COM20019 Datasheet, PDF (11/81 Pages) SMSC Corporation – Low Cost ARCNET (ANSI 878.1) Controller with 2K x 8 On-Board RAM

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
COM20019 to start sending a message in
response to a received message) which is
approximately 101.6 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 248 S translates
to a distance of about 32 miles. The flow chart in
Figure 1 uses a value of 597.6 S (248 + 248 +
101.6) to determine if any node will respond.
Idle Time
The Idle Time is associated with a NETWORK
RECONFIGURATION. Figure 1 illustrates 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 656 S. This 656
S is equal to the Response Time of 597.6 S
plus the time it takes the COM20019 to start
retransmitting another message (usually another
INVITATION TO TRANSMIT).
Reconfiguration Time
If any node does not receive the token within the
Reconfiguration Time, the node will initiate a
NETWORK RECONFIGURATION. The ET2 and
ET1 bits of the Configuration Register allow the
network to operate over longer distances than the
32 miles stated earlier.
The
logic levels on these bits control the maximum
distances over which the COM20019 can operate
by controlling the three timeout values described
above. For proper network operation, all
COM20019's 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. On a
312.5 Kbps network, each byte takes exactly 11
clock intervals of 3.2 S each. As a result, one
byte is transmitted every 35.2 S and the 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 1.6 S
duration. A transmission starts with an ALERT
BURST consisting of 6 unit intervals of mark (logic
"1"). Eight bit data characters are then sent, with
each character preceded by 2 unit intervals of
mark and one unit interval of space. Five types of
transmission can be performed as described
below:
Invitations To Transmit
An Invitation To Transmit is used to pass the
token from one node to another and is sent by the
following sequence:
An ALERT BURST
An EOT (End Of Transmission: ASCII code
04H)
Two (repeated) DID (Destination ID) characters
ALERT
EOT
DID
DID
BURST
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