. During NETWORK RECONFIGURATION, activity will appear on the line every 82 S. This 82 S is equal to
the Response Time of 74.7 S plus the time it takes the COM20051I to start retransmitting another message (usually
another INVITATION TO TRANSMIT).
Reconfiguration Time (ET1, ET2)
If any node does not receive the token within the Reconfiguration Time, it will initiate a NETWORK
RECONFIGURATION. The ET2 and ET1 bits of the Configuration Register allow the network to operate over longer
distances than the 4 miles stated earlier. The logic levels on these bits control the maximum distances over which
the COM20051I can operate by controlling the three timeout values described above. For proper network operation,
all nodes connected to the same network must have the same Response Time, Idle Time, and Reconfiguration Time.
LINE PROTOCOL
The ARCNET line protocol is considered isochronous because each byte is preceded by a start interval and ended
with a stop interval. Unlike asynchronous protocols, there is a constant amount of time separating each data byte.
Each byte takes exactly 11 clock intervals that are defined by nPULSE1 and nPULSE2 signals (at 2.5 Mbps one byte
takes 4.4 ms). As a result a time to transmit a message can be precisely determined. The line idles in a spacing
(logic "0") condition. A logic "0" is defined as no line activity and a logic "1" is defined as a negative pulse of 200nS
duration.
SMSC DS – COM20051I
Page 15
Rev. 03/27/2000