14
14
all other nodes by incrementing the destination
Node ID value.
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 (1-255).
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.
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
COM20051 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, ET3)
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, ET3)
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
COM20051
to
start
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