Cost Competitive ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip
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 3V's
internal microcoded sequencer. A processor or intelligent peripheral transmits data by simply loading a
data packet and its destination ID into the COM20019I 3V's internal RAM buffer, and issuing a command
to enable the transmitter. When the COM20019I 3V 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 3V to generate an interrupt to the processor when
selected status bits become true. Figure 3.1 - COM20019I 3V OPERATION is a flow chart illustrating the
internal operation of the COM20019I 3V connected to a 20 MHz crystal oscillator.
4.2
DATA RATES
The COM20019I 3V 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
TIMEOUT SCALING FACTOR
(MULTIPLY BY)
CLOCK PRESCALER
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 3V 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 3V is turned on (creating a new active node on the network), or if the COM20019I 3V has not
received an INVITATION TO TRANSMIT for 6.72S, or if a software reset occurs, the COM20019I 3V
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 3V senses an idle line for greater than 656μS, which occurs only when the token Is
lost, each COM20019I 3V starts an internal timeout equal to 1.168mS times the quantity 255 minus its own
ID. The COM20019I 3V starts network reconfiguration by sending an invitation to transmit first to itself and
Rev. 10-31-06
Page 12
SMSC COM20019I 3.3V Rev.C
DATASHEET
SMSC [ SMSC CORPORATION ]
