Sysmac Link System Communications - Omron CS1W-SLK11-21 - 06-2004 Manual

SYSMAC LINK System Communications

3-1

SYSMAC LINK System Communications

Token Bus
Communications
Polling Node
Startup Node
Communications Cycle
34
SYSMAC LINK Units communicate through the SYSMAC LINK Network, a
token bus LAN. This section describes the token bus communications used in
the SYSMAC LINK Network.
In token bus communications, individual nodes are connected to a main trunk
line as a common bus line, as shown in the figure below.
To avoid having two nodes attempt to transmit simultaneously, only one node
has the right to transmit at a time. The right to transmit is held by the node that
holds what is called the token, which is passed in sequence from node to
node, beginning with the node with the lowest node address (the polling node,
see below).
If the token is passed to a node that does not have data to transmit, the token
is immediately passed to the node with the next higher node address. The
node with the highest node address passes the token back to the node with
the lowest node address. In this way, each node in the network has its turn to
transmit data without interfering with other nodes.
Node
Connecting cable
(trunk line)
Node
In a SYSMAC LINK Network there is one node, called the polling node, that
controls communications in the network. Normally, the Unit with the lowest
node address is the polling node. If the polling node fails, the node with the
next higher node address automatically becomes the polling node, preventing
the whole network from failing.
The node that starts the data link in a SYSMAC LINK Network is called the
startup node. The data link operates using the parameter settings in the DM
Area at the startup node. With automatic allocation of link words, the DM
parameter area settings are used, and with manual allocation, the common
link parameters in the startup node are used.
The token in a SYSMAC LINK Network is first passed from the polling node.
The token is passed in sequence from node to node by node address until it is
finally returned to the polling node. One complete circuit around the network is
known as a token cycle.
At the end of each token cycle, the polling node polls all of the nodes in the
network to check whether any nodes have been added to or removed from the
network. The updated information on the network is transmitted to all nodes.
This polling and updating process is known as the polling cycle.
When the control cycle is completed, the polling node once again passes the
token, beginning the next token cycle. Together the token cycle and control
Node Node
Node
Highest node address
Section 3-1
Lowest node address
Node
Node
Control
sequence