Modbus Server 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 Server
RAPIDSEA implements Modbus server where multiple clients can connect and communicate with it to obtain information.
Supported Functionalities
Following services are supported in the Modbus Stack
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 server stack can handle most of the functionality such as client request validation, if it has error request, proper intimation to the client. The RAPIDSEA Modbus server 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.
Function |
Description |
|---|---|
rs_modbus_server_open |
To Initialize the serial/socket configuration. |
rs_modbus_server_run |
To set the state to be open |
rs_modbus_server_process |
To be called periodically to prepare the client response. |
rs_modbus_server_reconfig |
To restart the server application by reconfiguring modbus server |
The below table contains rcb calls to perform the server stack.
Function |
Description |
|---|---|
rcb_mb_set_holding_register |
Called to set the holding register value |
rcb_mb_set_coil_status_value |
Called to set the coil status value |
rcb_mb_get_coil_status_value |
Called to get the coil status value |
rcb_mb_get_holding_reg_val |
Called to get the holding register value |
rcb_mb_get_input_register_value |
Called to get the input register value |
rcb_mb_get_discrete_input_value |
Called to get the discrete input value |
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 Modbus server task from the application layer.
Then the server task state is in idle condition, until Modbus server starts.
Modbus server starts change the state from idle to state open.
In Modbus server process, process the client request and prepare the response for the client.
In case of a request failing, error code will be sent from the server response.
The below diagram captures the high level sequence of operations associated with the Modbus server RTU.
High level flow chart is depicted below for Modbus server:
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)
This version of Modbus is implemented for communications through TCP/IP networks linking over port 502. This variant does not require any kind of checksum calculation as lower levels itself provides this. Modbus TCP protocol uses a 10 Mbps Ethernet standard to convey the entire structure of Modbus messages.
Socket communication header details refer to the socket interface page for more details Socket Interface
Modbus Server Stack Memory Usage
The Modbus server stack has a size of 22588 bytes.
Modbus Server Stack RAM Consumption
The RAM consumption is different for each of the interfaces.
Interface |
Size |
|---|---|
RTU |
944 bytes |
ASCII |
1716 bytes |
TCP |
3232 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
176 bytes - other configuration related memory
ASCII Interface
- The memory (1716 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 (3232 bytes) needed can be separated as follows:
2600 bytes - buffer space for RX/TX in Modbus for 10 client connections
632 bytes - other configuration related memory
Modbus Server Header Details
Documentation from the relevant server header as follows:
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