Schneider Electric TSXCUSBMBP Configuring the Modbus Plus Routing, Addressing, Methods, Before

Introduction

When interfacing to an application through the Virtual Serial Port, the TSXCUSBMBP

operates as a serial Modbus to Modbus Plus converter, similar to the BM85 Bridge

Multiplexer. The Virtual Serial Port captures serial Modbus RTU messages sent by a

Modbus application and redirects them to the TSXCUSBMBP driver. The driver then

converts each message from the Modbus RTU format into the Modbus Plus format

before sending it out onto the network and waiting for a response.

Addressing

When converting a Modbus RTU request to Modbus Plus, the driver must convert a

Methods

single Modbus ID into a 5-byte Modbus Plus routing path. The TSXCUSBMBP driver

supports three different methods of doing this:

1. Explicit Addressing,

2. Direct Addressing,

3. Implicit Addressing.

The first method, known as Explicit Addressing, uses the TSXCUSBMBP driver's

Modbus Routing Table. A unique Modbus ID (also known as Modbus Slave Address

or Modbus Slave ID) is assigned by the user to each Modbus Plus device that will

be accessed from a Modbus application. An entry is then made into the Modbus

Routing Table that specifies the Modbus Plus Routing path to use whenever a

Modbus application sends a Modbus RTU message to that Modbus ID.

The second and third methods use the value of the Modbus ID to generate the

Modbus Plus routing path according to a predefined set of rules. These methods

include Direct Addressing and Implicit Addressing, where the actual addressing type

used is determined by the range of values that the Modbus ID falls into. Direct

Addressing and Implicit Addressing are only used when an entry does not exist in

the Modbus Routing Table for a Modbus ID.

The Explicit, Direct, and Implicit addressing methods are the same methods used by

the BM85 Bridge Multiplexer.

Before

Before configuring the Modbus Plus routing

configuring

1. make a complete list of all Modbus Plus nodes that will be accessed from Modbus

applications. Do not include any Modbus Plus nodes that will be accessed only

with the Diagnostic functions of the TSXCUSBMBP,

2. determine which nodes can be accessed using Direct Addressing or Implicit

Addressing (see below). The Modbus Routing Table has a maximum size of 64

entries and using Direct or Implicit Addressing where possible will limit the

number of table entries that must be used,

3. assign Modbus IDs to all remaining Modbus Plus nodes before entering them into

the Modbus Routing Table where Explicit Addressing will be used.

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