Modbus Client Protocol

MODBUS Overview

Modbus is a communication protocol used to transmit data across serial lines between various electronic devices. The protocol is open-source and can be used for major in industrial applications such as PLCs, variable speed drives, HMIs, Data acquisition systems etc. This is probably utilized for the connection of a system controlling using an RTU in SCADA in the power domain.

Embien offers Modbus Stack for both use in server and client configurations. Our implementation supports all the function codes defined by the protocol and supports extension via APIs too. Physical layers supported include RTU, ASCII and TCP/IP.

Some of the salient features of RAPIDSEA Modbus protocol stack are

• Single/Multiple Read/Write functionalities.

• Coils/Holding Registers and Input Registers.

• Supports major baud rates from 9600 to 115200 bauds and beyond

• RTU/ASCII modes can be operated with and without parity and configurable Stop bits.

• Server and Client Mode of operation.

• Royalty Free Licensing Model.

• Highly Optimized for MCU’s.

• Uses less amount of RAM and ROM.

• Runs on Linux, Windows and with/without RTOS.

The implementation is fully configurable and is offered for both server and client mode of operation.

Modbus Client

RAPIDSEA implements Modbus client where different types of requests such as READ_HOLDING, WRITE_INPUT etc can be orginated to the server address configured.

Supported Functionalities

Following services are supported in the Modbus Stack

Supported Functionalities

Function Code	Register Type	Value Type	Access Type
01 (0x01)	Read Coil Status	Discrete	Read
02 (0x02)	Read Input Status	Discrete	Read
03 (0x03)	Read Holding Registers	16 Bit	Read
04 (0x04)	Read Input Registers	16 Bit	Read
05 (0x05)	Write Single Coil	16 Bit	Read
06 (0x06)	Write Single Holding Register	16 Bit	Write
15 (0x0F)	Write Multiple Coils	Discrete	Write
16 (0x10)	Write Multiple Holding Registers	16 Bit	Write

Application Interface

While the RAPIDSEA Modbus client stack can handle most of the functionality such as server response validation, validate client request formation, once submit the request, properly release the given request,etc. The RAPIDSEA Modbus client clearly defines API and callback functions that are essential for the user to use/implement. The below table captures the function that are to be called from the application logic.

The below table contains API calls to perform the client stack.

API Functions

Function	Description
rs_modbus_client_open	To Initialize the serial/socket configuration.
rs_modbus_client_run	To set the state to be open
rs_modbus_client_process	To be called periodically to process the request.
rs_modbus_client_get_free_req	To get a free available request from Modbus channel.
rs_modbus_client_submit_req	To submit the given request for the given channel.
rs_modbus_client_is_req_done	To return the current status of the given request.
rs_modbus_client_put_req	To release the given request for further use.
rs_modbus_client_reconfig	To restart the client application by reconfiguring modbus client.
rs_modbus_client_close	To close the Modbus client.
rcb_modbus_serial	Called to handle serial timeouts

Only a millisecond timer is sufficient for the stack to run.

Implementation Guide

This section explains how to implement the Modbus client stack using the RAPIDSEA stack, the steps to be followed are

• Initially start the client init to start the Modbus stack.

• Then client task state is in idle condition, until Modbus client start.

• Modbus client start change the state from idle to state open.

• In Modbus client process, initially get the free available request from the Modbus channel by giving the Modbus handle.

• Modbus client process called periodically to form the client request.

• Once form the request, Submit the given request to the given channel.

• Properly release the given request for further use.

The below diagram captures the high level sequence of operations associated with the Modbus client RTU

High level state machine of modbus client is depicted below

Modbus Supported Modes

Modbus RTU (Remote Terminal Unit)

To establish protocol communication, this represents data in a binary format and is mostly utilized in serial communication. The messages in this version are divided by idle periods. The format that is followed in the RTU version is a cyclic redundancy check to verify mechanism and this makes sure of data reliability.

Serial communication header details refer to the serial interface page for more details Serial Interface

Modbus TCP/IP (Transmission Control Protocol)

To establish protocol communication, this represents data in a binary format and is mostly utilized in internet communication. The messages in this version are divided by idle periods. The format that is followed in the TCP version is a longitudinal redundancy check to verify mechanism and this makes sure of data reliability.

Socket communication header details refer to the socket interface page for more details Socket Interface

Modbus Client Stack Memory Usage

Modbus Client stack has the size of 28780 bytes.

Modbus Client Stack RAM Consumption

The RAM consumption is different for each of the interfaces.

RAM Consumption

Interface	Size
RTU	964 bytes
ASCII	1736 bytes
TCP	372 bytes

RTU Interface

The memory (944 bytes) needed can be separated as follows:

• 256 bytes - buffer space for RX/TX in Modbus

• 256 bytes - buffer space for handling the RX FIFO buffer by the interface layer

• 256 bytes - buffer space for handling the TX FIFO buffer by the interface layer

• 196 bytes - other configuration related memory

ASCII Interface

The memory (1736 bytes) needed can be separated as follows:

• 513 bytes - buffer space for RX/TX in Modbus

• 513 bytes - buffer space for handling the RX FIFO buffer by the interface layer

• 513 bytes - buffer space for handling the TX FIFO buffer by the interface layer

• 197 bytes - other configuration related memory

TCP Interface

The memory (372 bytes) needed can be separated as following:

• 260 bytes - buffer space for RX/TX in Modbus

• 112 bytes - other configuration related memory

Modbus Client Header Details

Documentation from the relevant client header as follows:

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