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9
When any COM20020 senses an idle line for
greater than 82µS, which occurs only when the
token is lost, each COM20020 starts an internal
timeout equal to 146µs times the quantity 255
minus its own ID. The COM20020 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
COM20020 starts sending INVITATION TO
TRANSMIT with the Destination ID (DID) equal
to the currently stored NID. Within a given
network, only one COM20020 will timeout (the
one with the highest ID number). After sending
the INVITATION TO TRANSMIT, the
COM20020 waits for activity on the line. If there
is no activity for 74.7µS, the COM20020
increments the NID value and transmits another
INVITATION TO TRANSMIT using the NID
equal to the DID. If activity appears before the
74.7µS timeout expires, the COM20020 releases
control of the line. During NETWORK
RECONFIGURATION, INVITATIONS TO
TRANSMIT are sent to all NIDs (1-255).
Each COM20020 on the network will finally have
saved a NID value equal to the ID of the
COM20020 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 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 8 illustrates the
position of each byte in the packet with the DID
residing at address 0X01 or 1 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 (see Table 6) to
a logic "0".
EXTENDED TIMEOUT FUNCTION
There are three timeouts associated with the
COM20020 operation. The values of these
timeouts are controlled by bits 3 and 4 of the
Configuration Register and bit 5 of the Setup
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 COM20020 to start sending a
message in response to a received message)
which is approximately 12.7µS. 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 31µS translates to a distance of about 4
miles. The flow chart in Figure 1 uses a value
of 74.7µS (31 + 31 + 12.7) to determine if any
node will respond.