Modbus RTU and File Descriptor Support for Parallel Interface Monitorin

Modbus RTU and File Descriptor Support for Parallel Interface Monitorin - Printable Version

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Modbus RTU and File Descriptor Support for Parallel Interface Monitorin - Blooddah - 21.01.2025

Hello,

I am currently working with the LuaModbus library to communicate with Modbus RTU devices and have successfully implemented basic register reading functionality. Below is a simplified example of my current approach.

Code:
require('luamodbus')

-- Modbus RTU communication parameters
local port_rtu = "/dev/RS485-2"
local baud_rate = 9600
local parity = 'N'
local data_bits = 8
local stop_bit = 1
local slave_id = 78
local register_address = 19
local timeout = 5

-- Establish Modbus RTU connection
local mb = luamodbus.rtu()
mb:open(port_rtu, baud_rate, parity, data_bits, stop_bit)
mb:connect()
mb:setslave(slave_id)
mb:setresponsetimeout(timeout)

-- Read the register
local value, err = mb:readregisters(register_address, 1)

-- Display the result
if err then
  print("Read error: " .. err)
else
  print("Value read at register " .. register_address .. ": " .. value[1])
end

-- Close the connection
mb:close()

However, I would like to extend this functionality to periodically monitor multiple interfaces in parallel without relying on the Copas library, as it does not meet my performance requirements.
Specifically, I am interested in achieving behavior similar to MQTT, where I can use "socket" to monitor multiple file descriptors and trigger events when data is available. Is it possible to retrieve a file descriptor for a Modbus RTU connection (similar to the MQTT socket), which I can integrate into an event loop?

Code:
--[[...]] -- Client initialization

while true do
    connect_mqtt()

    local timerfd = socket.fdmaskset(timer:getfd(), 'r')
    local mclientfdset = mclient and socket.fdmaskset(mclient:getfd(), mclient:want_write() and 'rw' or 'r')

    local res, timerstat, mclientstat = socket.selectfds(10, timerfd, mclientfdset)

    if res then
        if timerstat then
            handle_timer_event()
        end

        if mclient and mclientstat then
            if socket.fdmaskread(mclientstat) then
                mclient:loop_read()
            end
            if socket.fdmaskwrite(mclientstat) then
                mclient:loop_write()
            end
            mclient:loop_misc()
        end
    end

    os.sleep(1)
end

This would allow me to efficiently manage multiple Modbus interfaces concurrently within the same script.
Any guidance or advice on how to implement this functionality with LuaModbus would be greatly appreciated.

RE: Modbus RTU and File Descriptor Support for Parallel Interface Monitorin - admin - 21.01.2025

Not possible. Modbus library functions are blocking. You can't perform any other actions until read/write is finished. The only solution I see is to split each Modbus port handler into a different process.