COM20051 Datasheet, PDF (51/82 Pages) List of Unclassifed Manufacturers – Integrated Microcontroller and ARCNET (ANSI 878.1) Interface

network. Once this node finds an ID and joins
the network, it brings its ID line low to allow the
next node to join. This process continues until all
nodes have joined the network. Figure 16 shows
an example of how this is accomplished.
Method 4 - Duplicate ID Software Search
The Duplicate ID software search is a software
only method of locating available node ID values
on a given network. The basic algorithm
involves a sequential search of ID values and
checking their availability by using a combination
of diagnostic bits located within the ARCNET
core of the COM20051. This method is not well
suited for the initial power-up sequence of a
network since there are instances in which the
algorithm will assign the same ID to two nodes
when they power-up simultaneously. This
occurs due to basic constraints imposed by the
ARCNET protocol itself. The Duplicate ID
algorthim is very useful for adding nodes to
already existing networks that use a mix of node
ID selection methods. Please refer to the section
on Duplicate ID Detection for details regarding
the algorithm.
Entering the Network
After establishing a unique Node ID, the
COM20051 can join the network. Prior to
actually joining the network, several diagnostic
bits should be checked to ensure that the node is
properly functioning and that the network is
functioning properly. The DUPID, RCVACT, and
TOKEN bits should all be checked to ensure that
the network is operating properly. A read of the
Diagnostic Status register should be done to
clear previous data prior to checking the status
bits. This will ensure that the Diagnostic Status
bits are reflecting the latest status of the network.
A period of time lasting 840ms should elapse
prior to checking the status bits. The 840ms
period is a worst case situation based on a full
255 node network with every node transmitting a
512 byte packet. The actual wait time will be
less for most systems and should be calculated
based on the number of nodes and maximum
packet size. Once the wait period is over, the
Diagnostic Status bits should read as follows:
DUPID = 0, RCVACT = 1, and TOKEN = 1. Any
other combination indicates that a problem exists
on the network. Once a valid status condition is
established, the TXEN bit of the Configuration
Regsiter should be programmed to a '1' to allow
the transmitter output to drive the network. The
node is now a member of the network.
Initializing Receptions and Interrupt Masking
Normally the receiver is enabled immediately
after joining the network. Since a node never will
know when a packet will be transmitted to it, it is
advisable to have the receiver enabled at the
earliest time possible. Generally, Enable to
Receive commands are given at the end of the
initialization. For systems using the Command
Chaining feature, two receive commands should
be given. For Non-Command Chaining systems,
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