com20020-5 Standard Microsystems Corp., com20020-5 Datasheet - Page 8

• Manufacturer Part Number: com20020-5
• Description: Com20020-5 Ulanc Universal Local Area Network Controller With 2k X 8 On-board Ram
• Manufacturer: Standard Microsystems Corp.

NETWORK PROTOCOL
Communication on the network is based on a
token passing protocol.
network configuration and management of the
network protocol are handled entirely by the
COM20020-5's internal microcoded sequencer.
A processor or intelligent peripheral transmits
data by simply loading a data packet and its
destination ID into the COM20020-5's internal
RAM buffer, and issuing a command to enable
the transmitter.
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.
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 COM20020-5 to generate an
interrupt to the processor when selected status
bits become true.
illustrating
COM20020-5 connected to a 40 MHz crystal
oscillator.
If the packet is received successfully, the
the
When the COM20020-5 next
internal
Figure 1 is a flow chart
Establishment of the
operation
PROTOCOL DESCRIPTION
of
the
8
DATA RATES
The COM20020-5 is capable of supporting data
rates from 312.5 Kbps
following protocol description assumes a 5
Mbps data rate.
internal clock divider scales down the clock
frequency. Thus all timeout values are scaled
up as shown in the following table:
PRESCALER
Example: IDLE LINE Timeout @ 5 Mbps = 41
* 16 = .66 ms
NETWORK RECONFIGURATION
A significant advantage of the COM20020-5 is
its ability to adapt to changes on the network.
Whenever
deactivated,
RECONFIGURATION is performed.
new COM20020-5 is turned on (creating
new
COM20020-5 has not received an INVITATION
TO TRANSMIT for 420mS, or if a software reset
occurs, the COM20020-5 causes a NETWORK
RECONFIGURATION
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
TRANSMIT, destroy the token and keep any
other node from assuming control of the line.
s. IDLE LINE Timeout for 312.5 Kbps is 41 s
CLOCK
÷128
÷16
÷32
÷64
÷8
active node on the network), or if the
with
a
W/20MHz XTAL
new
DATA RATE
the
312.5 Kbps
1.25 Kbps
625 Kbps
2.5 Mbps
5 Mbps
For slower data rates, an
next
node
a
by
to
INVITATION
is
5 Mbps. The
sending
(MULTIPLY BY)
activated
NETWORK
TIMEOUT
SCALING
FACTOR
When a
16
1
2
4
8
TO
a
or
a