Configuring Modbus TCP Explicit Messaging Using DATA

Introduction

When you use the DATA_EXCH block to create an explicit message for a Modbus TCP device, configure this block the same way you would configure it for any other Modbus communication. Refer to the Control Expert online help for instructions on how to configure the DATA_EXCH block.

Configuring ADDM Block Unit ID Settings

When you configure the DATA_EXCH block, use the ADDM block to set the DATA_EXCH block’s Address parameter. The ADDM block presents the configuration format ADDM(‘rack.slot.channel[ip_address}UnitID.message_type.protocol’) where:

The parameter...	Represents...
rack	the number assigned to the rack containing the communication module
slot	the position of the communication module in the rack
channel	the communication channel—set to a value of 0
ip_address	the IP address of the remote device, for example, 192.168.1.7
Unit ID	the destination node address, also known as the Modbus Plus on Ethernet Transporter (MET) mapping index value
message_type	the three character string TCP
protocol	the three character string MBS

The Unit ID value in a Modbus message indicates the destination of the message. The manner in which the communication module handles the Unit ID value depends upon its role as either a server or a client. When the communication module acts as a:

server: a message with Unit ID value of 255 is directed to and processed by the communication module itself. Other messages are passed to the CPU.
client: a message with Unit ID value of 255 is directed to and handled by the communication module itself. Other values are passed to the CPU.

Modbus function code 3 provides easy access to a variety of diagnostic functions, including basic network diagnostics, Ethernet port diagnostics, and Modbus port 502 diagnostics.

To access the function code 3 diagnostics from the local device, set the Unit ID to 255.

Contents of the Received_Data Parameter

The Received_Data parameter contains the Modbus response. The length of the response varies, and is reported by Management_Param[3] after the response is received. The format of the Modbus response is described, below:

Offset (words)	Length (bytes)	Description
0	2	First word of the Modbus response: High byte (MSB): if successful: Modbus Function Code if not: Modbus function code + 16#80 Low byte (LSB): if successful: depends on the request if not: Modbus exception code
1	Length of the Received_Data parameter – 2	Remainder of the Modbus response: depends on the specific Modbus request)

NOTE: Structure the response in little endian order.