COM20051I Datasheet, PDF (13/74 Pages) SMSC Corporation – Integrated Microcontroller and ARCNET (ANSI 878.1) Interface

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 COM20051I's internal microcoded sequencer
called ARCNET network core. The 80C32 controller core transmits data by simply loading a data packet and its
destination ID into the network core's RAM buffer, and issuing a command to enable the transmitter. When the
ARCNET core 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 ARCNET core to generate an
interrupt to the processor when selected status bits become true. Figure 4 is a flow chart illustrating the internal
operation of the ARCNET core. All timing details in the discussion of ARCNET protocol are based on the 2.5 Mbps
data rate.
DATA RATES
The ARCNET core is capable of supporting data rates from 156.25 Kbps to 5 Mbps. For slower or faster data rates,
an internal Programmable clock divider scales down the clock frequency. Thus all timeout values are scaled up
as shown in the following table:
CLOCK
PRE-
SCALER
!8
!16
!32
!64
!128
DATA
RATE
40 MHz CLOCK
DIV. TO 20 MHz
40 MHz UN-
DIVIDED
25Mbps
1.25Mbps
625Kbps
3125Kbps
156.25
Kbps
5Mbps
25Mbps
1.25Mbps
625Kbps
3125Kbbps
TIMEOUT SCALING
FACTOR
(MULTIPLY BY)
40 MHz CLOCK
DIV. TO 20 MHz
40 MHz UN-
DIVIDED
1
.5
2
1
4
2
8
4
16
8
NETWORK RECONFIGURATION
A significant advantage of the ARCNET 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 ARCNET node is turned on
(creating a new active node on the network), or if the COM20051I has not received an INVITATION TO TRANSMIT
for 840ms, or if a software reset occurs, the ARCNET node 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 ARCNET node senses an idle line for greater than 82 S, which occurs only when the token is lost, it starts
an internal timeout equal to 146 s times the quantity 255 minus its own ID. The COM20051I starts network
reconfiguration by sending an invitation to transmit (TOKEN) first to itself and then to all other nodes by
incrementing the destination Node ID value. If the timeout expires with no line activity, the ARCNET core starts
sending INVITATION TO TRANSMIT with the Destination ID (DID) equal to the currently stored NID. Within a given
network, only one node will timeout (the one with the highest ID number). After sending the INVITATION TO
TRANSMIT, the COM20051I waits for activity on the line. If there is no activity for 74.7 S, the COM20051I
increments the NID value and transmits another INVITATION TO TRANSMIT using the NID equal to the DID. If
activity appears before the 74.7 S timeout expires, the COM20051I releases control of the line. During NETWORK
RECONFIGURATION, INVITATIONS TO TRANSMIT are sent to all NIDs (1-255).
SMSC DS – COM20051I
Page 13
Rev. 03/27/2000