Modbus-SL Functions

General Description

The Modbus messaging service provides a client/server communication between devices connected to a Modbus Serial Line (Modbus-SL) network.

The client/server model is based on four types of messages:

Modbus Request, the message sent on the network by the client to initiate a transaction.
Modbus Indication, the request message received on the server side.
Modbus Response, the response message sent by the server.
Modbus Confirmation, the response message received on the client side.

The Modbus messaging services (client/server model) are used for real time information exchange between:

Two device applications.
Device application and other device.
HMI/SCADA applications and devices.
A PC and a device program providing on line services.

On Serial Line, the Modbus Application Data Unit (ADU) encapsulates the Protocol Data Unit (PDU) adding the server address and a Cyclic Redundancy Check (CRC). A dedicated header is used on TCP/IP to identify the Modbus Application Data Unit. It is called the MBAP header (Modbus Application Protocol header).

Table of Modbus Functions

The following table describes in detail the functions supported by the Modbus-SL devices supported by the EcoStruxure Panel Server:

Function code	Function name
03	Read n words
04	read input register
06	Write 1 word
16	Write n words
43-14	Read device identification (refer to Function 43-14: Read Device Identification)
100-4	Read non-adjacent words where n ≤ 100 (refer to Function 100-4: Read Non-Adjacent Registers)

IMPORTANT: Function codes 1 and 2 are not supported.