TMC2074-NE SMSC, TMC2074-NE Datasheet - Page 26

• Manufacturer Part Number: TMC2074-NE
• Description: Network Controller & Processor ICs Standalone Mode CircLink Cntlr
• Manufacturer: SMSC

Specifications of TMC2074-NE

• Product: Controller Area Network (CAN)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

2.3
Revision 0.2 (10-23-08)
CircLink Network Communication Protocol Overview
CircLink Protocol is derived from the ARCNET protocol. This section explains the ARCNET basic
communication protocol.
A token (ITT: Invitation to Transmit) is a unique signaling sequence that is passed in an orderly fashion
among all the active nodes in the network. when a particular node receives the token, it has the sole right
to initiate a transmission sequence or it must pass the token to it’s logical neighbor. This neighbor can be
physically located anywhere on the network and has the 2nd highest address. Once the token is passed to
the recipient, it has the right to initiate transmission. This token-passing sequence continues in a logical
ring fashion serving all nodes equally. Node addresses must be unique and can range from 0 – 255 with 0
reserved for broadcast messages. In a transmission sequence the node with the token becomes the
source node and any other node selected becomes the destination node. First the source node inquires if
the destination node is in a mode to receive a transmission by sending out a free buffer enquiry (FBE). The
destination node responds by returning an Acknowledgement (ACK) meaning that the buffer is available or
by returning a negative Acknowledgement (NAK) meaning that no buffer is available. Upon receiving the
ACK, the source node sends out the data transmission (PAC) with either 0 – 507 bytes of data (PAC). If
the data was properly received by the destination node as evidenced by a successful CRC test, the
destination node sends another ACK. If the transmission was unsuccessful, the destination node does
nothing causing the source node to timeout. The source node will therefore, infer that the transmission
failed and will retry after it receives the token on the next token pass. The transmission sequence
terminates and the token is passed to the next node. If the desired message exceeds 507 bytes the
message is sent in a series of packets-one packet every token pass.
The ARCNET protocol comprises the reconfiguration process to ensure the complete token passing for
every node linked to the network.
ARCNET has the ability to reconfigure the network automatically if a node is either added or removed from
the network. If a node joins the network it does not automatically participate in the token passing
sequence. Being excluded from receiving the token, the new node will generate a reconfiguration burst that
destroys the token passing sequence. Once the token is lost all nodes will cease transmitting and begin a
timeout sequence (Priority Timer, (255-ID) x 146 μs ,based on their own node address. The node (Node
ID=N) with the highest address will timeout first and pass the token to the next higher address (Node
ID=N+1). If that node does not respond, it is assumed that node does not exist. Then the node address is
incremented (Node ID=N+2) and the token resent. This process is repeated until a node responds. At that
time the token is released to the responding node and the address of the responding node is noted as the
logical neighbor of the originating node. This process is repeated by all nodes until each node learns its
logical neighbor. This eliminates wasting time in sending datagrams to absent addresses once the network
has been re-established.
When a node leaves the network the reconfiguration process is slightly different. When a node releases
the token to its logical neighbor, it expects its logical neighbor will respond within the response time out
window (78 μs) .If no response within the response time out window, it assumes that its neighbor has left
the network and immediately begins a search for a new logical neighbor by incrementing the node address
of its logical neighbor and initiating a token pass. Network activity is again monitored and the increment
process and resending of the token continues until a new logical neighbor is found. Once found the
network returns to the normal logical ring routine of passing token to logical neighbors.
These reconfiguration sequences of the network are automatic and seamless without software intervention
required.
DATASHEET
Page 26
Dual Mode CircLink™ Controller
SMSC TMC2074
Datasheet