Low Cost ARCNET (ANSI 878.1) Controller with 2K x 8 On-Board RAM
Chapter 4 PROTOCOL DESCRIPTION
4.1
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 COM20019I's internal
microcoded sequencer. A processor or intelligent peripheral transmits data by simply loading a data packet
and its destination ID into the COM20019I's internal RAM buffer, and issuing a command to enable the
transmitter. When the COM20019I 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 COM20019I to generate an interrupt to the processor when
selected status bits become true. Figure 3.1 - COM20019I OPERATION is a flow chart illustrating the
internal operation of the COM20019I connected to a 20 MHz crystal oscillator.
4.2
DATA RATES
The COM20019I is capable of supporting data rates from 156.25 Kbps to 312.5 Kbps. The following
protocol description assumes a 312.5 Kbps data rate. For slower data rates, an internal clock divider
scales down the clock frequency. Thus all timeout values are scaled as shown in the following table:
Example: IDLE LINE Timeout @ 312.5 Kbps = 656 μs. IDLE LINE Timeout for 156.2 Kbps is 656 μs * 2 =
1.3 ms
INTERNAL CLOCK
FREQUENCY
CLOCK
PRESCALER
TIMEOUT SCALING FACTOR
(MULTIPLY BY)
DATA RATE
20 MHz
Div. by 64
Div. by 128
312.5 Kbps
156.25 Kbps
1
2
4.3
NETWORK RECONFIGURATION
A significant advantage of the COM20019I 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
COM20019I is turned on (creating a new active node on the network), or if the COM20019I has not
received an INVITATION TO TRANSMIT for 6.72S, or if a software reset occurs, the COM20019I 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 COM20019I senses an idle line for greater than 656μS, which occurs only when the token Is
lost, each COM20019I starts an internal timeout equal to 1.168mS times the quantity 255 minus its own
ID. The COM20019I starts network reconfiguration by sending an invitation to transmit first to itself and
then to all other nodes by decrementing the destination Node ID. If the timeout expires with no line
SMSC COM20019I
Page 11
Rev. 03-07-06
DATASHEET
SMSC [ SMSC CORPORATION ]
