rmodbus - Modbus for Rust

A framework to build fast and reliable Modbus-powered applications.

Cargo crate: https://crates.io/crates/rmodbus

What is it

rmodbus is not a yet another Modbus server. rmodbus is a set of tools to quickly build Modbus-powered applications.

Why yet another Modbus lib?

• rmodbus is transport and protocol independent

• rmodbus is platform independent

• can be easily used in blocking, async (non-blocking) applications

• tuned for speed and reliability

• provides a set of tools to easily work with Modbus context

• supports server frame processing for Modbus TCP/UDP and RTU

So the server isn't included?

Yes, there's no server included. You build the server by your own. You choose protocol, technology and everything else. rmodbus just process frames and works with Modbus context.

Here's an example of simple TCP blocking server:

use std::io::{Read, Write};
use std::net::TcpListener;
use std::thread;
use rmodbus::server::{ModbusFrame, ModbusProto, process_frame};

fn tcpserver(unit: u8, listen: &str) {
    let listener = TcpListener::bind(listen).unwrap();
    println!("listening started, ready to accept");
    for stream in listener.incoming() {
        thread::spawn(move || {
            println!("client connected");
            let mut stream = stream.unwrap();
            loop {
                let mut buf: ModbusFrame = [0; 256];
                if stream.read(&mut buf).unwrap_or(0) == 0 {
                    return;
                }
                let response: Vec<u8> =
                    // the function will process Modbus frame, read/write
                    // context and generate ready-to-send response
                    match process_frame(unit, &buf, ModbusProto::TcpUdp) {
                        Some(v) => v,
                        None => {
                            println!("frame drop");
                            continue;
                        }
                    };
                println!("{:x?}", response);
                if stream.write(response.as_slice()).is_err() {
                    return;
                }
            }
        });
    }
}

There are also examples for Serial-RTU and UDP in examples folder (if you're reading this text somewhere else, visit rmodbus project repository.

Modbus context

The rule is simple: one standard Modbus context per application. 10k+10k 16-bit registers and 10k+10k coils is usually more than enough. This takes about 43Kbytes of RAM, but if you need to reduce context size, download library source and change CONTEXT_SIZE constant in "context.rs".

rmodbus server context is thread-safe, easy to use and has a lot of functions.

Every time Modbus context is accessed, context mutex must be locked. This slows down a performance, but guarantees that the context always has valid data after bulk-set or 32-bit data types were written. So make sure your application locks context only when required and only for a short period time.

There are two groups of context functions:

• High-level API: simple functions like get_coil automatically lock context but do this every time when called. Use this for testing or if the speed is not important.

• Advanced way is to use low-level API, lock context and then call proper functions, like set, set_f32 etc.

Take a look at simple PLC example:

use rmodbus::server::context;

fn looping() {
    loop {
        // READ WORK MODES ETC
        let mut ctx = context::CONTEXT.lock().unwrap();
        let _param1 = context::get(1000, &ctx.holdings).unwrap();
        let _param2 = context::get_f32(1100, &ctx.holdings).unwrap(); // ieee754 f32
        let _param3 = context::get_u32(1200, &ctx.holdings).unwrap(); // u32
        let cmd = context::get(1500, &ctx.holdings).unwrap();
        context::set(1500, 0, &mut ctx.holdings).unwrap();
        match cmd {
            1 => {
                let _ = context::save_locked("/tmp/plc1.dat", &ctx).map_err(|_| {
                    eprintln!("unable to save context!");
                });
            }
            _ => {}
        }
        drop(ctx);
        // does the same but slower
        //let _param1 = context::get_holding(1000);
        //let _param2 = context::get_holding_f32(1100);
        //let _param3 = context::get_holding_u32(1200);
        // ==============================================
        // DO SOME JOB
        // ..........
        // WRITE RESULTS
        let mut ctx = context::CONTEXT.lock().unwrap();
        context::set(0, true, &mut ctx.coils).unwrap();
        context::set_bulk_with_context(10, &(vec![10, 20]), &mut ctx.holdings).unwrap();
        context::set_f32(20, 935.77, &mut ctx.inputs).unwrap();
    }
}

To let the above program communicate with outer world, Modbus server must be up and running in the separate thread, asynchronously or whatever is preferred.