Installation guide

38 Introducing the Modbus Plus Network

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Modbus master devices connected to the Bridge Multiplexer can access

any controller node on the Modbus Plus network, including nodes on

remote networks through Bridge Plus devices. A master device can

also access a slave device connected to another port on the local Bridge

Multiplexer, or one connected to a remote Bridge Multiplexer node on

Modbus Plus.

Modbus slave devices can be accessed by a master at the local Bridge

Multiplexer, or by a master at a remote Bridge Multiplexer node, or by

program functions in your Modbus Plus application.

For example, in Figure 22 the Modbus master devices connected to

ports 1 and 2 of the Bridge Multiplexer can attach to: (a) any controller

on the Modbus Plus network; (b) any slave device on the modem

network at port 3; or (c) the slave controller at port 4. Application

program function blocks in the controllers on Modbus Plus can access:

(a) the slave devices on the modem network at port 3; and (b) the

controller at port 4. Each Modbus port can be configured for the

communication mode (ASCII / RTU) and parameters suitable for its

port devices.

1.13.3 Modbus Port Mapping

Each Modbus port has an address mapping table that allows messages

and commands received at the port to be routed as needed. The table

converts the Modbus address in the message to a routing path, allowing

it to be routed to a device on the local Modbus Plus network, to a device

on another Modbus Plus network, or to a device at another Modbus

port. The mapping table allows the devices to be uniquely addressed,

even when two or more ports have devices using the same Modbus

address.

For example, in Figure 23 both ports 1 and 3 have networks of Modbus

slave devices. Devices on the two networks can have the same

addresses. Commands or messages originated at the controllers on

Modbus Plus can be routed to a unique device on either of the two

Modbus networks. Similarly, the Modbus master device at port 2 can

attach to a unique device on either network.