Installation guide
20 Introducing the Modbus Plus Network
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1.6.3 Point to Point Message Transactions
While a node holds the token, it sends its application messages if it has
any to transmit. Each message can contain up to 100 controller
registers (16bit words) of data. The other nodes monitor the network
for incoming messages.
When a node receives a message, it sends an immediate
acknowledgment to the originating node. If the message is a request
for data, the receiving node will begin assembling the requested data
into a reply message. When the message is ready, it will be transmitted
to the requestor when the node receives a subsequent token granting it
access to transmit.
Nodes can also transact messages containing local and remote
operating statistics. These include information such as identification of
active nodes, current software version, network activity, and error
reporting. If a node transmits a request to read statistics in another
node, the entire transaction is completed while the originating node
holds the token. The remote node’s statistics are imbedded in its
acknowledgement. It is not necessary for the remote node to acquire
the token to transmit the statistics.
After a node sends all of its messages, it passes the token on to the next
node. Protocols for token passing and messaging are transparent to the
user application.
1.6.4 Global Database Transactions
When a node passes the token, it can broadcast up to 32 words (16 bits
each) of global information to all other nodes on the network. The
information is contained in the token frame. The process of sending
global data when transmitting the token is controlled independently by
the application program in each node.
The global data is accessible to the application programs at the other
nodes on the same network. Each node maintains a table of global data
sent by every other node on the network. Although only one node
accepts the token pass, all nodes monitor the token transmission and
read its contents. All nodes receive and store the global data into the
table.
The table contains separate areas of storage for each node’s global data.
Each node’s application program can selectively use the global data
from specific nodes, while other applications can ignore the data. Each
node’s application determines when and how to use the global data.