15
15
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 COM20051 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.
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 (ITT)
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 ITT (Invitation To Transmit: ASCII code
04H)
• Two (repeated) DID (Destination ID)
characters
ALERT
BURST
ITT
DID
DID
Free Buffer Enquiries (FBE)
A Free Buffer Enquiry is used to ask another
node if it is able to accept a packet of data. It is
sent by the following sequence:
• An ALERT BURST
• An FBE - Free Buffer Enquiry: ASCII code
85H)
• Two (repeated) DID (Destination ID)
characters
ALERT
BURST
FBE
DID
DID
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