COM20051I Datasheet, PDF (50/74 Pages) SMSC Corporation – Integrated Microcontroller and ARCNET (ANSI 878.1) Interface

DUPID Algorithm Exception Handling
There are two cases that require special processing that the normal Duplicate ID algorithm does not handle. The first
case is when the node is the first node to join the network. The second case is when the node is the second node to
join the network.
When a node is the first to join a network it will detect no receive activity and detect no tokens (i.e. RCVACT = 0 and
TOKEN = 0). If this is the case, a special routine is entered that monitors the MYRECON bits and constantly polls the
RCVACT, TOKEN, and DUPID bits. If any of the RCVACT, TOKEN, or DUPID bits gets set prior to the MYRECON
bit, then another node has entered the network just prior to this one. If the MYRECON bit sets and the RCVACT,
TOKEN, and DUPID bits remain reset then the node takes the ID value of FFh. Node ID FFh is reserved for the first
node to join the network.
The second case involves a node that is the second node to join the network. In this case the node will detect the
presence of tokens and receive activity (RCVACT & TOKEN = 1), and no duplicate IDs (DUPID = 0). The algorithm
will detect that RECON bit has set. This is because the only existing node on the network is undergoing constant
reconfigurations because it cannot pass tokens to another node. In order to prevent an endless loop from occuring
due to the RECON test failure, a count of RECON bit tests is kept. If three consecutive polls of the RECON bit are
positive it can be safely assumed that this node is the second node to join the network and takes the ID value of FEh.
Basic Transmit and Receive Service Routines
Transmit and Receive Service routines are often found together as part of an single interrupt service routine used to
service the ARCNET interrupt. The COM20051I provides a 1Kx8 RAM for buffering of both transmit and receive
packets and accommodates two types of packet formats: A short format which takes a maximum of 253 bytes of data
and a long format which takes a maximum of 507 bytes of data. Most industrial/embedded applications use the short
(256 byte) format, thus allowing the RAM to buffer four packets. The following examples will use a two packet buffer
for receptions and a two packet buffer for transmissions. Examples are shown for both Command and Non-
Command Chaining systems. Figure 18 shows how a typical program will run.
Upon entry into the service routine all relevant 8051 registers are pushed onto the stack including the PSW and
DPTR. The state of the COM20051I address registers is also saved so that any procedures that were interrupted in
the process of accessing the COM20051I RAM are not corrupted. The COM20051I status register is then read and
stored in a variable. A series of case statements then check the bit settings and branch to the appropriate service
routine.
Service Priority
The ARCNET core of the COM20051I can generate a single interrupt from multiple sources including reception of
packets, transmission of packets, EXcessive NACK counts, RECONfigurations, and New Next ID generation. When
testing status bits to determine the source of the interrupt it is best to use the following priority:
1) RI - Receiver Inhibited (packet received)
2) TA - Transmitter Available (packet transmitted)
3) EXNACK - Excessive Number of NACK's to FBE's
4) RECON - either a full or mini RECON was detected
5) New Next ID - the node to which this node is passing the token to has not responded to the token pass and this
node has found a new id to pass the token to.
SMSC DS – COM20051I
Page 50
Rev. 03/27/2000