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8
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
COM20020's internal microcoded sequencer. A
processor or intelligent peripheral transmits data
by simply loading a data packet and its
destination ID into the COM20020's internal
RAM buffer, and issuing a command to enable
the transmitter. When the COM20020 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 COM20020 to generate an
interrupt to the processor when selected status
bits become true. Figure 1 is a flow chart
illustrating the internal operation of the
COM20020 connected to a 20 MHz crystal
oscillator.
DATA RATES
The COM20020 is capable of supporting data
rates from 156.25 Kbps to 2.5 Mbps. The
following protocol description assumes a 2.5
Mbps data rate. For slower data rates, an
internal clock divider scales down the clock
frequency. Thus all timeout values are scaled
up as shown in the following table:
Example: IDLE LINE Timeout @ 2.5 Mbps = 82
µs. IDLE LINE Timeout for 156.2 Kbps is 82 µs
* 16 = 1.3 ms
NETWORK RECONFIGURATION
A significant advantage of the COM20020 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 COM20020 is turned on (creating a new
active node on the network), or if the
COM20020 has not received an INVITATION
TO TRANSMIT for 840mS, or if a software reset
occurs, the COM20020 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.
CLOCK
PRESCALER
DATA RATE
W/20MHz
XTAL
TIMEOUT
SCALING
FACTOR
(MULTIPLY BY)
÷8
÷16
÷32
÷64
÷128
2.5 Mbps
1.25 Mbps
625 Kbps
312.5 Kbps
156.25 Kbps
1
2
4
8
16