12
12
PROTOCOL DESCRIPTION
NETWORK PROTOCOL
Communication on the network is based on a
token passing protocol.
Establishment of the
network configuration and management of the
network protocol are handled entirely by the
COM20051's internal microcoded sequencer
called ARCNET network core.
The 80C32
controller core transmits data by simply loading
a data packet and its destination ID into the
network core's RAM buffer, and issuing a
command to enable the transmitter. When the
ARCNET core next receives the token, it verifies
that the receiving node is ready by first
transmitting
a
FREE
BUFFER
ENQUIRY
message.
If the receiving node transmits an
ACKnowledge message, the data packet is
transmitted followed by a 16-bit CRC.
If the
receiving
node
cannot
accept
the
packet
(typically its receiver is inhibited), it transmits a
Negative
AcKnowledge
message
and
the
transmitter passes the token. Once it has been
established that the receiving node can accept
the packet and transmission is complete, the
receiving node verifies the packet. If the packet
is received successfully, the receiving node
transmits an ACKnowledge message (or nothing
if it is not received successfully) allowing the
transmitter to set the appropriate status bits to
indicate successful or unsuccessful delivery of
the packet.
An interrupt mask permits the
ARCNET core to generate an interrupt to the
processor when selected status bits become
true.
Figure 4 is a flow chart illustrating the
internal operation of the ARCNET core.
All
timing details in the discussion of ARCNET
protocol are based on the 2.5 Mbps data rate.
DATA RATES
The ARCNET core is capable of supporting data
rates from 156.25 Kbps to 5 Mbps. For slower or
faster data rates, an internal Programmable
clock
divider
scales
down
the
clock
frequency.
Thus all timeout values are scaled
up as shown in the following table:
DATA
RATE
TIMEOUT SCALING
FACTOR
(MULTIPLY BY)
CLOCK
PRE-
SCALER
40 MHz
CLOCK DIV.
TO 20 MHz
40 MHz UN-
DIVIDED
40 MHz
CLOCK DIV.
TO 20 MHz
40 MHz
UN-
DIVIDED
÷8
÷16
÷32
÷64
÷128
25Mbps
1.25Mbps
625Kbps
3125Kbps
156.25
Kbps
5Mbps
25Mbps
1.25Mbps
625Kbps
3125Kbbps
1
2
4
8
16
.5
1
2
4
8
NETWORK RECONFIGURATION
A significant advantage of the ARCNET 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 ARCNET node is
turned on (creating a new active node on the
network), or if the COM20051 has not received
an INVITATION TO TRANSMIT for 840ms, or if
a software reset occurs, the ARCNET node
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 ARCNET node senses an idle line for
greater than 82 S, which occurs only when the
token is lost, it starts an internal timeout equal to
146 s times the quantity 255 minus its own ID.
The COM20051 starts network reconfiguration
by sending an invitation to transmit
(TOKEN)
first
to
itself
and
then
to
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