COM20051 Datasheet, PDF (53/82 Pages) List of Unclassifed Manufacturers – Integrated Microcontroller and ARCNET (ANSI 878.1) Interface

a single Enable to Receive command should be
issued.
Interrupt Masking is a critical function in Non-
Command Chaining systems. The interrupt mask
is critical because it provides the only method of
releasing interrupts in Non-Command Chaining
systems. For Non-Command Chaining systems,
the RI interrupt should be unmasked right after
the Enable to Receive commands to ensure that
an interrupt will be generated. Note that the
interrupt must be masked to release the interrupt
once the packet has been received. For
Command Chaining systems, the Receive
Interrupt mask should be enabled and left
enabled prior to the first Enable to Receive
Commands. The Command Chaining mode
utilizes a software Clear Receive Interrupt
command to release the interrupt thus the
interrupt can be left unmasked.
DUPLICATE ID DETECTION (AUTO NODE ID
SELECTION)
The Duplicate ID algorithm was introduced
previously as a software only method of
obtaining unique ID values to join the network.
The algorithm uses many of the diagnostic
features found in the COM20051 including the
RECON, RCVACT, TOKEN, DUPID, and
MYRECON diagnostic bits to locate a unique ID
value. Caution should be used when using this
algorithm in that it cannot isolate a unique value
when two nodes power-on simulatneously or
within close proximity of each other. Two nodes
using the same ID value will cause the network
to fail.
The basic algorithm operates as follows: The
COM20051's Node ID is temporaily initialized to
FEh. A 52us waiting period is then entered to
allow the core to read the new node ID value.
This is necessary to allow time for the ARCNET
microsequencer to read the value from the
ARCNET Node register. A one second timing
loop
is then entered in which
several diagnostic bits are sampled in order to
determine if the selected node ID is being used
or not. The first determination made is if the
RCVACT (RECEIVER ACTIVITY) and the
TOKEN bits are set. If both are reset, this node
is most likely to be the first node to join the
network. If one of these bits is reset, then a
timing loop is entered and the bits are sampled
again. Once both bits are found to be set then
the DUPID bit is sampled. If this bit is set then
the current node ID is decremented and the
timing loop is restarted. If the DUPID bit is reset
then the program continues in the timing loop
and the RECON bit tested after the loop is exited
to determine if a RECON had occurred during
the sampling of the diagnostic status bits. If the
RECON bit is reset then another loop is entered
to synchronize the COM20051 with its Token
Rotation timer in order to prevent multiple nodes
from using the same Node ID. This loop
samples the MYRECON bit which only gets set
when the node has not seen a token to itself for
840ms. While the program is sampling the
MYRECON bit, the DUPID bit is checked to
prevent two nodes which have reconfigured
close together from using the same ID. If the
DUPID bit is set before the MYRECON bit is set
then another node ID must be found. Once the
MYRECON bit is found to be set then the
TOKEN bit is checked again to make sure that
another node has not RECONed just prior to this
node. If all these conditions are satisfied then
the TXEN bit of the Configuration Register is set
and a transmission is sent out to the same Node
ID. The TA and TMA bits are polled for 840ms. If
the TMA bit is sampled as set within 840ms then
the node ID is not valid and the selection process
starts again. In the event that the MYRECON bit
is not found to be set, an 840ms timer has been
incorporated into the polling loop to account for
this. The only time the MYRECON bit will not get
set is when there is a single node network.
Once a valid node ID has been located then the
transmitter is enabled and the initialization
process is completed.
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