10 Mbps ARCNET (ANSI 878.1) Controller with 2Kx8 On-Board RAM
Datasheet
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 COM20022I 3V's
internal microcoded sequencer. A processor or intelligent peripheral transmits data by simply loading a
data packet and its destination ID into the COM20022I 3V's internal RAM buffer, and issuing a command
to enable the transmitter. When the COM20022I 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 COM20022I 3V to generate an interrupt to the
processor when selected status bits become true. Figure 3.1 is a flow chart illustrating the internal operation
of the COM20022I 3V connected to a 20 MHz crystal oscillator.
4.2
Data Rates
The COM20022I 3V is capable of supporting data rates from 156.25 Kbps to 10 Mbps. The following
protocol description assumes a 10 Mbps data rate. To attain the faster data rates, the clock frequency may
be doubled or quadrupled by the internal clock multiplier (see next section). 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 @ 10 Mbps = 20.5 μs. IDLE LINE Timeout for 156.2 Kbps is 20.5 μs * 64 =
1.3 ms
INTERNAL
CLOCK
FREQUENCY
CLOCK
PRESCALER
TIMEOUT SCALING
FACTOR (MULTIPLY BY)
DATA RATE
80 MHz
40 MHz
20 MHz
Div. by 8
Div. by 8
10 Mbps
5 Mbps
1
2
Div. by 8
2.5 Mbps
4
Div. by 16
Div. by 32
Div. by 64
Div. by 128
1.25 Mbps
625 Kbps
312.5 Kbps
156.25 Kbps
8
16
32
64
4.2.1 Selecting Clock Frequencies Above 2.5 Mbps
To realize a 10 Mbps network, an external 80 MHz clock must be input. However, since 80 MHz is the
frequency of FM radio band, it is not practical for use for noise emission reasons. Therefore, higher
frequency clocks are generated from the 20 MHz crystal as selected through two bits in the Setup2
register, CKUP[1,0] as shown below. The selected clock is supplied to the ARCNET controller.
SMSC COM20022I 3V
Page 11
Revision 02-27-06
DATASHEET
SMSC [ SMSC CORPORATION ]
