5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
The EF bit in the SETUP2 register must be set when the data rate is over 5 Mbps.
5.3 Network Reconfiguration
A significant advantage of the COM20020I 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 COM20020I is turned on
(creating a new active node on the network), or if the COM20020I has not received an INVITATION TO TRANSMIT
for 420mS, or if a software reset occurs, the COM20020I 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 COM20020I senses an idle line for greater than 41μS, which occurs only when the token Is lost, each
COM20020I starts an internal timeout equal to 73μs times the quantity 255 minus its own ID. The COM20020I starts
network reconfiguration by sending an invitation to transmit first to itself and then to all other nodes by decrementing
the destination Node ID. If the timeout expires with no line activity, the COM20020I starts sending INVITATION TO
TRANSMIT with the Destination ID (DID) equal to the currently stored NID. Within a given network, only one
COM20020I will timeout (the one with the highest ID number). After sending the INVITATION TO TRANSMIT, the
COM20020I waits for activity on the line. If there is no activity for 37.4μS, the COM20020I increments the NID value
and transmits another INVITATION TO TRANSMIT using the NID equal to the DID. If activity appears before the
37.4μS timeout expires, the COM20020I releases control of the line. During NETWORK RECONFIGURATION,
INVITATIONS TO TRANSMIT are sent to all NIDs (1-255).
Each COM20020I on the network will finally have saved a NID value equal to the ID of the COM20020I that it
released control to. 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 12 to 30.5 mS.
5.4 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 4 illustrates the position of each byte in the packet with the DID residing at address 0X01 or
Hex 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
to a logic "0".
5.5 Extended Timeout Function
There are three timeouts associated with the COM20020I operation.
The values of these timeouts are
controlled by bits 3 and 4 of the Configuration Register and bit 5 of the Setup 1 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 COM20020I to start sending a message in response to a
received message) which is approximately 6.4 μS. 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 15.5 μS translates to a distance of about 2 miles. The flow chart in Figure 1 uses a value of 37.4
S (15.5 + 15.5 + 6.4) to determine if any node will respond.
Revision 12-06-06
12
SMSC COM20020I 3.3V
DATASHEET
SMSC [ SMSC CORPORATION ]
