Installation guide

Introducing the Modbus Plus Network

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1.3 Overview of the Logical Network

Network nodes are identified by addresses assigned by the user. Each

node’s address is independent of its physical site location. Addresses

are within the range of 1 to 64 decimal, and do not have to be

sequential. Duplicate addresses are not allowed.

Network nodes function as peer members of a logical ring, gaining

access to the network upon receipt of a token frame. The token is a

grouping of bits that is passed in a rotating address sequence from one

node to another. Each network maintains its own token rotation

sequence independently of the other networks. Where multiple

networks are joined by bridges, the token is not passed through the

bridge device.

While holding the token, a node initiates message transactions with

other nodes. Each message contains routing fields that define its

source and destination, including its routing path through bridges to a

node on a remote network.

When passing the token, a node can write into a global database that is

broadcast to all nodes on the network. Global data is transmitted as a

field within the token frame. Other nodes monitor the token pass and

can extract the global data if they have been programmed to do so. Use

of the global database allows rapid updating of alarms, setpoints, and

other data. Each network maintains its own global database, as the

token is not passed through a bridge to another network.